NATL INST OF ^ ||||||| A1 11 03088706

Mechanical Failures/The role of coat^^^^^ QC100 .U57 N0.452, 1976 C.2 NBS-PUB-l^ 1» Q NBS SPECIAL PUBLICATION 452

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

The Role of Coatings

in the Prevention of

Mechanical Failures

MFPG

23rd Meeting
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MFPG

The Role of Coatings in the Prevention of

Mechanical Failures v . vv Vc^w^

Proceedings of the 23rd Meeting of the

Mechanical Failures Prevention Group,

held at the National Bureau of Standards,

Gaithersburg, Maryland, October 29-31, 1975

Edited by

T. R. Shives and W. A. Willard

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

The 23rd meeting of MFPC and these proceedings were

sponsored by the Institute for Materials Research of the National Bureau of Standards, Washington, D.C. 20234;

the Office of Naval Research, Department of the Navy,

Arlington, Va. 22217; the Naval Air Development

Center, Department of the Navy, Warminster,

Pa. 18974; the Frankford Arsenal, U.S. Army,

Philadelphia, Pa. 19137; and the Federal Aviation Administration, Department of Transportation,

Washington, D.C. 20591.

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secrefary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

2 NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued September 1976
Library of Congress Cataloging in Publication Data

Mechanical Failures Prevention Group. MFPG--the role of coatings in the prevention of mechanical failures. (National Bureau of Standards special publication ; 452)

Supt. of Docs, no.: C 13.10:452

1. Fracture-mechanics-Congresses. 2. Coatings-Congresses. I. Shives,

T. R. II. Willard, WiHiam A. III. United States. National Bureau of Standards. Institute for Materials Research. IV. Title. V. Title: The role of coatings in the prevention of mechanical failures. VI. Series: United

States. National Bureau of Standards. Special publication ; 452. QC100.U57 no. 452 [TA409] 602'. Is [620.n26] 76-608258

National Bureau of Standards Special Publication 452

Nat. Bur. Stand. (U.S.), Spec. Publ. 452, 199 pages (Sept. 1976) CODEN: XNBSAV

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FOREWORD

The 23rd meeting of the Mechanical Failures Prevention Group was held

October 29-31, 1975, at the National Bureau of Standards in

Gai thersburg, Maryland. The program was organized by the MFPG

committee on the Mechanisms of Failure under the chairmanship of

Professor Elmer E. Klaus of The Pennsylvania State University. The

Mechanisms committee, the session chairmen, and especially the speakers

are to be commended for the excellent program.

The papers in these Proceedings are presented as submitted by the

authors on camera ready copy, except for some minor editorial changes.

In addition to the papers, the Proceedings include the discussions of

the talks. The discussions were recorded at the meeting and have been

edited to improve readability.

Appreciation is extended to Mr. T. Robert Shives and Mr. William A.

Willard of the NBS Metallurgy Division for their editing, organization,

and preparation of the Proceedings, to Mrs. Sara R. Torrence of the

NBS Office of Information Activities for the meeting arrangements, to

Mr. Harry C. Burnett of the Metallurgy Division for general coordination and registration, to Mr. Paul M. Fleming of the Metallurgy

Division for handling financial matters, and to the entire staffs of

the Metallurgy Division and the NBS Institute for Materials Research

for their assistance in many ways. Special thanks are accorded to

Mrs. Marian L. Slusser of the Metallurgy Division for her diligent

efforts in transcribing the recorded discussions.

ELIO PASSAGLIA

Executive Secretary, MFPG

Chief, Metallurgy Division

National Bureau of Standards

III
TABLE OF CONTENTS

FOREWORD

SESSION I: POLYMER COATINGS

1. The Friction and Wear of Polymeric Coatings on Metal.

K. C. Ludema

2. Performance Studies of Polymer Processed Mechanical

and Structural Components. M. J. Devine

3. Nylon II as a Lubricant and Corrosion Preventative

Coating. R. G. Baker, G. H. Kitchen* and

R. R. Wells

4. Mechanical Design Concepts for Coated Nuclear Fuel

Particles. J. M. Tobin

SESSION II: METAL COATINGS

1. Nickel-Phosphorus Alloy Coatings for Wear Resistance.

C. E. Johnson* and F. Ogburn

2. Chromium Deposit Structure Has a Profound Influence

on Properties. W. H. Safranek

3. Plated Coatings for Electric Contacts.

M. Antler

4. Compatibility Effects When Soft Metals Are Used as Solid Lubricants. E. Rabinowicz

5. Reduction of Sliding Wear by Metallic Coatings.

S. Jahanmir* and N. P. Suh

SESSION III: SOLID FILM LUBRICANTS

1. The Search for Factors to Prevent Mechanical Failures

Due to Corrosion with Solid Film Lubricants.

M. K. Gabel* and M. B. Peterson

2. Rust-Inhibited Nonreactive Perfl uorinated Polymer

Greases. J. Messina

IV
3. The Role of Coatings in the Prevention of Erosion 120

Damage. A. P. Thiruvengadam* and A. A. Hochrein, Jr.

4. Solid Film Lubricant to Prevent Fretting Damage in 124

Titanium Engine Components. B. D. McConnell

5. A Study of Solid Lubricated Gears at Elevated Temperatures. 132

A. J. Haltner, D. K. Snediker* and K. E. Demorest

SESSION IV: CHEMICAL COATINGS

1. Vapor Deposition of Wear-Resistant Coatings. 151

W. J. Wilson,* M. F. Browning and J. M. Blocher, Jr.

2. Barrier Films for Miniature Bearings. 170

M. K. Bernett and H. Ravner*

3. Potential of Sputtered Coatings for Improved Gas Bearing 177

Instrument Performance. Maj. L. L. Fehrenbacher,*

B. D. McConnell and Capt. C. Pellerin

4. Ferrography of Polymer Formation Under Lubricated 187

Rubbing Contact—an Exploratory Investigation.

A. A. Reda

LIST OF REGISTRANTS FOR THE 23rd MFPG MEETING 192

* Indicates speaker when a paper had more than one author.

V
ABSTRACT

These Proceedings consist of a group of eighteen submitted papers

and discussions from the 23rd meeting of the Mechanical Failures

Prevention Group which was held at the National Bureau of Standards in

Gai thersburg, Maryland on October 29-31, 1975. The central theme of

the Proceedings is the role of coatings in the prevention of mechanical

failures, with emphasis on polymer coatings, metal coatings, solid film

lubricants, and chemical coatings.

Key words : Barrier film; chemical coatings; failure prevention;

mechanical failure; metal coatings; plated coatings; polymer coatings;

solid film lubricants; sputtered coatings; wear

UNITS AND SYMBOLS

Customary United States units and symbols appear in many of the

papers in these Proceedings. The participants in the 23rd meeting of

the Mechanical Failures Prevention Group have used the established units

and symbols commonly employed in their professional fields. However,

as an aid to the reader in increasing familiarity with and usage of the

metric system of units (SI), the following references are given:

NBS Special Publication, SP330, 1974 Edition, "The International System

of Units."

ISO International Standard 1000 (1973 Edition), "SI Units and Recommendations for Use of Their Multiples and of Certain Other Units."

E380-76 ASTM Metric Practice Guide (American National Standard Z210.1).

Disclaimer:

Certain trade names and company products are identified in order

to adequately specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply that the products are necessarily the best available for the purpose.

VI
SESSION I

POLYMER

COATINGS

Chairman: E.E.Klaus

The Pennsylvania State University

THE FRICTION AND WEAR OF POLYMERIC COATINGS ON METAL

K.C. Ludema

Mechanical Engineering Department

The University of Michigan

Ann Arbor, Michigan 48lOU

Abstract

Polymeric coatings on metals wear and/or debond when rubbed. Information on wear is available from tests using bulk polymers which us- ually have a different structure than do thin films. The bond strength

requirements for rubbing surfaces appear to be greater than that developed by the technology centered in adhesives and composite materials

industries. Many of the factors involved in wear life and bonding are summarized in the paper. Recent and broad ranging references are given

for each of the major points in the paper.

Keywords: Adhesion; coatings; friction; metal; polymers; wear,

A frequently quoted ancient author of papers on friction is

Amontons [1] a French engineer. More often than not his conclusions are cited so that they may be differed with in the modern paper. It is not

usually mentioned, however, that many of the conclusions of Amontons

were based on research using a lubricant. In fact the lubricant used

was pork fat, or "suet" [2], and this produced a coefficient of friction

of 0.2. This number has been faithfully carried down to the modern day

by countless handbooks and textbooks, and is now taken as the best estimate for dry friction.

Polymeric films stand in the long succession of lubricants beyond

suet. This evolution has included lubrication: by liquids alone, by

liquids containing boundary additives, by liquids containing solid lubricants, by solid lubricants attached to surfaces by various adhesives,

and finally by polymeric films themselves. Obviously each step marked

progress toward the achieving of some goal including economy.

Polymeric coatings can be considered either protective coatings or lubricants depending upon the industry. They are protective when applied to optical surfaces where scratching must be prevented. They are lubricants when they prevent seizure of machine elements. They can be

considered mere conveniences in other instances. They are used because

they do not collect dust as does a liquid lubricant film. They can be

touched by clothing or hands without undesirable asthetic consequences.

They can be used near food without adding visible "dirt" or toxicity to

the food. They do not migrate or evaporate as does a liquid. They can

be operated over a wider range of temperature than many liquids.

5
Finally they can add corrosion protection, electrical resistivity, color,

and liquid barrier capabilities to surfaces.

Historically thin polymeric films had two separate uses as lubricants in machinery. The first is as a binder for such solid lubricants

as graphite, M0S2, and others. The second is as a film without fillers.

The two films can be distinguished, however, in terms of the severity of

service each is subjected to. Solid lubricants are usually attached to

a metal surface by a thermo-setting polymer and more recently by silicone resin [3]. This is a consequence of the fact that solid lubricants

find themselves in situations where high temperatures are generated and

only thermo-setting or silicone polymers will survive. However, thermosetting polymers are usually mildly abrasive to the counter surface. In

terms of wear of the counter surface this is usually not of severe con- sequence since graphite and M0S2 also wear the counter surface [4] by

way of impurities. Simple polymeric layers on the other hand are often

of the linear polymer or thermo-plastic polymer group. These are cheaper than composite layers and very useful for many light duty applications .

In some instances solid lubricants are bonded with linear polymers

[5] and in other instances films of linear polymer are lubricated by

liquids [6] . It is not clear that these formulations offer long wear

life. In this paper we will consider only the simple films of a few

linear polymers.

Linear Pol3rmer Films

o Thin films of polymer on metal are usually on the order of 100 A to

4000 A thickness. This is to be distinguished from the wall thickness

of polymer sleeve bearings which is often in the order of 1/10 of the

shaft diameter. Since thin films are so very thin, the system must be

constructed to avoid too many episodes of local high wear rate. Furthermore, thin films rely upon good bonding with the adherend (substrate)

far more than do the polymer bearings. Thus, there are two problems.

Good bonding of films, and film integrity.

Very thin films of polymer on metal are sometimes used to achieve a low coefficient of friction. A linear polymer is very soft as compared

with a metal and its behavior is analogous to that of a thin film of

soft metal on a harder substrate [7]. The behavior of a soft film between a sphere and flat surface is described in the model of Tabor which

is expressed in terms of F = A x s. With a hard substrate the contact area is low and shear strength is high, and with a soft substrate the

contact area is high but the shear strength is low, both yielding a moderate friction force. On the other hand, when a thin soft film of low

shear strength is placed upon a hard backing the contact area is also

small, leading to a low friction force.

Whereas the model is described in terms of a sphere of radius about

1^
100 times the film thickness, the principle applies as well to contact

stress of flat surfaces in the order of the strength of the polymer.

Apparently the proper conditions for low friction are, a real contact area nearly equal the apparent area and a soft film which is too thin to

extrude from between the hard sliding bodies. This condition produces a coefficient of friction [9] of about 0.12 for coatings of several polymers (high density polyethylene, polypropylene, polyvinylidene fluoride,

FEP, and E.T.F.E.) and values as low as 0.03 for low density polyethylene and polytetrafluorethylene . However, when asperity radius becomes

smaller than 1/10 of film thickness or if contact pressures become much

less than about 50% of the strength of the polymer, the coefficient of

friction returns to values in excess of 0.35. Thus in the lightly loaded rubbing of close fitting parts a polymer film, other than Teflon, may

produce a higher friction than between two metal surfaces.

The Wear of Polymeric Films

Poljrmer films wear by several mechanisms, as follows:

1. A progressive, mild wear by non-repeating passage of paper, cloth,

metals, human hands, etc. The most likely mechanism of polymer loss in

this case is abrasion.

2. A relatively slower loss of polymer due to repeated passage of a counter surface. This is found in conveyors, hinges, etc. In these instances transfer of polymer from the deposited film to the counter surface is usually achieved, and this occurs in the first few passes [10]

.

Continued rubbing consists in an exchange of the transferred film between the rubbing surfaces and by shearing of the thin transferred film.

However, contaminants including water may produce a "debond" of transfer film [11,12] which then is removed from the system in long ribbons

and/or flakes. Retention of the transfer film is vital to long life of

the polymer coated component [13].

3. At high speeds and loads there may be thermal degradation of the

polymer. In this case, characteristic gases are evolved from the rubbing surfaces and a discontinuous or particulate wear debris is formed.

This case is not predicted by the well-known PV limit. (At this point

it may be useful to point out that the PV limit is not regarded as applicable to thin films of polymer whatever the loss mechanism [14].)

4. Loosening of the polymeric film from the substrate (adherend) causes large regions of polymer to be removed from the system, and these regions are often larger than a local area of contact distress. The loss

of large regions of film is usually the consequence of progressive debonding. Either there was poor strength originally due to improper displacement of adsorbed gas during the coating process, or the film may

have been permeated by a fluid which preferentially wet the interface.

Film Deposition

Highly adherent polymeric films have been achieved with only limited success. Most often it is attempted with a number of recipes for

5
"couplers" purporting to achieve the DeBruyne-Zisman criteria—namely

the liquid polymer must wet the solid completely and spontaneously [15,

16]. The majority of recent developments in coupling agents has apparently been in the field of composite materials where good bonding between fibers (or particles) and a matrix is thought to be desirable [16]

Whereas most papers on bond strength focus on any available improve

ment some researchers are searching for the theoretical maximum bond

strength. In some work bond strength equal to the cohesive strength

(tensile strength) of the polymer is reported. Higher bond strengths seem unnecessary, but interesting. For example, polymeric films can be- come attached so firmly to metals during sliding that upon separation of

the bodies some metal (aluminum) has been found attached to the polymer

(PTFE) [17]. The polymer-metal bond in vacuum was found to be chemical

in nature, not vanderWaals forces as is often supposed. In air the bond

is probably between the polymer and the metal oxide, and attack by water

upon the oxide is the probable cause of loss of bonding [18]. Here obviously are challenges for the industrial chemist.

Another concern in the search for high bond strength is the optimum

surface roughness of metals to be coated. Rough surfaces promote spread

ing of liquid, apparently fulfilling the DeBruyne-Zisman criterion. On

the other hand, rough surfaces trap more air between the polymer and

substrate than does a smooth surface and this decreases average bond

strength [19].

After surface preparation the polymer may be applied in many ways

such as by spraying or dipping. However, as mentioned above a polymer

film can also be applied by rubbing a block of polymer against the surface to be coated. In many instances rubbing apparently produces a stronger bond than other methods but the reason is not well understood.

The testing of film durability can be done by a number of methods.

Perhaps the best understood test is the peel test. This test is difficult to use as a manufacturing quality control test because a precise

setup is required. On the other hand, for production quality control a roller test is often used and this test may be called by several names including the blister test. Presumably the blister test tests a material in fatigue in some manner since blisters are found to grow by re- peated loading. It is not known whether this is a consequence of migration of contaminants under the film by the action of the roller, or whether the bond between the polymer and the metal substrate ruptures in

fatigue. Surprisingly little is known about the stress state in a blister test and whether it approximates practice. Very little of an analytical nature is published for the blister test. On the other hand, con- siderable information is available for the peel test [20], usually

treating the film as a visco-elastic substance.

Increasing the Life of Polymeric Films

The answer to questions on wear life depends upon the mechanism of

6
wear. If wear is primarily due to abrasion then a polymer film may be

improved by including in it a distribution of hard fillers such as glass

beads. If debond is the problem then the chemistry of interfaces must

be considered. If all else fails a little suet may help!

7
REFERENCES

1. Amontons, Histoire de I'Academie Royale des Sciences avec les

Memoires de Mathematique et de Physique, p. 206.

2. D. Dowson, Private communication.

3. R.J. Benzing, B.D. McConnell and W.L. Chow, "A Methyl Phenyl Polysiloxane Bonded Lubricating Film", International Conference on Solid

Lubrication, Denver, CO, 1971 p. 116-128, American Society of Lubrication Engineers.

4. J. P. Giltrow and J.K. Lancaster, "The Role of Impurities in the

Abrasiveness of M0S2", International Conference on Solid Lubrication

Denver, CO, 1971, p. 79-85, American Society of Lubrication Engineers .

5. H.S. White, "Small Oil-Free Bearings", J. of Research, N.B.S., 27,

p. 185-204, (1956).

6. B.J. Briscoe, V. Mustafaev and D. Tabor, "Lubrication of Polythene

by Oleamide and Stearamide", Wear, 19, p. 399-414, (1972).

7. C. Dayson, "The Friction of Very Thin Solid Film Lubricants on Surfaces of Finite Roughness", ASLE Trans., V 14, p. 105-115, (1971).

8. F.P. Bowden and D. Tabor, "The Friction and Lubrication of Solids,

II", Clarendon Press, 1964.

9. R.C. Bowers, "Coefficient of Friction of High Polymers as a Function

of Pressure", J. Appl. Phys., V 42, N 12, p. 4961-4970, (1971).

10. B.J. Briscoe, CM. Pooley and D. Tabor, "Friction and Transfer of

Some Polymers in Unlubricated Sliding", Advances in Polymer Friction and Wear, Vol 5a, p. 191-202, Ed. L.H. Lee, Plenum Press, 1974.

11. D. Summers-Smith, "Operating Experience with Filled PTFE Piston

Rings", (ASME) J. Lubrication Technology, V 93, p. 293-301, (April

1971).

12. J.K. Lancaster, "Lubrication of Carbon Fibre Reinforced Polymers",

Wear, V 20, p. 315-350, (1972).

13. J.H. Fuchsluger and R.D. Taber, "The Effect of Atmospheric Environment on the Wear Properties of Filled T.F.E. Materials", (ASME) J. Lubrication Technology, V 93, p. 423, (June 1971).

14. J.K. Lancaster, "Dry Bearings, A Survey of Materials and Factors

Affecting Their Performance", Tribology, V 6, p. 219-251, (Dec.

1973)

.

15. R.C. Bowers, E.G. Shafrin and W.A. Zisman, "Frictional Properties of

Adsorbed Halogenated Carboxylic Acids—Potential as Coupling Agents"

I.&E.C. Res. & Dev. V 14, p. 44-50, (1975).

16. P.W. Erikson, "Historical Background of the Interface—Studies and

Theories", J. Adhesion, V 2, p. 131-146, (July 1970).

17. D.H. Buckley and W.A. Brainard, "The Atomic Nature of Polymer-Metal

Interaction in Adhesion, Friction and Wear", Advances in Polymer

8
Friction and Wear, Vol 5A, p. 315-350, Ed. by L.H. Lee, Plenum Press^

1974.

18. W.D. Bascom and R.L. Patrick, "Chemistry of Bonding Metals with

Polymer Adhesives", Adhesives Age, p. 25, Oct. 1974.

19. C.W. Jennings, "Surface Roughness and Bond Strength of Adhesives",

J. Adhesion, V 4, p. 25-38, (1972).

20. S. Yamamoto and M. Hayashi, "Viscoelastic Analysis on Peel Adhesion

of Adhesive Tape by Matrix Method", J. Appl. Polymer Sci., V 19,

p. 2107-2118, (1975).

9
DISCUSSION

R. S. Miller, Office of Naval Research : At what temperatures or under

what conditions does the siloxane polymer change from organic to

inorganic and what is the size distribution of the particles?

K. C. Ludema : That subject is dealt with in a paper by Benzing,

McConnell and Chow. Mr. McConnell is here—perhaps he can comment.

B. U. McConnell, USAF Materials Laboratory : I don't know the

particle sizes of the material after the conversion, but the

temperature range for changing from the organic to the inorganic is

somewhere in the neighborhood of 800 F. This is based more on a high

temperature paint formulation than on our solid film work.

A. J. Babecki, NASA, Goddard Space Flight Center : A lot of the ball

bearings in spacecraft applications incorporate a Teflon ball cage,

and lubrication is brought about by transfer of the Teflon from the

cage to the balls and to the races. But one of the problems we have

with that kind of lubrication is the continual buildup of this

transferred material which eventually causes high friction within the

bearing, or at least roughness in the bearing. Can this continual

transfer be minimized by precoating the metal surfaces with Teflon or something else?

K. C. Ludema : I can point to the work by Tabor, et al , and a couple

of others who show that the nature of the transfer film in Teflon

varies according to strain rate and temperature. A buildup is due to

low operating temperatures or high strain rate in the polymer. This

buildup is also usually a consequence of the test configuration. If

a piece of polymer is kept pressed against a piece of glass and made

to go around in a circle, the buildup never develops. In a ball

bearing, there may be a particle carried around that gets into the

track.

E. E. Klaus, The Pennsylvania State University : Mechanistically, to

what do you attribute the order of magnitude change in friction

between high and low density polyethylene?

K. C. Ludema : I go to the explanation of Tabor, apparently supported

by others working in the polymer field. Tabor sees in low density

polyethylene and Teflon smooth molecular profiles. This means that

the molecules can shear or slip over each other much more easily than

those with bulky side groups.

E. E. Klaus : You mean crystall inity has nothing to do with it?

K. C. Ludema : That's correct. There isn't a great amount of

crystal 1 inity in either of those anyway.
E. E. Klaus : Generally, the difference in friction is attributed to

the difference in crystall inity. In fact the high density polyethylene has an average of 5 methyl side chains per thousand carbon

atoms versus 40 per thousand for the low density material. That

doesn't seem like a whole lot of side chains to blame it on.

R. S. Miller : If you have a service application in which the

temperature can vary and wear can occur at all temperatures, how do

you best pick a polymer to minimize the wear rate? Can you correlate

such things as the glass transition temperature of the polymeric

film with wear?

K. C. Ludema : I don't think there is a single criterion one can

use. If you consider the glass transition temperature you are totally frustrated by the fact that polyethylene with a glass

transition temperature of about -120 C lasts a lot longer than

polymethylmethacrylate with a glass transition temperature of +105

C. Wear rate is not correlatable with glass transition temperature

or with any other visco-elastic quantity.

S. A. Karpe, David Taylor Naval Ship Research and Development Center : What is the effect of pressure on the shear strength in these films?

K. C. Ludema : You may refer to the work of R. C. Bowers (Reference

no. 9 of this paper) who has done as much as anybody in the field.

S. A. Karpe : What is the effect of counterface surface topography?

K. C. Ludema : C. W. Jennings (Reference no. 19 of this paper) has

explored this, although not in sliding contact. He tested the glue

bond between two pieces of metal by running a series of tests varying

the surface roughness of the metals and testing the mechanical

strength of the bond. It is his conviction that surface roughness is

a factor in the strength of the bond largely because it influences

wettability. As far as the surface roughness of the counterface is

concerned, there is conflicting evidence as to the influence on wear.

S. A. Karpe : Most of our discussion here has been on bulk polymers.

Do thin films behave in a similar manner?

K. C. Ludema : All of our friction information is from bulk polymers.

Very little work has been done studying the wear mechanisms of thin

fi 1 ms

.

S. Jahanmir, Massachusetts Institute of Technology : You mentioned the

thickness of the films you used as ranging from 100 to 4,000 angstroms.

First, I would like to know why you selected this range, and second, is

there any optimum range that exists for a particular type of coating

and a particular substrate or for a particular sliding condition?

11
K. C. Ludema : I quote 100 to 4,000 angstroms because that's what

is in the literature. Now, for optimum film thickness, it appears

that film thickness has one effect when it comes to wear--the thicker

the film the longer the life--and has an opposite effect on bonding.

In the dynamics of debonding, there is an optimum film thickness

above which debonding is more likely to happen.

E. E. Klaus : You were non-commital on the surface roughness. A

year or two before the date you quoted, at the last ASLE-ASME International Lubrication Conference, there were three papers which all

seemed to indicate that a surface roughness of about 15 yin seemed

to be an optimum. Could you tell us what the surface roughnesses

were in the polymer studies?

K. C. Ludema : It was in that area, but I don't remember the specific

numbers

.

E. E. Klaus : Well, at least in one paper that dealt with solid and

liquid lubricants, one got the impression that if you considered

surface roughnesses on either side of 15 uin, the results were quite different from those obtained at 15 uin.

K. C. Ludema : I looked for the possibility that there might be a minimum or a maximum to the right of which and to the left of which

there would be opposite behavior or similar behavior and I could

find no correlation. Lancaster, with the Royal Aeronautical

Establishment in England, feels that surface roughness is of no consequence where there is transfer because transfer simply fills

up all the little holes and the surface becomes smooth. I don't

quite believe that, but it must be considered as a possibility.

Therefore, surface roughness would be thought to be more important

for thermosetting resins which do not form a transfer film very

readily and, again we didn't find any sensible correlation there.

A. W. Ruff, National Bureau of Standards : Does the wear rate or the wear mechanism or both depend on the nature of orientation of the

transferred polymer film?

K. C. Ludema : The effect of orientation is very transient. In

about the first 1/8 in of travel, original orientation makes a big

difference, but ever thereafter the surface is oriented in the

sliding direction. The sliding process orients the polymer so it

doesn't matter what the initial orientation was.

A. W. Ruff : I was thinking of reciprocal sliding.

K. C. Ludema : In reciprocal sliding, there is a reorientation

problem on either end. There are big globs of polymer worn off

on either end of the travel.

12
S. A. Karpe : I think that polyethylenes are two-phase materials;

i.e., they are partly amorphous and partly crystalline. If you use

: the amorphous shear strength in Tabor's adhesion theory of friction

' instead of the bulk shear strength, there is a beautiful correlation

for friction coefficients. In our work, we have found a range of

surface roughness of the metal counterface between about 15 and

35 yin where the coefficient of friction of the polymers is

constant. For very smooth surfaces, there are very slight increases

j in the coefficient of friction due, we believe, to an effect of

' reinforcing actions of the polymer asperities or surface contact

regions. At surface roughness values greater than 35 uin, there

is mechanical entrapment of the polymer in the metal surface causing

an increase in the coefficient of friction.

15
PERFORMANCE STUDIES OF POLYMER PROCESSED MECHANICAL

AND STRUCTURAL COMPONENTS

M. J. Devine

Naval Air Development Center

Warminster, PA 18974

A technique which has become known as "dry powder painting" is one of

the major advances made in the application of coatings. This technique

is based upon the deposition of specially formulated, thermoplastic or thermosetting, heat-fusible powders on metallic substrates. Since there

are no solvents present, the coating can be cured immediately after

application. One of the most obvious advantages of this method is the

absence of pollution and safety problems associated with organic solvents

that are used in most liquid paint systems. A second advantage is that

coatings may be obtained by this process from materials that cannot be

applied by liquid paint techniques because of their poor solubility in

organic solvents. Dry powder techniques are readily adaptable to current

production methods and are easily learned by product finishing personneL

Because of special requirements in particle size, particle size

distribution, and other properties, dry powder coatings are generally more expensive than liquid paint systems. However, in view of the

advantages of dry powder processes over wet painting techniques the over- all cost differences are probably minimal. In addition as much as 95%

of the dry powder material can be utilized if a powder recovery system

is used.

Advantages similar to those reported for dry powder techniques are also

claimed for electrocoating . This process can be compared to electroplating of metals, and, like the dry powder method, it also avoids the use of organic solvents.

Dry Powder Techniques

1. Fluidized Bed Method

Until now most of the powdered paints have been applied by fluidized

bed techniques. In the fluidized bed method a dense cloud of powder is

created by passing air through the perforated bottom of a powder

reservoir. At the correct flow rate the cloud of powder behaves like a liquid that tends to "wet" all surfaces of an object immersed in it.

An agitator or vibrator is normally employed to maintain the uniformity

of powder and to prevent channeling of air. Usually the part to be

coated is preheated so that the coating particles can fuse and adhere to

the hot surface. When large objects are coated, the cure must usually
be completed in separate ovens. The fluidized bed method Is best suited

for the application of relatively thick coatings—10 to 60 mil—to small

objects.

2. Electrostatic Deposition of Dry Powders

The second method of dry coating technology, electrostatic

deposition of dry powders, is carried out by two different procedures.

The first procedure employs a fluidized bed as described above and an electrostatic field that imparts a charge of the powder particles. The

fluidized bed is maintained by an air flow of low pressure (approximately

5 cfm/ft2 at 2 to 5 psi) , and the charge is supplied by electrodes at an applied voltage of 60 to 90 kv. The object to be coated is grounded and

passed over or through the fluidized bed of charged powder. It has been

found that even nonconductive materials can be coated by this method

without the use of conductive prime coats. A major inherent limitation

is the absence of uniformity of the charged cloud on a vertical plane.

As a consequence only articles that are relatively flat can now be

coated by this process. However, it is believed by some that this

process will eventually become the major means of depositing dry powder

materials

.

The most advanced method of electrostatic deposition of dry powder

is the spray procedure. In this method a metered amount of powder is

fed to a spray gun by compressed air, and, as the particles pass through

a region of air ionized by the d-c potential of an electrode, they

become charged. The part to be coated carries a charge opposite to that

of the coating particles and the powder is attracted to it. The sprayed

coating, which may be as thin as 1 mil, is then fused or cured in an

oven. This process is especially suited for large parts and parts with

complex shapes. Coating thickness is limited by the loss of attraction

of the powder to the substrate surface as the coating builds up.

3. Applications of Fluidized Bed and Electrostatic Spray Techniques

Numerous plastics have been formulated as dry powder coatings.

Epoxy resins are the most widely used at present, but the following

resins are also being employed or considered for use: poly (vinyl

chloride), poly (vinyl fluoride), polyester, nylon, polyethylene, polypropylene, acrylic, chlorinated polyether, cellulose acetate butyrate,

f luorocarbons , and alkyd melamine. In industry, powder coatings are being used to coat a wide range of objects such as appliances, metal

furniture, electrical equipment, and pipes.

It has been shown that powder coatings technology has considerable

promise for numerous aircraft applications. In fact, a few Local

Process Specifications (LPS) covering fluidized bed and electrostatic

spray applications have already been issued at two of the seven Naval

15
Air Rework Facilities. In addition, the Naval Air Development Center

is carrying out numerous studies devoted to the evaluation of electrostatic spray method. In one study at the Naval Air Development Center,

spline assemblies were powder-spray coated with nylon 11 and examined

for performance life on a spline wear tester. Under the test conditions

the nylon-coated assembly remained operational four times longer than

the control. A coating formulation for spline applications is now

commercially available. Powder coating techniques have also been used

in studies to overcome other problem areas in aircraft maintenance.

Fatigue failure resulting from repeated stressing is a major problem in

the operation of aircraft. Tests have shown that powder coatings of

poly (vinyl chloride), nylon 11, and epoxy are effective in increasing

the resistance to flexure fatigue failure. A similar increase in

fatigue life was also observed for nylon 11 coated Ti-6A1-4V alloy

substrate. A number of components electrostatically coated with nylon 11 are already being field tested on operational aircraft. These include

aluminum and magnesium wheels, access panels, tie down chains, leading

edge panel^*, antenna covers, bell cranks, spline assemblies, and wingfold lock/unlock levers. Increased resistance to erosion and corrosion

are sought.

4. Plasma Spray Techniques

Although most of the dry powders which are organic polymers are applied by fluidized bed and electrostatic spray methods, steady

progress has been made in developing plasma spray equipment that can be

used with organic polymers. In this procedure a torch is used which

produces and controls a high velocity inert gas stream, usually argon,

at temperatures ranging from 5,000° to 15,000°F. The resultant gas

stream consists of a mixture of free electrons, positive ions, atoms,

and molecules. The plastic powder is fed into the plasma stream at an angle of approximately 20° so that the dwell time of the material in the

flame is exceedingly brief. The hot gas melts the plastic powder without decomposition and forces it onto the substrate. Dense, pore-free

coatings can be obtained by proper control of process parameters such as voltage, current, arc gas flow, powder feed rate, and spray distance.

An obvious advantage of this method is that the coating can be

applied and cured in one step thereby avoiding subsequent heat treatments characteristic of other dry powder processes. The heat treatments

are often objectionable if the temperature necessary to achieve cure is

sufficiently high to cause metallurgical changes in the substrate. For

example, tests have shown that 7075-T78 aluminum alloy loses 10 percent

of its tensile strength at 275°F. The application of similar coatings

by plasma spraying did not result in loss of mechanical properties of

the substrate. It has been observed that during plasma spraying

substrate temperatures were as low as 185°F for coating with epoxy

resin and 200°F for coating with nylon 11 resin. An additional

16
advantage is that by this method the coatings can be applied to parts

that are too large to be cured in an oven or which cannot be conveniently removed from a larger structure. The NADC mobile plasma spray

unit provides for repairs of plastic coatings in the field.

Table I presents a status report for several applications involving

polymer powder materials.

17
REFERENCES

1. H. Shapiro, "Epoxy Coatings for Corrosion Protection," NAVMIRO

Manufacturing Technology Bulletin, September, 1972.

2. Abraham Mann, "Powder Coatings," Paint and Varnish Production , September, 1970, pp. 25-31.

3. John A. Mock, "Plastics Powder Coatings Cut Costs, Up Performance,"

Materials Engineering , March, 1969, p. 50.

4. K. C. Pettigrew, "Fluidized Bed Coating," Modern Plastics , August,

1966, p. Ill and September, 1966, p. 150.

5. Frank E. Pegg, "Applying Plastic Coatings with the Fluidized Bed

Process," Plastic Design and Processing , October, 1970, p. 22.

6. R. R. Sharetts and D. S. Richart, "A Report on thfe Flui.dized Bed

Coating System," Plastic Design and Processing , June, 1962, p. 15,

and July, 1962, p. 26.

7. "Fluidized Beds Take on New Life," Chemical and Engineering News , December 14, 1970, p. 46.

8. A. H. Landrock, "Fluidized Bed Coating with Plastics: Technology

and Potential for Military Applications," Plastic Report No. 13,

Plastics Technical Evaluation Center, Picatinny Arsenal, Dover, N.J.,

1964.

9. Daniel R. Savage, "Look Where Dry Powder Finishing is Heading,"

Plastics Design and Processing , August, 1972, p. 12.

10. E. P. Miller, "Electrostatic Powder Coatings," Industrial Finishing , May, 1967.

11. R. W. Bright and R. A. Coffee, "Electrostatic Powder Coatings,"

Institute of Metal Finishing , Volume 41, 1964.

12. Hani T. Azzam, "Coatings Without Solvents," Machine Design , March,

1971, p. 91.

13. "Dry Powder: The Clean Coating," Chemical Week , March 29, 1972,

p. 30.

14. Proceedings 2nd North American Conference on Powder Coatings,

Toronto, Canada, March, 1972.

18
15. A. J. Koury, A. A. Conte, and M. J. Devine, "Aircraft Applications

of Powder Coatings Technology" in Proceedings 2nd North American

Conference on Powder Coatings, Toronto, Canada, March, 1972.

16. Norfolk, LPS No. 480, "Organic Coatings, Fluidized Bed Techniques,"

31 March 1970. Norfolk, LPS No. 480, Amendment 1, "Organic

Coatings, Fluidized Bed Techniques, Application of Epoxy Coating,"

2 April 1970. Norfolk, LPS No. 480, Amendment 2, "Organic Coatings,

Fluidized Bed Techniques, Application of Nylon Coating," 4 June

1970. Alameda, LPS/AL 02-2-0120, "Epoxy Coating Applied by

Fluidized Bed and Electrostatic Spray Techniques," 17 September

1970. Powder processing facilities are also being planned for

other Naval Air Rework Facilities.

17. A. A. Conte, "Application and Properties of a Nylon 11 Polymer

Coating Deposited on Metallic Substrates via an Electrostatic Spray

Method," Report No. NADC-MA-7123, 13 April 1971.

18. A. A. Conte, "Coatings to Improve Wear Resistance," ASME Design

Engineering Conference, Chicago, Illinois, May, 1972.

19. M. L. Valtierra and P. M. Ku, "Research on the Mitigation of Spline

Wear by Means of Plastic Coatings," Southwest Research Institute,

Report No. RS-539, 5 January 1970.

20. Dana Corporation, 45OO Dorr Street, Toledo, Ohio.

21. R. D. Kremith and J. W. Rosenberry, "Plasma Spray Application of

Plastic Materials," 12th National SAMPE Symposium.

22. A. E. Katzer and R. 0. Houghtaling, "Method of Plasma Spraying

of Tetraf luoroethylene-Hexafluoropropylene Copolymer," U. S.

3,510,337, 5 May 1970.

23. "Spray Gun Coats with PTFE," Chemical Engineering, 5 October 1970.

24. R. J. Janowiecki, M. C. Willson, and G. F. Schmitt, "Plasma

Sprayed High-Temperature Polymeric Coatings, SAMPE Journal , June/

July 1968.

25. D. V. Minuti, "Investigation Conducted Under the Analytical Rework

and Maintenance Engineering Support Program," Report No. NADC72119-VT, 28 June 1972, pp. 21-24.

19
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20
DISCUSSION

A. J. Babecki , NASA, Goddard Space Flight Center : The coating of

magnesium with nylon sounds like a good idea for spacecraft

applications. We use a lot of magnesium, and we do have a corrosion problem. How thick are these nylon coatings?

M. J. Devine : Our experience indicates that with the plasma

deposition of nylon, a film of 1 to 2 mils in thickness can be

produced. With the fluidized bed process, normally the thickness

is from 5 to 11 mils. With the electrostatic process, the

thickness is in the range of 4 to 5 mils. Without the nylon

coating on the control lever that I showed, corrosion developed on

the inner bore as well as on the outer surface in the proximity of

the bearing, but once we applied the nylon to the entire assembly

and pressed the bearing in, there was no evidence of corrosion

after 36 months.

N. Glassman, David Taylor Naval Ship Research and Development Center : What was the thickness of the paint on the treated panel on the

ground support equipment?

M. J. Devine : About 4 mils.

N. Glassman : How thick was the nylon?

M. J. Devine : The nylon was about 8 to 10 mils thick. It was applied

by the fluidized bed method.

N. Glassman : Did I understand you to say that you were using an

anti fretting compound on the strands of the wire rope cable?

M. J. Devine : The compound that we introduced into the internal area was MIL-G-81322, a synthetic hydrocarbon. It is probably the best

available antifretting compound for such applications.

N. Glassman : Is it an oil or a grease?

M. J. Devine : It is a nonsoap thickened grease. During the

impregnation process, however, it's cut back to a slurry

consistency with about 50% hexane. The hexane is driven off before

the polymer is applied.

J. t. Stern, NASA, Goddard Space Flight Center : Did you have any

special process for cleaning the cables before you coated them after

you nad impregnated them with the antifretting compound?

M. J. Devine : The only procedure that we followed was to wipe the excess grease off with a cloth saturated with hexane.

21
J. E. Stern : Were there any problems in bonding or in getting a dense

coating without any cracks, breaks, or openings?

M. J. Devine : No, I think that is what is unique about the process.

It violates the concept that we all follow that the surface must be

very, very clean. There is no evidence that that coating, which is

then fused, can be easily removed from the cable.

J. E. Stern : What was the effect on the stiffness and the structural

behavior of the cable after it was coated?

M. J. Uevine : There was no significant effect on stiffness. We

exposed both the treated and untreated cable to a salt fog environment.

At various intervals we ran load tests by pulling the cables over a 1/4 in diameter pulley with a 300 pound load. After 600 hours the

conventional cable broke during the load test, whereas after 1670

hours, the treated cable showed no evidence of damage.

E. E. Klaus, The Pennsylvania State University : Can you comment on the

quantities of the antifretting material in the cable?

M. J. Devine : No, we fill all those internal areas between individual

wires. Part of the fretting problem comes from the interaction of

the wires. Therefore, if the process is going to be effective, it

must protect those individual wires, so all the internal spaces must

be filled. There must be a tight film around the cable to eliminate

the material from being pumped from the internal areas. To have it

there initially is one thing, but to maintain it there continually

is more important.

L. F. Fehrenbacher, USAF Materials Laboratory : Have you done any

work with higher temperature polymers such as polyimides which may

be more environmentally resistant?

M. J. Devine : We have written a number of papers about polyimides

in solid sections, especially for antifriction bearing applications,

both as retainers and as land inserts. We have not attempted to apply

the polyimides by any of the techniques presented here. We are using

polyphenylene sulfide for higher temperature applications. We do

have articles out for field tests, but we don't have any field test

data as yet. We are also working with a number of Teflon or modified

Teflon- type products, but again, there are no significant field test

resul ts

.

L. F. Fehrenbacher : How do you determine which process--plasma,

fluidized bed, or el ectrostatic--to use?

M. J. Devine : Sometimes it will depend on the size of the article,

but usually the available clearances will determine the process.

22
If the clearance is small, the plasma process will probably have to

be used.

L. F. Fehrenbacher : If all of the three techniques could be used in

a given application, do you have enough data to indicate which would

give the best results?

M. J. Devine : There is just a little indication. A very important

test is the water vapor transmission test in which an attempt is made

to determine how much moisture will penetrate a film in a given

period of time. Test results on polyurethane paints indicate moisture

penetration of about 5 grams per 100 centimeters per 24 hours. With

the plasma applied nylon, it runs about 3 and with the electrostatic

processed nylon, it's down to somewhere between 1 and 2. It appears

for a very non-porous film that, based on the water vapor

transmission data, the electrostatic process gives the best film.

L. F. Fehrenbacher : Do you get a denser film with the electrostatic

process?

M. J. Devine : It appears to be, based on limited data.

A. J. Babecki : What kind of temperatures do the substrates reach with

each of these application processes?

M. J. Devine : The highest substrate temperatures will be encountered

with the fluidized bed process. In this process a layer of powder is

held in suspension in a drum-like arrangement. The part to be coated

is heated in an oven above the melting point of the powder before the

part is inserted into the powder to fuse the coating on. In the case

of nylon-11, the temperature is above 400 F for the fluidized bed

process. For the electrostatic process, the temperature is between

385 and 400 F. For the plasma process, the temperature of the

substrate is significantly lower. In this process, the heat is used

to melt the boundary layer of the plasma. The melt is then flowed

onto the substrate surface. It is quite possible that nylon-11 could

be applied to ordinary newspaper or cardboard without damaging the

substrate.

E. J. Deemer, Naval Air Systems Command Representative, Atlantic : Is there any possibility of glass bead impregnation in the nylon-ll

coatings to increase resistance to erosion?

M. J. Devine : Your point is well taken, but at present we do not

know. Professor Ludema may have something to add.

K. C. Ludema, University of Michigan : In one proprietary process

that I know of, where paper slides over these polymers, glass beads

will increase the life of the film by a factor of 10.

23
J. E. Stern : Have you done any experimentation with sputtered

coatings? If you did, what kind of coatings were they and what

kind of results did you get from them?

M. J. Devine : In October of 1973, Gentner and Newhart presented a paper at the Mechanical Failures Prevention Group meeting in Boulder,

Colorado (The Role of Cavitation in Mechanical Failures, Proceedings

of the 19th Meeting of MFPG, Boulder, Colorado, 1973, NBS Special

Publication 394). They discussed the application of tungsten carbide

and silicon carbide blended with type 304 stainless steel sputtercoated on aircraft compressor blades. The coatings reduced the

effects of erosion on the leading edge, while the geometry of the

blade, both on the fore side and the back side, was maintained. They

found that sputtered coatings on titanium prevented the formation of

titanium oxides for thousands of hours at elevated temperatures. The

major problem with sputtered coatings is in setting up a production

facility. For compressor blades, we are talking about processing

only one or two a day.

J. E. Stern : Have you tried any sputtered polymers on any of your

devices?

M. J. Devine : Wo, I have not. Spalvins at NASA Lewis has done work

in that area.

W. F. Byrnes, Naval Air Engineering Center : When the tractor panels

that had been fluidized bed-coated were damaged, could they be

repaired without removing the first coating?

M. J. Devine : They can be repaired. In fact, the plasma process

can be used to repair coatings applied by either the fluidized bed or the electrostatic process. However, we are trying to develop a much simpler technique to effect repairs. The main drawback in

repairing with the plasma process is blending the colors so they will

match.

W. F. Byrnes : Would this be done at the depot level?

M. J. Devine : We would have to do this at the depot level using the

plasma process. However, the technique that we are working on in the

laboratory is very close to being completed. This technique would

enable repairs to be made at any level of maintenance.

2k
NYLON 11 AS A LUBRICANT

AND CORROSION PREVENTATIVE COATING

R.G. Baker and George H. Kitchen

Bell Telephone Laboratories

Murray Hill, New Jersey O772U

and

R.R. Wells

Western Electric Company

Greensboro^ North Cai olina 27^09

The working bearing surfaces of Sea Plow IV, a vehicle

designed to lay cable in a 4-ft trench in the ocean floor,

must be protected from corrosion, internal friction plus

abrasion from sand, mud and silt. Examination of this problem suggested that a coating of nylon 11, applied electrostatically as a dry powder, could afford protection against

all of these conditions. Conventional application techniques were modified to permit application of the coating at

dockside. Results have been excellent as evidenced by Sea

Plow IV 's more than one year of troublefree operation.

Keywords: Abrasion; Corrosion protection; Friction; Marine

environment; Powder coating; Salt water; Solid lubrication.

American Telephone & Telegraph company and its overseas

partners began laying voice communications cables on the

ocean floor over twenty years ago. Currently a new cable,

known as TAT 6, is being put into place between the United

States and France. Early cables were merely placed on the

ocean floor. But this subjected sections near each coast

to damage from fishing trawlers. Now the cables are buried

at each end, essentially along the continental shelf, in a

trench about 4-ft deep. An ocean-floor traversing vehicle

called Sea Plow IV is performing this operation off the

coasts of France and Rhode Island. This paper deals with

the materials and techniques used to protect the working

bearing surfaces of this machine from corrosion, friction

and abrasion.

Bell Telephone Laboratories were consulted on this problem. The essential requirements were that the Sea Plow IV

operate reliably in 1,000 fathoms of sea water. In addition

to the chemical attack by sea water , the machine had to be

protected against many other types of corrosion including

stress, galvanic, crevice and pitting. To operate satisfactorily the lead screw and attitude control mechanism required lubrication and protection from tolerance destroying

25
abrasion from the sand, silt and mud plowed to form the

trench. Finally the working surfaces also had to be protected against abrasion by the cables themselves.

Several years previously we had consulted the Naval Air

Development Center about the problem of protecting and lubricating bearing surfaces required to operate under high

load conditions in a sea water environment. The Center

demonstrated a method of electrostatically applying a dry

powder coating of nylon 11 on aircraft splines. We contacted the manufacturer of the electrostatic equipment,

Interrad Corporation, and talked to various powder vendors.

Based on the Navy's experience in which nylon 11 has withstood 100,000 psi stress in an undersea environment for

years, we selected this nylon 11 from Rilson Corporation.

The problem of application was less straightforward. In

the Navy applications relatively small parts are coated in

the shop where conditions for surface preparation, priming,

application and curing could be closely controlled. Our

problem was to coat large sections of a 30-ft long vehicle

on a Baltimore dock, with only torches and a heat-treating

furnace available for curing the powder.

Fortunately the equipment manufacturer was able to provide a recently developed, portable, fully transistorized

powder spray unit. In addition, the miniaturized high

voltage generator necessary to charge the powder particles,

was located right in the spray gun. This eliminated the

need for stringing a bulky 70 KV cable along the exposed

dock.

Personnel at Bell Telephone Laboratories Guilford Center

further adapted this equipment. Wands were developed to extend the spray gun nozzles in order to deliver powder inside several tubular parts.

Although more conventional methods of metal pretreatment

such as phosphating or chromate rinsing may have been preferable, our location and the equipment available limited

the pretreatment to sandblasting.

Because we were unable to closely control preheat and

cure temperatures, the degree of cure was checked by eye

and by feel. After sandblasting parts were preheated at

650 F for one hour. The heated parts were primed, by dip

or brush and immediately sprayed with nylon 11 powder. The

coating was cured by reheating for 20 minutes at 650 F. If

the coating appeared under cured, the part was heated again

until we were satisfied that cure was complete.

Among the parts coated in this manner was a lead screw

and its 15-ft tube, the U-bolts through which the vehicle

is towed by the surface vessel. Clearance between the mating threads of the lead screw was 30 mils. The coating

could not exceed this thickness. Minimum coating thickness

was established at 8 mils.

26
Compared to the controlled conditions of a conventional

in-plant coating line, our application technique was rather

crude. Nevertheless, thanks to the ingenuity of everyone

involved, including equipment and material suppliers, the

coating has proved to be effective. It permitted the designers of Sea Plow IV to use tool steel for critical parts

instead of much more costly aluminum-bronze. Perhaps more

importantly, it has enabled the machine to operate reliably

in a most difficult environment. The Sea Plow IV has been

in almost continuous service for over a year. During that

time there have been no reports of malfunction or corrosion.

27
28
DISCUSSION

R. A. Moll 1 cone, USAF Materials Laboratory : Have you done any work

with composites?

G. H. Kitchen : Marty, do you want to respond to that?

M. J. Devine, Naval Air Development Center : We did do a few experiments

with composite materials. One was actually made into a structural

component--a fuel tank section. It was tested to failure in bending

fatigue. It had approximately 95% leakage, mostly at the seams. We

took this essentially expended specimen and coated the entire surface

area including all the seams externally with the nylon by plasma

deposition. It was again tested in fatigue; the leakage had been

reduced to about 5% of what it was before.

It is also possible to apply these powders on fibers to improve their

ultraviolet radiation resistance.

R. A. Mollicone : Does anyone have any background information in

regard to high temperature (300-350 F) applications with loading?

M. J. Devine : I think 250 F is about the upper temperature limit

for nylon-11 for continuous service. At higher temperatures (500 to

700 F) materials such as aromatic polyimides and the polyphenylene

sulfides must be considered.

R. A. Mollicone : What is the effect of a 250 F temperature on moisture absorption of the structural graphite epoxies or boron

epoxies?

M. J. Devine : At 250 F, nylon will be effective for extended periods.

J. Kruger, National Bureau of Standards : How good is the erosion or abrasion resistance of this coating?

G. H. Kitchen : The abrasion resistance even to sand is excellent.

J. Kr'ijer : Do you have any information regarding the bio-fouling of

this coating?

G. H. Kitchen : We have found that there is definitely a significant

reduction in the buildup of inorganic salts on metal parts.

J. Kruger : What about barnacles?

G. H. Kitchen : The things that we bring up are fairly clean. Marty,

do you want to comment on barnacles? Your equipment is in shallow

water.

29
H. J. Devine : After three years, our equipment was clean.

H. Glassman, David Taylor Naval Ship Research and Development Center : What is the function of the primer that you mentioned?

G. H. Kitchen : The primer is a plastic-like material that is used to

coat the substrate to a thickness of about half a mil. It has a very

tacky, rough surface that permits a mechanical bond of the nylon-11

to it rather than to the metal substrate.

li. Glassman : You talked about using this material on electroless

nickel. That wasn't because of corrosion, was it?

G, H. Kitchen : No.

G. E. DeLong, Naval Air Systems Command Representative, Pacific : Was

your primer inhibited?

G. H. Kitchen : No

.

M. J. Devine : George DeLong made a very important point. Nylon-11

is not a chemical conversion coating. It is really a barrier film.

If the coating is disrupted so that water can penetrate to the base

metal, there will be a site for corrosion to take place if there is no inhibited primer. For maximum corrosion protection, it is very

important to include an inhibited primer prior to the application

of a polymer film.

G. H. Kitchen : If the nylon-11 coating is disrupted and moisture

penetrates to the metal, corrosion will start. But the corrosion

doesn't spread under the nylon-11 like it does under other coatings.

A. J. Babecki , NASA, Goddard Space Flight Center : From the standpoint

of spacecraft applications, do either of you gentlemen know anything

about the coefficient of friction of this nylon surface against itself

or against metal s?

M. J. Devine : We have run comparative experiments using nylon on

steel and grease on steel and found no difference in the heat inputs.

30
I MECHANICAL DESIGN CONCEPTS FOR COATED NUCLEAR FUEL PARTICLES

J. M. Tobin

WesHnghouse Astronuclear Laboratory

PiHsburgh, Pennsylvania 15236

Abstract: Retention of fission products in microspherical coated particles depends

upon the mechanical integrity of the coatings. Mechanical design concepts have

been developed over a 15 year period which led to the fabrication of highly successful multiple coated particles such as the TRISO coated particles. An over- view of this development is described.

Key words: Computer modeling on a microscale; elasto-plastic micropressure

vessels; failure predictability through small size; microencapsulated coated fuel

partic les.

Millions of curies of radioactive Isotopes are the product of fissions in a typical

power reactor. These fission products are isotopes of dozens of elements from the

periodic table, representing solid, vapor, and gas species.

Ideally, there should be a zero release of fission products to the coolant and to the environment. Great progress has been made in fulfilling that goal through the use of many independent containment components (fission product release barriers)

in a reactor system. Failure of one fission product barrier should not compromise

the goal of zero release to the environment by the system.

The primary barrier responsible for fission product retention is the fuel element.

Typically it retains 99. 999 percent of the fission products generated. Some

reactors have had retention exceptional to the requirements, such as the Peach

Bottom gas cooled reactor, a test reactor of 40 MWth, Its performance was evaluated by the reactor operator, the Philadelphia Electric Company (1) and showed

the release to be 0. 003 fraction of the AEC required limits.

Liquid cooled reactors generally use fuel elements having long tubular metallic

cladding with welded enclosures to assure retainment, or fuel cermet layers

bonded between metal sheets with side enclosures (picture frames). A typical

light water reactor may have 1,000 to 40,000 individual elements. This approach

to fission product retainment is the oldest and most developed.

51
Another retainment concept is the subject of this paper, the use of tiny microspherical fuel particles, each with its own coating for fission product retention.

This concept has been applied to gas cooled reactors such as the helium cooled,

graphite moderated Ft. St. Vrain reactor in Colorado. The total number of

coated particles in the 851 MWthcore is particles. Failure prediction is improved because of the great number of nominally identical particles. Failure

of one particle is found to be essentially independent of failure of another. The

individual failed particles with coatings not structurally intact release only a

fraction of their fission product inventory, largely the fission gases krypton and

xenon.

A typical example of a multilayer coated particle for use in a helium cooled

graphite reactor is shown in Figure 1, "Cross section of a Typical Multi-coated

Particle". Many variations are possible for specific applications, but the design

concepts are basically like this, it is the mechanical integrity of the outer three

layers that is important.

Prevention of mechanical failure and loss of structural integrity of the coatings on

fuel particles determines the fractional release of fission products to the helium

coolant of a nuclear reactor. Coated particle development over a 15 year period

improved the fraction failed in a long operating time from 0. 8 to 0. 00001. The

history of this remarkable improvement shows the value of applying mechanical

analysis and statistics to help in performance testing evaluation and production

process development.

This coated microspherical fuel particle has been successfully developed over a

period of approximately 15 years by many laboratories in several countries. A few

of the major contributors to this development will be mentioned, but many will not

for practical reasons. The omission is not meant to be a reflection on the value of

their contributions.

The history of the development of this reliable concept began with attempts to blend ceramic (oxide or carbide) fuel powders into a graphite matrix in the early

concepts of ROVER, HTGR, and MGCR reactors.

The design requirements for the ROVER nuclear rocket emphasized tight dimensional

tolerances of extruded graphite fuel, as well as strength and density. The fraction

of the uranium fissioned was very low, and the fission product retainment without

fuel particle coating was adequate for tests at the rocket test site for periods of one hour or less. However, moisture in air during storage, assembly, shipping,

and test preparation was found to deteriorate the fuel strength. The individual

fuel grains were than spheroidized and given a single pyrocarbon layer to protect

the uranium dicarbide kernels from hydrolysis. This was the major solution to the

problem of loss of strength by hydrolysis. A ROVER type particle is shown in Figure 2. 52
Further development sponsored by the USAEC showed that the simple one layer

pyrocarbon coating fractured readily with greater uranium bumup than required for

ROVER but required for power reactors, such as the Gas Cooled Reactors. One

early mechanical model was devised and was described by Prados and Scott (2) of

ORNL. Sources of stress were defined. The fuel kernel swelled since each uran- ium atom fissioned produced two fission product atoms. Further, 14 percent of the

fission product atoms at high bumups are the inert gases krypton and xenon. The

gases can create tremendous internal pressure inside the pyrocarbon coating if there

is no provision for gas expansion space inside the coating.

Thus the two layered, or BISO fuel, was conceived. The cross section of a typical

one is given in Figure 3. A somewhat sooty porous inner buffer layer is deposited on the fuel microsphere kernel, and the buffer layer is coated with a gas tight

outer pyrocarbon layer. The porous buffer layer provides gas expansion space.

The first core loading in the Peach Bottom reactor used the single pyrocarbon layer

coated particles as in Figure 2 and had particle failures in 0. 8 of the particles.

Core 2 operated-more successfully using the BISO coated particles. It is estimated

that less than 0.01 fraction of these particles failed.

The main function of the intermediate porous buffer coat in Figures 1 and 3 is to reduce mechanical stresses and structural damage to the outer layers due to recoil

fission product ions from the fission events near the surface of the fuel kernel. The

outer pyrocarbon coat was modeled as a microspherical hollow pressure vessel.

Thus, it was necessary to measure its creep strength at operating temperatures.

Price and Bokros of OA (3) reported creep properties and densification and related

them to irradiation. Isotropic pyrocarbon was found to be the most desirable pyrocarbon. They measured the effect of a high fluence of fast neutrons on dimensional

changes of pyrocarbon of various densities, etc. There appears to also be an enhanced mechanical creep due to irradiation. Prados and Godfrey (4) of ORNL

continued evolving a more complicated code called STRETCH to accommodate

these observations.

Another material effect was observed. Some fission product elements such as Ba

and Sr are not as well retained by isotropic pyrocarbon as other fission products.

DeNordwall and Sayers (5) reported a 10^ to 10^ improvement in retention of

these fission products if a silicon carbide layer is introduced. Barr (6) also observed cesium retention to be improved when sandwiched between two pyrocarbon

layers. Silicon carbide was also found to be stronger than pyrocarbon. Considering these and other factors, the multicoated particle concept known in the USA

as a TRISO coated particle was developed. Figure 1 shows one example of this

type.

33
The principal purpose of the inner pyrocorbon coat is to be able to coat the silicon

carbide over a buffer coat without silicon carbide penetrating the buffer and filling

its desirable porosity.

Mechanical analysis then became much more complicated. Multicoat analytical

modeling by Kaae (7), Walther (8), and Stevens (9), capable of analyzing this four

coated particle significantly advanced earlier codes. The current analytical code

(7) is designated TRISO II. It was necessary to account for the differential thermal

expansion between silicon carbide and pyrocorbon during processing and operating

temperature changes. The strength of the silicon carbide is greater and has less creep than pyrocorbon. Complex stress and strain relationships were developed.

The current status is that several versions of mechanical design codes based on

elastic, plastic, and irradiation behavior at operating temperatures are capable of

predicting coated particle failure rates for a broad range of dimensional and property variations as well as the uranium bumups.

The eventual successful development of the TRISO coated particle was achieved

through an iterative process of mechanical design analytical concept definition,

mechanical materials property measurements, testing particles in a reactor, evaluating coating failure, evaluating fission product retention, and remodeling the

analytical models for redesign. Several modern analytical codes are based on

elasto -plastic behavior in fabrication and operation of multi -layer coated particles.

Through iterative comparison of analytical models with irradiation test evaluation,

the basic mechanical concepts have been retained, but modifications to the material

behavior functions were necessary. A recent treatment by Gulden (10) of GA is particularly interesting since it recognizes the effect of variations of buffer coating

thickness and kernel diameters on the prediction of particle coating failure as compared to experimental results.

The time dependence of stresses analyzed by Gulden using the GA TRISO II code is given in Figure 4. The silicon carbide layer in this nominal design with average

dimensions is always in compression and will not foil. Initial compressive stress in the silicon carbide layer is desirable and is obtained by use of differential contraction of the outer pyrocorbon layer upon the silicon carbide. The combined effects

of stress relief through creep, shrinkage of the outer pyrocorbon with irradiation,

and internal gas pressurization led to on early stress reversal in the silicon carbide

followed by a decreasing compressive stress behavior. It is critical to the coated

particle coating integrity that the silicon carbide not be in tension. The effective

strength before rupture of the silicon carbide is estimated by Gulden to be 30,000

psi through comparison with empirical results. When the silicon carbide tensile

stress exceeds 30,000 psi the particle coating fails.

5^^
The variation of buffer thickness and kernel diameters was found to be critical

through analysis by Gulden (10). The total porous volume in the buffer layer, and

thus the internal gas pressure at high uranium burnup fractions depends upon these

dimensional variations determined at the time of fabrication. Variations in the

silicon carbide tensile stresses resulted. The smaller the porous volume the greater

the tensile stress in the silicon carbide. A high probability of failure occurs above

30, 000 psi stress.

Two failure modes are postulated. When the tensile stress In the silicon carbide

exceeds 30,000 psi, the stored elastic energy in the multicoated shell is released

and the crack propagates through all three outer layers. The buffer coat serves to decouple mechanical reactions between the kernel and the outer coatings. It will

not propagate a crack initiated in the outer coatings nor transmit strain from the

expanding kernel to the outer coats. Therefore it is not mechanically involved in a failure mode analysis since it does not store elastic energy or transmit a crack.

However, it can release stored fission gases If the outer coatings crack. Bursts of

fission gas activity release have been experimentally associated with intentional

cracking of individual irradiated coated particles. The other failure mode begins

with the chemical attack of the inner pyrocarbon layer by a fuel kernel moving

("amoeba" effect) up a temperature gradient and results in an unbalanced stress. This second mode of failure also leads to a sudden cracking of all three outer layers

and a burst of fission gas release.

Following the burst of fission gas release is a slower release of a wide variety

of gas, vapor, and solid fission products.

The main criterion is to maintain the integrity of a very high fraction of the fuel

particle coatings throughout their life in spite of severe conditions of:

• temperature • internal gas pressure • radiation damage.

1

2

The use of 10 of individually coated fuel particles permits the use of statistical

techniques in analyzing fuel bead performance that results in specification of fuel

particle coating tolerances in production. The failure prediction of 10*^ to 10^

clad tubular fuel pins gives less predictability, and a greater release fraction with

each failure.

Test results have been applied in developing computer codes which are able to evaluate the effects of changes in coating layer designs and predict coated particle

performances.

• codes also consider the variability of coatings on a statistical basis.

35
Fuel particle coating developments are continuing to improve the coating performance and also to improve the understanding of the design parameters of importance.

References

1. Birely, W. C. , in "Gas-Cooled Reactors: HTGR and GCFBR", ANS

Topical Meeting, May, 1974, CONF-740501.

2. Prados, J. W. and Scott, J. L , Nucl. Appl. 2, 402 (1966).

3. Price, R. J. and Bokros, J. C. , J. Nucl. Mater, 22, 158 (1967).

Bokros, J. C. and Schwartz, A. S. , Carbon , 5, 481 (1967).

Bokros, J. C. and Price, R. J., Carbon, 3, 503 (1966).

4. Prados, J. W. and Godfrey, T. G. , ORNL-TM-21 27, December, 1967.

5. DeNordwall, H. J. and Sayers, J. B. , Met. Soc. Conf, 42, 65 (1966).

6. Barr, P., et al, DRAGON Project Fuel Symp. , Bournemouth, England,

November 4, 1969.

7. Kaae, J. L. , J. Nucl. Mat. , 32, 322 (1969).

8. Walther, H. , Nucl. Engr. Design, 1_8, II (1972).

9. Stevens, D. W. , Nucl. Tech ., 10, 301 (1971).

10. Gulden, T. D. , et al, Nucl. Tech. , 16, (1972).

36
LiU

03

O > CD

CD

Q

CD

J)

"ODOOtoo

'5.co

(U3

57
58
59
FAST FLUENCE [lO^' n/cm^E >0.18 MeV)]

STRESS INCREASE WITH FAST FLUENCE

Figure 4. Coating Stresses in TRISO Coated Particles with

Increasing Fast Neutron Exposure (and burnup).

ho
DISCUSSION

R. M. Thomson, National Bureau of Standards: Why did you use silicon carbide, which is a ceramic, for the strengthening agent

instead of something like tungsten or stainless steel which wouldn't

have the thermal shock properties that the ceramic has?

M. Tobin : The substitution of zirconium carbide for silicon carbide

is under current development. The main reason for this planned

replacement pertains to the limit of the processing temperatures

to 1800 C for 2 hours or less for TRISO particles containing silicon

carbide. Beyond this limit, the particle will be chemically

destroyed. The particle kernal will attack the silicon carbide.

Silicon carbide at 2000 C has a very high silicon pressure. Zirconium

carbide has a melting point in contact with carbon as a ZrC-carbon

eutectic of 2890 C. It is much more refractory than SiC. Tungsten

carbide is another possible replacement for silicon carbide.

R. S. Miller, Office of Naval Research : How were creep properties

of the beads measured?

M. Tobin : Jack Bokros made little coin-like discs of carbon and

fluidized them in a conical bed of graphite. He coated pyrocarbon

on the disc, and then assumed that the structure was the same as would be obtained on a little tiny ball. Most of his creep work was done with the coatings from these discs. The coatings were removed

from the discs and tested in graphite furnaces.

A. J. Babecki , NASA, Goddard Space Flight Center : Is the hammer

made just of pure carbon or is some kind of binder used?

M. Tobin : Just pure carbon. It is produced by Pure Oil Company of

Decatur, Texas. It is a reconstituted graphite that has been heat

treated at 2500 C. There are two major kinds of manmade graphite.

Primary graphite is produced by heating pitch to form coke. The coke

is then crushed to powder and mixed with more pitch and then heated

to a very high temperature. Most graphites are of this type. Highly

specialized graphites are made by grinding primary graphite into a powder, classifying it, and possibly treating it. Sometimes it is

then mixed with a thermoplastic or a thermosetting binder and molded

or extruded and then thermally processed into another graphite which

is more isotropic and stronger than the primary graphite. This

material is a reconstituted or secondary graphite with superior

properties. All the graphite parts used in the nuclear rocket

program were of that type.

A. J. Babecki : Do you know if there are any ball bearing components

made of this material?
M. Tobin : I heard it works fine as raceways. It is a dimensional ly

stable material and therefore may be desirable for some bearing

applications. This secondary graphite costs about 25 times as much

as primary graphite, and for this reason, it is not used unless

necessary.

A. J. Babecki : Is the application of this material to bearings done

under NASA sponsorship?

M. Tobin : No, this was a very minor effort sponsored by the

manufacturer.

k2
SESSION II

METAL

COATINGS

Chairman: J.Kruger

National Bureau Of Standards

NICKEL-PHOSPHORUS ALLOY COATINGS FOR WEAR RESISTANCE

Christian E. Johnson

Fielding Ogburn

National Bureau of Standards

Washington, DC 2025^

Electroless nickel coatings are nickel alloys generally with a nominal composition of 8 wt % phosphorus, applied to metal substrates

from a nickel solution by an autocatalytic chemical process. They are used for protecting steel from corrosion and wear, and are generally

more corrosion and wear resistant than electroplated nickel.

Heat treatments can increase significantly the wear resistance and

hardness of electroless nickel. This occurs by precipitation hardening

and the precipitation of a nickel phosphide. The hardness depends on

the heat treatment and on the alloy composition and often attains a Knoop hardness of 900-1000, equivalent to the hardness of electrodeposited chromium.

Tests by several laboratories show improved wear resistance with

appropriate heat treatment and in some circumstances, the electroless

nickel coatings have a higher wear resistance than electrodeposited

chromium. Wear resistance of electroless nickel can also be improved by

the incorporation of small particles of hard material into the coating

during the plating process.

Keywords: Auto-catalytic nickel; auto-catalytic nickel, hardness of;

auto-catalytic nickel, wear of; electroless nickel; electroless nickel,

hardness of; electroless nickel, wear of; nickel, auto-catalytic;

nickel, electroless; nickel-phosphorus alloy; and wear resistant alloy.

Coatings of nickel-phosphorus alloys are used to provide hard,

wear resistant surfaces and with other characteristics have contributed

to their use in a wide variety of applications

The coatings are formed by a chemical process in which sodium hypophosphite reacts with nickel ions in a solution on a catalytic surface

to produce nickel metal. Since nickel is a catalyst for the process,

the formation of nickel can continue after coverage of the original surface and coatings can be built up to substantial thicknesses. In the

plating trade, both the process and the coating are referred to as electroless nickel, though the proper designation is now considered to be

^5
autocatalytic nickel. Actually these terms include processes which produce materials other than nickel-phosphorus alloys, but will not be discussed further.

These nickel-phosphorus coating materials contain up to 15 wt %

phosphorus. As-plated, they are supersaturated solutions of phosphorus

in nickel and are often considered to be amorphous or a metallic glass

though it has been shown by electron diffraction that at some compositions they are very fine grained. Subsequent heat treatments at appropriate temperatures will convert the metastable material to Ni3P and

nickel to form a substantially harder and more wear resistant material.

Economically, electroless nickel plating is more expensive than

conventional electroplating; but the process has the advantage that the

coating has a more uniform distribution and it can be applied to com- ponents whose shapes are not suitable for electroplating. In addition,

the nickel-phosphorus alloys are more corrosion resistant, harder, and

more wear resistant than electrodeposited nickel. Because of these

characteristics of the process and the material, electroless nickelphosphorus coatings are used on such items as heat exchangers, pumps,

oil field drilling equipment, high pressure gas bottles, shipboard

equipment, hydraulic cylinders, gears, printing press beds, and as a protective surface for stainless steel. A complete list would include

many other applications where corrosion resistance and/or wear resistance are required.

Gutzeit and Mapp [1] reported data on the rate of penetration of

electroless nickel-phosphorus coatings by over 100 different solutions.

Generally, these coatings are corroded by reagents which attack unalloyed nickel but usually at a slower rate. This corrosion resistance

should be dependent on the alloy composition and heat treatment, but

most corrosion data do not specify the composition or are only for one composition.

Because of the relation between hardness and wear resistance, and

because of the relative ease of measuring hardness, hardness data is

often used in lieu of wear data and an understanding of the hardness

characteristics of the nickel-phosphorus alloy is pertinent to the un- derstanding of the wear characteristics.

As deposited, electroless nickel-phosphorus coatings vary in Knoop

hardness from about 500 to 700 for a lOOg load depending primarily on

the phosphorus content (Fig. 1). After a simple heat treatment in air

at 200°C for 8 hours, there may be some hardening of the material, again

depending on the phosphorus content. After heat treatment at 400°C for

1/2 hour, the hardness is generally in the range of 900 to 1000 HK^qo

(Fig. 1) resulting from precipitation hardening in which nickel phosphide, Ni3P, precipitates. The precipitation hardening process is further illustrated in Fig. 2 in which hardness is plotted as a function of

1+6
the time at 400°C. The falling off of hardness of the low phosphorus

alloy after the initial hardening is probably associated with recrystallization of the nickel [2] and agglomeration of nickel phosphide as shown by Brenner [3] for heat treated alloys higher in phosphorus content. The high phosphorus material then consists essentially of small

grains of nickel dispersed in a matrix of nickel phosphide, Ni3P.

Hardness as a function of the heat treatment temperature is shown

in Fig. 3. It is evident that the alloys generally encountered, with

6 to 10 wt % phosphorus, reach maximum hardness with heat treatments

at about 400°C. Above 400°C, the hardness decrease becomes less pronounced with increasing phosphorus content. Heat treatments of 400°C

are generally used for hardening electroless nickel-phosphorus and the

hardness obtained is of the same order as that obtained on conventional

chromium coatings and sometimes harder. It is not unreasonable to

speculate that still harder material could be obtained with an appropriate cycle of heat treatment.

Wear tests have been reported by at least 5 laboratories [4,5,6,7,

and 8]. These results are not directly comparable, because each laboratory used a different test or different test conditions, although there

is no disagreement with the qualitative relationships between the heat

treatment and phosphorus content of the material and its wear performance .

Our own data, obtained with a Taber Abraser, are summarized in

Table 1. In brief: heat treatment at 400°C improved the wear resistance of the nickel alloy, although the wear resistance of electroplated

chromium was still better. In relating wear resistance of a material to

that of chromium, it should be borne in mind that the properties of

electroplated chromium are not single valued. That is, the wear resistance and other properties of chromium are dependent on the chromium

plating process.

According to Gutzeit [4] and Randin and Hintermann [5], the wear resistance of electroless nickel-phosphorus increases considerably as the temperature of the heat treatment is raised, reaching that of hard

chromium at 600° C.

Turns and Browning [6] report increasing wear resistance with increasing temperature of heat treatment up to about 480°C. Their Taber

Abraser test rated chromium better than heat treated nickel-phosphorus.

Weightman and Pearlstein [7] obtained similar results using a Falex

wear test except that nickel-phosphorus heated at 400°C was better than

chromium. Parker [8] and Cohen [9] (Table 3) obtained the same results.

During the deposition of electroless nickel-phosphorus, one can incorporate small particles of alumina or other hard material into the

hi
coating. Parker [8] tested the wear resistance of such composite coatings containing various hard particles. He reports that as-plated,

these coatings were as wear-resistant as chromium and aluminum hardcoat, and more wear-resistant after heat treatment at 260-290°C (Table

2).

In summary: The nickel-phosphorus coatings have been used in

pumps, compressors, molding and extruding dies for plastics, in order to

reduce galling type wear in machinery, etc. These applications have

evolved by experience and trial and error. In recent years interest in

these coatings for wear resistance have lead to laboratory testing and

developments which are expected to lead to better wear performance.

REFERENCES

[1] Gutzeit, G. and Mapp, E.T., Corrosion Technology, _3> 331 (1956).

[2] Graham, A.H., Lindsay, R.W. , and Read, H.J., J. Electrochem. Soc.

,

112 , 401 (1965).

[3] Brenner, A., and Riddell, G.E., Proc. Am. Electroplaters ' Soc,

156 (1947).

[4] Gutzeit, G., Trans. Inst. Metal Fin., 33, 383 (1956).

[5] Randin, J. P., and Hintermann, H.E., Plating, 54, 523 (1967).

[6] Turns, E.W., and Browning, J.W. , Plating, 60_, 463 (1973).

[7] Weightman, R.F., and Pearlstein, F. , Metal Fin., 7_2, No. 9, 53

(1974).

[8] Parker, K., Plating, 61, 834 (1974).

[9] Cohen, B., WADC Technical Note 57-425, January 1958.

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TABLE 2

COMPOSITE ELECTROLESS NICKEL

TABER WEAR RESISTANCE [8]

Particle

Deposit

Taber Wear Index

None

+ Graphite

+ Chromium Carbide

+ Tungsten Carbide

+ Aluminum Oxide

+ Titanium Carbide

+ Silicon Carbide

+ Boron Carbide

+ Diamond (Du Pont)

Hardness

Knoop

Soft

1735

2080

2100

2470

2500

2800

7000 Est,

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18

15

8

3

10

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2

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Heat

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8

8

2

2

5

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2

1

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Aluminum Hardcoat

Hard Chromium 1000

Taber Wear Index = weight loss mg/1000 cycles

(Average of 5000 cycles, CS-10 wheels — lOOOg load)

°10-16 hr. at 290°C (554°F)

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DISCUSSION

R. E. Marlnyer, Battel 1e, Columbus Laboratories : You stated that the

heat treatment of the electroless nickel plate is sometimes called a stress relief treatment. I believe that that is wrong. We did some experiments some years ago in which we took a thin sheet of prepared

beryllium and plated Kanigen electroless nickel on it. Our purpose

was to compare thermal expansion coefficients, but we found that after

it came out of the plating bath, it stayed perfectly straight. As soon as it was heat treated, it deformed. Instead of stress relieving

the material, the heat treatment built up the stress. For tmst

applications this doesn't make any difference.

C. E. Johnson : Were you heat treating in air?

R. E. Haringer : I believe so, although it may have been in argon.

C. E. Johnson : If the heat treating atmosphere was hydrogen, I am sure the stresses would increase. My statement about stress relieving

referred to the high phosphorus alloy.

A. J. babecki , NASA, Goddard Space Flight Center : At what phosphorus

content is there a change in the ferromagnetism of the nickel in

nickel -phosphorus alloys?

C. E. Johnson : Above about 8% phosphorus, the ferromagnetism of the

nickel diminishes significantly. After heat treating, the alloys we examined become ferromagnetic, regardless of the phosphorus content.

A. J. Babecki : Can the adhesion of the as-deposited plating be

improved by heat treatment?

C. E. Johnson: Adhesion does increase with heat treatment.

55
CHROMIUM DEPOSIT STRUCTURE HAS A PROFOUND

INFLUENCE ON PROPERTIES

W. H. Safranek

Battelle, Columbus Laboratories

Columbus, Ohio ^^5201

Abstract

The optimum structure of chromium electroplate should be specified for

each individual application. Crack-free chromium supplies good corrosion protection and good wear resistance for some applications. Customary hard chromium is preferred for resisting wear in other applications. High-crack-count chromium is mandatory for avoiding harmful

effects on the fatigue strength of high-strength steel substrates.

Each of these structures can be deposited by a new process that is 25

to 50 times faster than the customary rate for chromium plating.

Kejnvords : Corrosion; crack-free chromium; electroplating; fast

plating; fatigue strength; grinding; hardness; stress;

tensile strength; wear

Introduction

The chief functional uses for chromium electroplate involve resistance

to wear, erosion, friction or galling, and protection of substrate

metals from high-temperature oxidation or corrosion at ambient temperatures. For these purposes, chromium coatings characterized by a hardness of 800 to 1100 VHN and a crack count of 10 to 100 per cm (25 to

250 per inch) are usually applied on coating and drying rolls, cutting

tools, engine cylinders, extrusion dies, glass forming molds, gun-tube

bores, landing gear cylinders, piston rings, pump shafts, rods, and

many other products. Thickness ranges from 2.5 to > 250 ym (0.1 to

> 10 mils) . The annual added-on value of these products by chromium

plating is estimated at $250,000,000 to $500,000,000.

Chromium plate with a network of cracks numbering 5 to 100 per cm as intersects to any straight line on the surface is the most common of

three alternative types because it was the first type developed com- mercially and is currently the only type available from the large

majority of vendors equipped for chromium plating. These vendors and

their customers have been neglectful of the potential advantages of

two other types, which are termed (1) crack-free chromium and (2) hlghcrack-count chromium (> 250 cracks/cm or 650 cracks/inch).

56
Properties and Structure

Table 1 summarizes property data for three types of chromium. Crackfree chromium Is characterized by a high density, a relatively low con- centration of oxides (0.05 to 0.2 percent oxygen), and a high tensile

strength. Conventional and hlgh-crack-count chromium exhibit a low

density, a high concentration of oxides (0.4 to 0.88 percent oxygen),

and a low tensile strength.

Crack-free deposits are softer than conventional or hlgh-crack-count

chromium. Soft deposits exhibited a hexagonal structure, whereas

chromium with a hardness of 1000 kg/mm^ or more exhibited a bodycentered cubic structure. ^ ^ A mixture of bee and hexagonal chromium

was observed for deposits of intermediate hardness.

Some investigators have reported that wear resistance is independent

of hardness. Others observed that soft deposits showed less wear than

hard deposits, but a few reported Improved resistance to wear or scratching when hardness was increased to about 1050 kg/mm^.^^^ During

a recent study, crack-free chromium with a hardness of only 725 kg/mm^,

which was deposited on the bore surfaces of cylinders evaluated in an air conditioner assembly, showed only one-third as much wear as conventional chromium with a hardness of 875 to 925 kg/mm^.^^-^ Phosphated steel

piston rings were run in the chromium-plated, aluminum cylinders.

Applications

Where corrosion resistance is of primary Importance, consideration

should be given to crack-free chromium in preference to conventional

deposits. Available data show that crack-free chromium furnishes better

protection against humid, corrosion environments. Crack-free chromium

also should be used for high-temperature service, particularly when

parts such as molds for fabricating glassware are thermally cycled.

For good resistance to wear, either crack-free or conventional chromium

may be satisfactory. The best kind of chromium for maximum wear re- sistance in a specific application can only be determined by making com- parative tests. The nature of mating surface and its finish, the

chromium surface finish, and lubrication conditions appear to have

far more influence on wear than the hardness of the deposit. A lightly

honed crack-free chromium with a mlcronodular surface (20 to 40 microinches) is better than a ground and lapped surface for some purposes.

To avoid the catastrophic consequences of structural damage during

grinding, a low wheel speed, a relatively low pressure, and good lubrication are very Important. A lightly honed, thin deposit (0.002 to

0.004 inch) may be better than a thicker, surface-ground deposit, and

less costly. Failures attributable to surface grinding of 0.006 to

0. 015-lnch-thick deposits occur frequently. Chromium thickness on

57
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58
cutting tools is sometimes limited to only 2.5 or 5 \im (0.0001 or 0.0002 inch) to eliminate the need for any surface finishing after

plating.

Conventional chromium deposits are stressed in tension and reduce the

fatigue strength of steel (and aluminum alloy) substrates, as shovm in

Table 1. The reduction in fatigue strength depends on the stress level

in the chromium, and the fatigue limit of the steel. A high stress

(20 to 42 kg/mm^) in chromium deposited on a high-strength steel (112,

to 148 kg/mm^) reduces fatigue strength as much as 65 to 73 percent.^

On the other hand, high-crack-count chromium exhibits a compressive

stress (-1.2 to -12 kg/mm^) and has little or no harmful influence on

the fatigue strength of the substrate. Thus components subjected to

fatigue during service should be plated with high-crack-count chromium.

Plating Procedures

At least three procedures for depositing crack-free chromium have been

developed. One uses 2.5 M chromic acid solutions containing 2.0 to 2.5

g/1 of sulfuric acid and maintained at 80 to 85 C. ^-^^ Another uses 60-

cycle pulsating current. ^ The third deposits crack-free chromium at

a rate of approximately 20 ym/min, which is approximately 60 times

faster than the usual rate for plating chromium. This process uses high solution velocity (6 to 8 ft/sec) past cathode surfaces or a high

rate of cathode rotation to sustain high current densities of 3 to 6

amp/cm^ (20 to 40 amp/in^), which is 10 to 15 times higher than normal.

Cathode efficiency with these conditions is 45 to 55 percent. By comparison, crack-free or cracked chromium is deposited conventionally

with an efficiency of 10 to 15 percent.

Conventional chromium with 5 to 100 cracks/cm is usually deposited in

1.5 to 3 M chromic acid solutions maintained at 45 to 55 C at current

densities of 0.12 to 0.4 amp/cm^ (0.8 to 2.6 amp/in^). With a temperature of 50 C, variations in current density from 0.2 to 0.8 amp/cm^ or shifts in the concentration of chromic acid have only a slight effect

on hardness. Sulfuric acid concentration is usually 1 percent of

the chromic acid concentration unless fluosilicate ions are added to

replace a part of the sulfate ions. Deposits from solutions containing

fluosilicate ions are slightly harder than chromium deposited in

chromic-sulfuric solutions

.

Compressively stressed chromium with a high crack count is deposited in

40 C, 2.5 M chromic acid solutions at a current density of 0.2 amp/

cm^.(^'-'-O) A higher current density (0.4 amp/cm^) can be used to deposit this type of chromium in 40 C chromic-sulfuric solutions ^-'-^^ or

43 C chromic-sulfuric-fluosilicate solutions . ^"'""'"^ A fast plating method

also has been developed for plating high-crack-count chromium.

59
Fast Plating Processes

Fast plating procedures were developed recently at Battelle-Columbus for

depositing each of the three types of structures described in this

paper. Table 2 summarizes the conditions. Deposits obtained at high

current densities with fast solution flow (1.25 to 1.5 m/sec) were

similar in structure and hardness to deposits obtained on piston rods

rotated at a high rate (1.25 to 1.5 m/sec).

References

(1) Brenner, A., Burkhead, P., and Jennings, C. W. , "Physical Properties of Electrodeposited Chromium", Proceedings Am. Electroplaters

Society, 34, 32-73 (1947).

(2) Safranek, W. H. , "The Properties of Electrodeposited Metals and

Alloys", Am. Elsevier Publishing Co., 517 pp (1974).

(3) Arend, H. , "The Hardness of Hard Chromium Plates", Metalloberflaeche, 3B(5), 72-76 (1951), Brutcher Translation No. HB 3427.

(4) Battelle's Columbus Laboratories unpublished data.

(5) Hammond, R.A.F., "Some Engineering Aspects of Electrodeposition"

,

Proceedings Am. Electroplaters' Soc . , 51, 9-20 (1964).

(6) Williams, C. and Hammond, R.A.F., "The Effect of Chromium Plating

on the Fatigue Strength of Steel", Trans. Inst. Metal Finishing,

32 (1), 85-106 (1955).

(7) Stareck, J. E., Seyb, E. J., and Tulumello , A. C. , "The Effect of

Different Chromium Deposits on the Fatigue Strength of Hardened

Steel", Plating, 42, 1395-1402 (1955).

(8) Faust, C. L., Schaer, G. R, and Semones, D. E. , "Electroplating

Chromium Directly on Aluminum", Plating, .48, 605-612 (1961).

(9) Safranek, W. H. and Layer, C. H. , "Fast Rate Electrodeposition",

Trans. Inst. Metal Finishing, 53^ (autumn issue 1975).

(10) Aotani, S., "An Internal Stress of Chrome Plating and Its

Relaxation by Heating, Journal Metal Finishing Society of Japan,

16 (1), 7-11 (1965).

(11) Stareck, J. E. , Seyb, E. J., and Tulumello, A. C, "Stress in

Chromium Deposits", Plating, 41., 1171-1180 (1954).

60
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61
DISCUSSION

E . Rablnowicz, Massachusetts Institute of Technology : I have only

user's experience with chrome plating. But from what people tell me,

the reliability of chrome plating is notoriously low. There might be

a commercial outfit that does good chrome plating for you for years at

a time, and then one week suddenly everything goes bad. Is that your

experience?

W. H. Safranek : I have heard of a few similar cases. Chromium

plating is very highly dependent upon the quality of the substrate,

and this is one thing that is very rarely kept under control.

A. J. Babecki, NASA, Goddard Space Flight Center : When electrodepositing chromium, do you use the same intermediate layer to get

good adhesion on different substrates such as cast iron, steel or

al uminum?

W. H. Safranek : In the case of cast iron piston rings, the first

operation is wet blasting. Then they are vapor degreased. Next they

are cathodically activated at low voltage for a few seconds. Then they

are plated. For high silicon content aluminum, we use an etching type

alkaline cleaner in order to obtain a controlled micro-roughness on the

surface. This operation is followed by rinsing, nitric acid desmutting, rinsing and zincate. Then, the material is chromium plated.

F. K. Sautter, Watervl iet Arsenal : What is the residual stress in the

crack-free chromium?

W. H. Safranek : Published data for tensile stress in a crack-free

type chromium ranges from 1.4 to 4.2 kg/mm^.

F. K. Sautter : You mentioned that crack-free chromium was particularly

good for high temperature applications. It has been our experience

that crack-free chromium cracks as soon as heat is applied.

W. H. Safranek : This happens in some cases. The magnitude of the

difference in coefficients of expansion between the chromium and the

substrate determine whether the plating will crack. Our experience

for the crack-free type chromium relates primarily to its use in the

glass mold forming application, and in that application it did very

well. These are low chromium steel molds and the coefficients of

expansion for the steel and the chromium are similar.

W. G. Thelen, David Taylor Naval Ship Research & Development Center : There are proprietary processes that supposedly seal the cracks in

62
chromium with chromium oxide or other ceramics. Do these have any

effect on wear in the plucking or scratching mode such as in a cylinder with piston rings?

W. H. Safranek : I can't really answer the question directly.

Certainly they could have an effect and it might be a factor in

accounting for inconsistencies in reports relating hardness to wear

for a conventional type chromium--a low-crack-count type chromium. In some cases there could have been higher amounts of oxides retained in

the cracks i:han in other cases. But I don't know anybody who has

studied this particular aspect in any detail.

R. M. Thomson, National Bureau of Standards : I'm a little puzzled

that you have not seized on the cracks as the primary characteristic

of the quality of the chromium plate. One would think that the cracks

would be sites for corrosive attack on the underlying substrate and

that also they would be important stress raisers for the underlying

substrate where fatigue might initiate. What are the criteria used to

determine the quality of the plated product?

W. H. Safranek : Well, I didn't entirely neglect these subjects. I did point out that the crack-free type chromium does not provide paths

for corrosive elements to get to the substrate. The literature does

not contain a great deal of data demonstrating that the crack-free

type chromium is very much better than a low-crack-count chromium.

But we have quite a bit of data at Battelle. For chromium deposits on steel, in the 1/2 to 1 1/2 mil thickness range, the crack-free

chromium is very much superior "co the low-crack-count chromium. You

are quite right about the effect of the cracks in the chromium being

sites for fatigue cracks to start in the steel and this is exactly

what happens with low-crack-count chromium. Now, when the crack-count

goes up to 250 cracks per centimeter or more, the load is spread out

over so many potential sites that the effect on failure of the steel

no longer exists, or at least does not exist to the same degree. That

is why the high-crack-count chromium causes no large reduction in the

fatigue strength of the substrate.

G. H. Kitchen, Bell Telephone Laboratories : There are companies which

supply chromium plated coatings in which the cracks are filled with

molybdenum disulfide, graphite, Teflon, etc. Purportedly they heat

treat the coatings and seal the solid lubricants in the chromium

surface. Can you comment on that?

W. H. Safranek : Actually, I was not aware of it, so I can't comnent.

We have made some pretty sophisticated studies of the width of these

cracks in the as-plated material. They are about 2 to 3 yin wide. I find it hard to believe that you can get very much material (lubricant)

into these narrow cracks.

65
PLATED COATINGS FOR ELECTRIC CONTACTS

Morton Antler

Bell Telephone Laboratories, Incorporated

Columbus, Ohio 43213

ABSTRACT

Electrical contacts, such as those in separable connectors,

are required to have low and stable contact resistance.

For high reliability connector applications in low voltage

circuits, gold is the preferred contact material because

of its chemical inertness. Recent studies have shown that

the most common gold electrodeposits , which contain cobalt

or nickel as hardeners, are complex materials, containing

10% or more by volume of organic polymers, potassium

compounds, and other substances. Failure in service can

occur if the deposit is porous, since films may form on

the surface by reaction of the base substrate metal with

air pollutants at pore sites. The use of underplatings

and smooth substrates can reduce the thickness of gold

required for low porosity; inhibitor coatings may suppress

corrosion reactions, and galvanically inert substrates can

even eliminate pore corrosion. Wear-induced porosity is

also a failure process, and contact lubrication is effective in its control. Degradation by diffusion of substrate metals, such as copper, to the surface where films

form is controlled by barriers, like nickel underplate.

The high cost of gold has led to the search for alternate

contact materials. Palladium plate and the soft base

metals, tin and solder plate, have achieved limited

acceptance

.

Key words: Contact lubrication; corrosion inhibitors; electrical

contacts; gold plate; palladium; polymer in gold;

porosity in gold; solder plate; tin-nickle alloy;

tin plate; underplatings for gold; wear of gold.

6k
Plated metal coatings are widely used on the contacts of

current-carrying devices whose function it is to allow a circuit to be disconnected and reengaged, or to enable one

member to be moved while in continuous contact with another. Such devices include connectors, slip rings, and

switches. When voltage and current levels are low and

high reliability is required, gold and some other noble

metals, e.g., alloys high in gold content and rhodium

plate, have become nearly universal as the contact coating. We describe one of these devices, the separable

electronic connector, in some detail: design practices,

materials selection, and its mechanisms of contact failure. We then briefly consider some lower cost metals that

are candidate replacements for gold.

It is of interest to note that the value of separable

electronic connectors manufactured annually in the United

States is 0.5 billion dollars. About 1 million troy

ounces of gold are used in these devices and on the contacts of printed circuit boards. The manufacture and

servicing of modern telecommunications, computer, and

electronic instruments would be impossible without

connectors

.

Design Factors Connectors typically have several to 100

or more contacts separated by dielectric material, each

joined at their back ends to a conductor by solder, wire

wrapping, crimping, or welding. They usually are engaged

by a wiping action, although butt (nonsliding) contacts

are also used. Typical designs include the pin in split

tine socket, the printed circuit board edge connector

having cantilever contacts which deflect when pushed over

the board contacts, and forked contacts that spread

during engagement by flat blade members. Substrates must

be good conductors, and some of them that also are spring

materials include copper-nickel-tin alloys, phosphor

bronze, and beryllium copper. Copper, brass, and nickel

are other widely used substrate metals.

Design normal loads for connector contacts range from ca.

50-300g. Contact geometries usually have single- or

multi-point or line configurations. These devices are

required to carry current with low and stable resistance,

often limited to a few milliohms, for 5-40 years. The

required numbers of engagements and rematings is usually

in the range, 50-500. Service environments can be benign,

as in air conditioned offices, or aggressive, such as in

equipment that serves in sheltered out-of-doors locations,

e.g., in aircraft. Ambient temperatures may range from

65
subzero to 150°C or more, depending on the application.

Above all, high reliability is necessary, with contact

failure rates often having to be no greater than 10, or

even 1, contact in 10 operating hours (1140 years).

Connectors have more than 1 contact, and, therefore, would

experience higher failure rates than this. A failure is

an exceedance of contact resistance beyond some value

dependent on the circuit in which the connector is used.

Contact Materials The popularity of gold, the most common

contact material, originates in its nobility, for if

insulating surface films form, contact resistance could

increase. Gold is used mainly as electrodeposits on both

members, or as a plating vs. wrought gold weldments or

claddings on the opposite member. Wrought golds may be

pure or be alloyed with other metals, such as silver.

Pure gold' electrodeposits are unsatisfactory because of

poor wearing characteristics and high friction on engagem.ent. In practice, from ca. 0.1-0.5% of cobalt or nickel,

is codeposited with gold to give a two t^ four fold increase in hardness, to ca. 140-225 kg/mm , and improved

durability. Deposit thicknesses range from 0.5-5

micrometers

.

Structure and Composition of Hard Gold Electrodeposits In

the past few years it has been found that gold electrodeposits are complex2m^ter ials , containing (a) C, N, and 0

polymeric substances ' that originate in KAu(CN)2 from

the plating bath, (b) usually potassium in unknown form,

but conjectured to be occluded plating solution and (c)

hydrogen in minute voids, a few tens of angstroms across.

The density of these golds may be 90% or less than that of

pure gold. The hardener metal, Co or Ni, is mainly in solid

solution and is believed by some to be partly combined with

the organic polymers. Typical golds have ca . 0.2% each of

Co or Ni, C, and K . Metallographic examination reveals

that hard gold deposits have laminar structures. The

deposits are brittle, with elongations below 1%, and may be

so internally stressed that when thick, i.e., 10 micrometers, they may spontaneously crack. Observation of

fracture edges of gold electrodeposits with the scanning

electron microscope reveals the occluded polymer. On dissolving the gold in aqua regia, the polymer (or reaction

product of polymer with aqua regia) can be isolated as a

^ porous film. Transmission electron microscopic examination

of thinned gold shows them to have a fine structure with

polymer "molecules" about 25 K in size. Secondary ion mass

66
spectrometric analysis o| a polymer-containing gold iden-_

tified fragments of AuNCN , Au , AuC , AUN2 / Au(CN)2 , Au~

,

AuCN", AuC2N~, and CN . Undoubtedly, the unique compositions, structures, and

properties of gold platings contribute to their good wear

resistance, better than that of wrought golds having the

same hardness. But rigorously cleaned hard gold electrodeposits are still poor wearing, ^or the codeposited substances are not solid lubricants.

Porosity in Gold The chief determinant of the thickness

of gold which is selected for contact use is its porosity.

It is impossible to plate gold on practical substrates

much less than 1-3 micrometers thick without it having a large number of submicroscopic voids that expose the

substrate to the atmosphere. Because of the galvanic

activity of the large area of gold in contact with exposed

base metal, corrosion of the latter can occur at pore

sites in humid, polluted atmospheres to produce surfaces

spotted with corrosion product. The significance of

porosity in electrodeposits on contacts is controversial.

On the one hand, the high cost of gold makes it imperative

to keep to a minimum the amount that is used, and thin

gold tends to be porous. The cost trade-offs between

having a thick deposit which can provide corrosion resistance, and the cost of a failure because of the susceptibility of the contact to degrade by film formation in

the particular environment in which the connector serves

are largely unknown. Resistance changes attributable to

porosity are difficult to determine in the field because

of their statistical nature, and there is little confidence that they can be realistically simulated in the

laboratory by accelerated environmental testing.

A more positive approach is to learn how to reduce porosity by improved substrate preparation and gold plating

techniques, and toward this end it has been shown that

smooth substrates and underplatings may permit thinner

deposits to be used. Other ways to reduce the susceptibility of porous golds to corrode in severe atmospheres

are the use of post plating corrosion inhibiting treatments, such as coating with very thin films of alkoyl

sarcosinates , and galvanically inert underplatings, like

tin nickel alloy.

Wear of Gold The limited durability of gold is a restriction to its use. Gold wears by prow formation. Many

gold contacts are unlubricated, but apparently are successful because adventitious contaminants from the

67
atmosphere collect on the surfat^e in sufficient amount to

provide a lubricating function. Conservative practice,

however, involves the application of these lubricant films

to contacts, either liquids or plastic solids. A widely

used liquid lubricant is a mixture of 5-ring polyphenylethers, and a common solid lubricant is microcrystalline

wa^. These are applied at coverages of ca. 10-30 yg per

cm of surface. Liquids are less dependent than solids on

surface roughness for effectiveness. Roughness provides a

reservoir for the solid which is replenished as the surface becomes burnished during use. The polyphenylethers

do not wet gold well, having contact angles of 20-25°, and

thus their good lubricity is surprising. Nonwetting is

desirable to minimize creep with subsequent lubricant

starvation.

Diffusion Degradation Another consideration in the use of

gold platings is the possibility of film formation due

to diffusion to the surface of codeposited base hardener

metals or of substrate elements where they can form insulating films. These processes occur slowly at room

temperature, and it is unclear whether they are significant in degrading the contact. But at elevated temperature, say at 125°C, diffusion processes are rapid.

Nickel underplate is an effective diffusion barrier for

substrate metals. The use of minimal amounts of the

hardener and care in maintaining low levels of metal

impurities in the gold is an effective way to minimize

this failure process.

g Alternate Materials There is considerable interest in

finding alternate materials for gold. This work parallels

the growing use of selective coating techniques for cost

reduction which apply gold locally to the contact site,

i.e., not to nonfunctional adjacent surfaces. Palladium

has proven itself to be a viable alternative in some

applications. The contact must be designed to not permit

small movements of the mated contacts, because this would

allow palladium to form insulating friction polymers. An

opposing member of gold also reduces the amount of polymer

that is formed. Palladium tarnishes at high relative

humidity in atmospheres polluted with sulfur dioxide.

Otherwise, considerations of porosity and contact lubrication apply to palladium as with gold.

The other materials which are making some inroad into gold

as a contact material are the soft base metals, tin and

solder plate. They are relatively corrosion resistant,

but, more importantly, can tolerate some film formation

since their plasticity allows the films to be easily

68
fractured on making contact and low resistance to be

attained. On the other hand, they have numerous deficiencies, listed below:

Problem

1.. Whiskering (for tin)

2. Excessive wear.

3. Fretting corrosion due

to vibration or differential

thermal expansion of the

contacts

.

Possible Solution

1. Alloy the tin with

another metal; reflowing the tin.

2. Lubricate; avoid

applications requiring many cycles

of mating.

3. Design for rigid

contact; coat contact with lubricant

sealants

.

4. Creep. 4. Restrict use to room

temperature or below; avoid thick

platings and sharp

contacts

.

5. Cold welding. 5. Lubricate; restrict

use to room temperature or below.

6. Intermetallic compound

formation at the interface

with copper or nickel

substrates

.

6. Restrict use to

room temperature or

less

.

7. Formation of insulating

films by corrosion.

7. Design for contact

loads above 200g

(with blunt contact

geometries) ; use

corrosion inhibiting

surface treatments.

69
REFERENCES

1. For a fuller discussion of most of the points in this

review, see (a) F. H. Reid and W. Goldie, Eds., Gold

Plating Technology , Electrochemical Publications,

Ldt., Ayr, Scotland, 1974; chapters by M. Antler,

"Wear, Friction, and Lubrication," pp 259-276; "Contact Properties," pp 277-294; "Contact Resistance,"

pp 334-344; "Connectors," pp 478-494, and chapters by

S. M. Garte, "Porosity," pp 295-315, 345-359, and (b)

M. Antler, "Tribological Properties of Gold for

Electric Contacts," IEEE Transactions On Parts, Hybrids, and Packaging," PHP-9 , 4-14, (1973).

2. G. B. Munier, "Polymer Codeposited with Gold During

Electroplating," Plating, 5£, 1151-1157 (1969).

3. Ch. J. Raub, "The Properties of Gold Electrodeposits

Containing Carbonaceous Materials," Proc. Fifth

Plating in the Electronics Industry Symposium , American Electroplaters ' Society, New York, N.Y.,

1975; pp. 55-66.

4. M. Antler, "Structure of Polymer Codeposited in Gold

Electroplates," Plating, 60^, 468-473 (1973).

5. S. Nakahara and Y. Okinaka, "Transmission Electron

Microscopic Studies of Impurities and Gas Bubbles

Incorporated in Plated Metal Films," in Properties of

Electrodeposits - Their Measurement and Significance , Eds. R. Sard, H. Leidheiser, Jr., and F. Ogburn, The

Electrochemical Society, Inc., Princeton, N. J. , 1975;

pp 50-62.

6. A. Keil, "Investigation of Electrodeposited Gold

Layers with Secondary Ion Mass Spectrometry (SIMS),"

Proc. Holm Seminar on Electric Contacts , Illinois

Institute of Technology, Chicago, 111., Oct., 1975.

7. M. Antler, "Wear of Gold Plate: Effect of Surface

Films and Polymer Codeposits," IEEE Trans, on Parts,

Hybrids, and Packaging , PHP-10 , 11-17 (1974) .

8. M. Antler, "Gold Connector Contacts: Developments in

the Search for Alternate Materials," IEEE Trans, on

Parts, Hybrids, and Packaging , PHP-11 , 216-220 (1975)

.

70
DISCUSSION

A. J. babecki" , NASA, Goddard Space Flight Center : How does the

ductility of the tin-nickel compound compare with that of electroless

nickel?

M. Antler : It is just as brittle.

S. Jahanmir, Massachusetts Institute of Technology : Has any work been

done to relate porosity to thickness in various platings such as nickel, silver, cadmium, etc.?

M. Antler : You can get any porosity you want. Porosity is caused by

occluded nonmetallic impurities that usually cannot be removed from

substrate surfaces. These are called pore precursors. It is the

quality of the substrate surface more than anything else which

determines zhe porosity of a plating.

S. Jahanmir : With the same substrate and the same plating process,

would the porosity increase with the plate thickness, say for nickel?

M. Antler : We would expect that the porosity would decrease with

increasing thickness of nickel on a given substrate.

H. Ravner, Naval Research Laboratory : Have you noticed any chemical

degradation of the polyphenyl ethers after use on these contacts?

M. Antler : I have not noticed any. The reason they were chosen

for contact application, apart from the fact that they are good

lubricants, is that they have low volatility and they are chemically

stable.

71
COMPATIBILITY EFFECTS WHEN SOFT METALS ARE USED AS SOLID LUBRICANTS

Ernest Rabinowicz

Massachusetts Ii'istitute of Technology

Cambridge, Massachusetts 02139

Abstract

The purpose of this study has been to test, mainly by means of

metal cutting experiments, the extent to which the \<feav life of electroplated soft metal film coatings is governed by the metallurgical

compatibility between the coating and the other sliding surface. Experiments have been carried out using hard steel tools coated with various

soft metals, and cutting a number of different workpiece metals. Two

different configurations were used, namely milling and shaping, and in some cases the lubricant was only applied once at the beginning of the

test, while in other cases it was reapplied continuously. It was found

that long tool life resulted when the soft metal had low metallurgical

compatibility with the workpiece material but not otherwise. The other

parameter investigated, namely relative mechanical strength between

film and substrate was found to be much less significant. This is in

contrast to the case of the friction coefficient, which has been found

to be greatly influenced by the mechanical strength ratio.

Introduction

Soft metals are among the oldest and best established tvpes of

solid lubricants. As it happens, they do not at present enjov wide

usage as solid lubricants, since the friction and wear tend to be relatively high in comparison to the layer lattice substances (1) . However, they are used to some extent in high vacuum applications, in which

case many layer lattice lubricants are unsuitable (2). Furthermore,

they have been found to be promising in certain metal cutting situations, in which case good performance at high temperatures and pressures

and the ability to re-form the solid lubricant continuously during sliding are important assets (3) . The recently developed technique of ion

plating, which allows soft metals to be deposited in such a x^ay as to

avoid a sharp interface between the soft metal and its substrate, has

also stimulated interest in the tribological behavior of soft metal

films (A, 5).

It is generally realized that, in order to function as a solid lubricant, a metal film should be softer than the substrate on which it is

deposited, should have a suitable thickness, and adhere well to the substrate (6). In this paper, we hope to examine one additional factor

which makes one soft metal more suitable than another, when used as a solid lubricant, namely the factor of compatibility. The experimental

testing has been confined almost exclusively to the use of the lubricants in metal cutting applications, but it is believed that the results

are also applicable to other sliding situations.

72-
The concept of compatibility assumes that if two metals sliding

against each other have low metallurgical compatibility against each

other, as evidenced by low solid solubility and poor liquid miscibility, the sliding characteristics will be good, (i.e. low friction, low

wear, and good resistance to seizure will prevail) . If on the other

hand the surfaces have high metallurgical compatibility the sliding

will be poor. This compatibility expresses the basic attraction

between atoms of the sliding materials, and is best determined by a systematic evaluation of binary phase diagrams (7)

.

To show the application of these compatibility ratings to a metal

cutting operation, consider the case of a nickel workpiece being cut by

a steel tool, which may be assumed to be essentially iron. Examination

of a compatibility chart (7) indicates that nickel is fully compatible

against iron, and thus the nickel-steel combination is expected to give

high friction and wear. Now consider some common soft metals, for

example, lead, tin, cadmium, zinc, copper and silver which might be

considered for application to the steel tool in order to function as solid lubricants. Assuming that the soft metal adheres to the steel

tool, the sliding interface will consist of the soft metal sliding

against nickel, and the compatibilities of these combinations become

relevant

.

On looking at a compatibility chart (7) it may be seen that copper

and zinc are fully compatible against nickel, x^7hile lead and silver are incompatible, and tin and cadmium are partially compatible. Thus, the

compatibility factors suggest that silver and lead are specially good

soft metal coatings for a steel tool to be used against nickel because

they greatly improve the compatibility rating of the sliding combination, while copper and zinc are less suitable, because they maintain

tht original unsatisfactory compatibility rating. The other two metals

should fall somewhere in between.

Based on published data in the literature, the above argument

should be regarded as a theoretical speculation, because there has been

no systematic evaluation of soft metal coatings to see if indeed their

wear properties are governed by compatibility relationships. Accordingly, a series of studies of various metal cutting operations has been

undertaken in which soft metal coatings were used, and their influence

on the cutting process was measured.

Apparatus

The experiments to assess the soft metal lubricants were carried

out using two different cutting geometries, namely milling and shaping.

Both continuously formed metal films, and films applied only once at

the beginning of the tests were used. Fortunately, the results obtained

with the two tests agreed quite well with each other and thus proved to

be alternative ways of studying the lubricating ability of thin metal

films

.

The milling experiments were carried out in a milling machine using

a standard end mill. A continuous jet of electrolyte from a positively

75
charged nozzle was directed at the flutes of the negativelv charged end

mill as they emerged from the cutting zone and in this way the soft

metal lubricant film was continuously replated (3)

.

The shaping tests were carried out in a horizontal milling machine,

using a high-speed-steel tool. The tool was fixed to the machine overarms and the workpiece was mounted in a dynamometer bolted to the milling machine table. The horizontal feed was used to pass the workpiece

under the tool, thus making a cut. Horizontal and vertical cutting

forces were recorded, and wearing out of the lubricating film was detected by an Increase in the cutting forces and by a reduction in

curl of the chips formed.

Materials used

Besides testing the compatibility criterion, one of the purposes

of the study was to find suitable lubricants for machining difficult to

machine materials, such as titanium alloys and nickel-based turbine

alloys and hence these materials were included in the testing program.

Another purpos.e was to see if the performance of the soft metal coatings was determined by other criteria such as relative mechanical

strength, and in order to extend the hardness range of the materials

used some tests were also carried out with pure nickel and with an aluminum alloy

.

The soft metal lubricants used were lead, tin, cadmium, zinc, silver and copper, and in addition a few tests were carried out using iron

and cobalt films. All these metals were deposited by electroplating

techniques. For most of the workpiece materials, the plated metals

covered a wide range of compatibilities, ranging all the way from fully

compatible to fully incompatible. However, in the case of the titanium the range of compatibilities tested was narrower, since no metal is

fully incompatible .against titanium.

In all the experimental tests, an attempt was made to keep the

thickness of the electroplated soft metal layer constant for all the

tests in that series, and reasonably close (within a factor of three)

to a thickness value of 10"-^ mm. This 10"^ mm represents the thickness

value which is generally found to give the lowest value of friction

coefficient (1)

.

Results

^ • Lubricant reapplied continuously

In these tests, the performance of the lubricant was determined by

the length of cut which could be undertaken before a sufficiently large wear land was found on the tool. Thus, figure 1 shows the results

obtained in machining a nickel-based turbine alloy. On the horizontal

axis is plotted the length of cutting required to wear out each tool

(wear out being defined as the formation of a wear land of length .023

mm), while on the vertical axis the degree of compatibility of the soft

7*+
metal lubricant against nickel is indicated. As will be seen there is a very good correlation between length of cut and degree of incompatibility.

UJ

_i ^ mg

So

21-

O

O I-z

1x1 <

33UJ

(El.

Q-O

1

\ NICKEL ALLOY WORKPIECE

/

/©

/ Cd

/

/

Sn

Cu Zn ^ Co

o o

10 20 50 100 LIFE OF TOOL, INCHES

Fig. 1 Life of tool plotted against relative compatibility of the soft

metal lubricant against nickel. The rank correlation coefficient for

these data is +0.88.

Perhaps the best way of reducing this diagram to a single number

is by computing the rank correlation coefficient between life and

compatibility, as has been done in an earlier study w). For the data

shown in figure 1, r^^^^j^ is found to be +0.88 (on a scale in which a perfect rank correlation is 1.0, and a complete lack of correlation is

0).

Similar data were obtained for titanium with a number of different

titanium alloys (3) , and averaged life values have been computed (fig- ure 2). In this case the r^.^^]^ value is much lower, namelv +0.42, and

for the limited number of tests run this figure is probably not significant. The reason for the poorer correlation in the case of titanium

may lie in the fact that only a limited range of compatibility could be

tested.

B. Lubricant applied once at the beginning of the test

In these experiments the friction during the cutting was monitored, and it was found that initially friction was low and the cutting

chips were well curled, but as cutting proceeded the lubricant wore off

and friction and chip radius of curvature increased. The length of cut

required to wear off the solid lubricant was determined, and for each

lubricant a relative life was determined, this being the length of cut

obtained with the lubricant divided by the geometric mean length of cut

with all the lubricants. Nickel and alumium workpieces were used, and

the lubricants were the plated metals lead, tin, cadmium, copper and

75
silver. A plot of relative life against compatibilitv for the ten com- binations tested is shown in figure 3, and for these data the rank cor- relation coefficient was +0.80.

UJo

i-q:

QQO

IP

UJ<

o

0--Z

H a; <o

Q-O

_ TITANIUM

1 1 ALLOY WORKPIECE1

Cd

" Zn

^-""o, o

Pb Fe

1 1 o

Sn

1

1 2 3

RELATIVE LIFE OF TOOL

Fig. 2 Life of tool (relative to the life in the presence of a commer- cial cutting fluid) plotted against relative compatibilitv of the soft

metal lubricant against titanium. The rank correlation coefficient is

+0.42.

LUO

mo

ie

oOH

z

ljj<

>-

\-°^ <m

o

o

o

5°

ALUMINUM AND NICKEL /

WORKPIECES •

/^

/

9^

h CD/

J 1

.25 .5 I 2 RELATIVE LIFE OF LUBRICANT FILM

Fig. 3 Life of the lubricant film (relative to the average life for

the whole series of shaping tests) plotted against relative compatibility of the soft metal lubricant against the workpiece metal. The rank

correlation coefficient is +0.80.

76
Discussion

These experimental tests were undertaken to see if compatibility

factors governed the performance of lubricating metallic coatings. It was found that, indeed, metallurgical compatibility has a strong influence on the performance of the coatings. The correlation coefficients

between performance and the compatibility of the materials at the sliding interfaces were always positive, and if the three rank correlations

coefficients values are averaged, the mean coefficient was found to be

0. 70.

As it happens, in an earlier study of unlubricated sliding metals

(8) it was found that in wear tests, (as opposed to friction tests

which gave lower correlation values) , a correlation coefficient of

about +0.6 between wear and compatibility was generallv observed. Thus

the data obtained here for metals acting as solid film lubricants are

right in line with those obtained earlier for unlubricated metals, and

it is likely that the same factors influence the wear processes in both

cases

.

Why is the correlation coefficient not higher, more specifically,

why does it not approach the theoretical maximum value of +1.0? The answer may lie in the fact that the real correlation is between wear and

surface energy of adhesion, rather than between wear and compatibility.

What is being measured then, in tests such as ours, is the correlation

between surface energy and compatibility. This correlation is positive

(8), but apparently does not approach the optimum value of 1.

Is there a correlation between life of a coating and its mechanical

strength? In order to see if there is an effect of this type, we have

for all our tests computed the ratio hardness of lubricant/hardness of

workpiece. Figure 4 shows the result of plotting relative wear life,

1. e. the ratio wear-life-of-f ilm/median-wear-life , for all the data

plotted in figures 1 to 3, against the hardness ratio. The compatibility rating for each point is indicated. The data indicate clearly the

presence of a compatibilitv effect, as seen in the fact that most of the

incompatible combinations give longer lives than the compatible ones. However, the effect of varying the hardness ratio is much less marked.

We may get a better idea of the hardness effect if we replot the

data of figure 4, so as to try to eliminate the compatibility effect and

reduce the experimental scatter. To do this it has been assumed that

the lives for the incompatible combinations is higher by a factor of twq

and thus to eliminate the effects of compatibility the lives should be

reduced by a factor of two. Similarly, the lives of the fully compatible combinations must be raised by the same factor of two. The result

of this adjustment is shown in figure 5, and now, with the experimental

scatter approximately halved, it is quite clear that the hardness ratio

has relatively little influence on the wear life of the coatings, but

there is an optimum wear life when the hardness ratio is about 0.1.

The wear relationships shown in figure 5 are in some disagreement

with the frictional relationships for soft metal films as a function of

the (hardness lubricant/hardness substrate) ratio. Figure 6, based on earlier experiments (9,10), shows that in friction tests using a pin-on77
disk tester, (using incompatible pin-film combinations throughout), the

friction coefficient is reduced continuously and monotonically as the

hardness ratio is reduced. The reason why this effect does not show up

in the wear results must be that reducing the hardness of the soft

metal film has two effects.

a) It leads to a lower friction coefficient, which, other things being

equal should give a longer wear life.

b) The hardness of the surface layer is reduced, which for constant

Archard wear coefficient, will give a greater wear volume and thus a shorter wear life.

.01 .02 .05 .1 .2 .5 1 HARDNESS LUBRICANT / HARDNESS WORKPIECE

Fig. 4 Plot of life of lubricated tools against the ratio of lubricant

hardness to workpiece hardness. There seems to be a very small influ- ence of hardness ratio on wear life, but a strong influence on compatibility, since the films incompatible with the workpiece (full black

circles) give exceptionally long life, while the compatible films (open

circles) give very short lives.

Not surprisingly, the two factors tend to cancel out.

One of the purposes of the study was to determine the possible uses

of soft metal lubricants. Past experience has suggested that the lowest friction coefficient is observed when the film thickness is very

small (10"-^ mm), but then the wear life is small. ^-Jhen much thicker

films are used, there is a tendency for the whole film to be spalled

off the surface and again the wear life can be very small (1). This

suggests two main uses of soft films, first in situations in which a thin film can be replenished continuously, and second in situations in

which a film resistant to spalling can be produced, as might be the case with techniques like ion plating. Assuming that reasonably long

life can be achieved, then the concept of compatibility constitutes a useful tool for selecting the soft metal to be used with any specified

workpiece material.

78
HARDNESS LUBRICANT / HARDNESS WORKPIECE

Fig. 5 Replot of the data of figure 4, but reducing? the lives for the

black circle combinations by a factor of two, and raising the lives

for the open circles by the same amount. The experimental scatter is

reduced considerably.

z 0-5- y

u.

LlUJO 0-2 zg

® DATA OF BOWDEN AND TABOR

©DATA OF RABINOWICZ

Pb-Cu^

^

Pb-Ti ©^ ^

Pb-Ni© ^X^®Pb-Cu

In - Ni© ^

®ln-Ag

In - Fe — 1 —1 —I —I •OOl -01 ! FILM hardness/ SUBSTRATE HARDNESS

Fig. 6 Plot of friction coefficients for hard metals lubricated bv soft

metal film against the f ilm/substrate hardness ratio. A hard steel

rider was used for all the tests. The friction is reduced monotonically

as the hardness ratio is reduced.

Acknowledgements

The authors wish to thank the National Science Foundation for sup79
porting this work under grant GK-36964, and to thank Messrs. S. Loutrel

and F. H. Anderson for carrying out much of the experimental work.

References

(1) Rabinowicz, E., "Variation of Friction and Wear of Solid Lubricant Films with Film Thickness," ASLE Trans., 10^, 1-9 (1967).

(2) Buckley, D. H. , "Friction, Wear and Lubrication in Vacuum," NASA

SP-277, Washington, D.C., 158-173 (1971).

(3) Cook, N. H. , Rabinowicz, E. , and Vaughn, R. L., "Metal Cutting

Lubrication through Continuous Electroplating," Lubrication Engineering, 22, 447-452, (1966).

(4) T. Spalvins, "Energetics in Vacuum Deposition Methods for Depositing Solid Film Lubricants," Lubrication Engineering, 2_5, 436-441,

1969.

(5) N. Ohmae, T. Nakai and T. Tsukizoe, "On the Application of Ion

Plating Techniques in Tribology," Japanese J. Applied Phvsics,

Supplement 2, Part I, 451-454, 1974.

(6) Clauss, F. J., "Solid Lubricants and Self-Lubricating Solids,"

Academic Press, N. Y., (1972).

(7) Rabinowicz, E. , "The Determination of the Compatibility of Metals

through Static Friction Tests," ASLE Trans., 14, 198-205, (1971).

(8) Rabinowicz, E., "The Influence of Compatibility on Different Tribological Phenomena," ASLE Trans., 14, 206-212, (1971).

(9) F. P. Bowden & D. Tabor, "The Friction and Lubrication of Solids,"

Oxford University Press, 1950, Chapters V and VI.

(10) E. Rabinowicz, Discussion in J. of Lubrication Technology, Trans.

ASME, F, 91, 555, (1969)

80
DISCUSSION

R. M. Thomson, National Bureau of Standards : Could another reason for

the scatter be the presence of voids in the interface? This may also

be a random variable because the surface roughness is not necessarily

given simply by looking at the compatabi 1 i ty of the effects.

E. Rabinowicz : Yes, there are other reasons for scatter apart from

the fact that the theory may not be correct. I would not have

expected the correlation coefficient to be better than .85 under ideal

conditions. I did expect a better coefficient than was obtained. I interpret the poorer coefficient as meaning that I was not correlating

against the ideal parameter.

A. W. Ruff, National Bureau of Standards : Did you use the hardness

value for the bulk material or did you actually measure the hardness

of the thin films?

E. Rabinowicz : In one or two cases, we measured the thin film

hardness. In the soft layers, like lead and tin, we did not make

measurements.

A. W. Ruff : Perhaps the films would be somewhat harder than the bulk

material. It might affect the scatter.

E. Rabinowicz : Yes.

A. W. Ruff : You commented on interactions between the cutting

material and the film from the point of view of adhesive energy.

Perhaps one should also consider the surface energy of the film

material and some ratio of adhesive energy between the two dissimilar

materials to the surface energy if you are pulling a fragment out of

the film. I suppose both the new surface energy and the adhesive

energy would be involved?

E. Rabinowicz : Yes. The expressions that are easiest to obtain

mathematically involve the surface energy itself. In a dimensional

analysis, it becomes very convenient to use the ratio of surface

energy of adhesion to self energy of adhesion. This ratio does not,

however, turn up in studies trying to produce a model of a process.

A. W. Ruff : I was thinking really of the physics of what is going on if you have two interfaces that are competing with one another; the

dissimilar-metal interface and the like-metal interface. They are competing on an energetic basis in the simplest sense.

E. Rabinowicz : That is perhaps a new way of looking at it.

81
A. W. Ruff : There is not a single metal that is incompatible in some sense with, for example, titanium. I was wondering about ternary

situations where one might consider a binary metal film interacting

with titanium in such a way that the binary combination--lead, tin, or whatever--woul d be incompatible with titanium. Have you extended your

ideas to ternary systems?

E. Rabinowicz : I have never looked at the three phase situation. In general, the indications are that when a binary system slides against

another metal, it is either the higher compatabili ty one that takes over or the softer of the components of the binary alloy that tends to

take over.

R. M. Thomson : The definition of the surface of two materials that

are diffusing into one another becomes a very complicated affair. It

becomes time dependent, for example. I wonder if you are not using

the right parameter after all instead of the surface energy.

E. Rabinowicz : That is ,'true. I have seen that argument used the

other way around against using compatabil i ty . Compatabil i ty theory

tells what the equilibrium state will be, but it does not tell the

rate at which the equilibrium is reached, and that is more significant. Possibly the diffusion coefficient would be the most

logical parameter because that has rate built into it, whereas a phase

diagram simply tells you what happens if you wait long enough. What

you are really looking for is a parameter that tells you how the atoms

at the surface are going to view each other.

J. Kruger, National Bureau of Standards : Just to introduce another

complicating factor, we have been concerned for a number of years with

the oxide films that form on a metal surface and their rate of

reformation once a bare metal surface is produced. What effect would

this have on evaluating compatabil ity effects? For example, I notice

with titanium, where you had problems, the rate of repassivation that we have measured is extremely rapid. It is in the order of

microseconds. Would this have any effect on the process?

j

I E. Rabinowicz : I think the only person who has studied the same , material both in vacuum and in air in the same series of tests was Coffin in the early 1950 's. Unfortunately he only used six i combinations that are really comparable for this purpose. For his

limited number of tests, it does turn out that the compatabil ity

effects correlate much better with performance in vacuum than in air.

The metals seem to know whether they would be soluble in each other if

they reach the melting point. They seem to be able to detect what the

metals underneath the oxide layers are doing. So presumably there is

j enough cracking or breaking or puncturing of the oxide layer to permit !

82
some metallic interaction. The compatabil ity relationship for oxides

is also a significant factor. Compatabili ties in oxides and other

nonmetals are much smaller than in metals.

85
REDUCTION OF SLIDING WEAR BY METALLIC COATINGS

S . Jahanmir and N . P . Suh

Department of Mechanical Engineering

Massachusetts Institute of Technology

Cambridge, Mass. 02139

Abstract

The role of soft metallic coatings in sliding wear has been

examined experimentally. The results indicate that the tribological

behavior of soft coatings is consistent with the delamination theory

of wear, especially the critical nature of the plating thickness.

It is shown that a reduction in wear rate of three orders of magnitude

is possible when the coating material is softer than the substrate and

thinner than a critical thickness. The optimum plate thickness was

found to be less than 1 ym for cadmium, silver, gold or nickel plated on various steels. As the plate thickness is increased, wear by delamination within the plate occurs. Environmental effects are important

in coating utilization.

Introduction

Soft metallic coatings have been used quite extensively in many

applications (1-13) to reduce friction and wear of sliding surfaces.

However, the exact nature and mechanism by which these coatings improve

the tribological behavior has not been investigated completely.

Previous attempts (12,13) have been made to determine the optimum

thickness of these coatings, but the results and the theoretical con- siderations were insufficient and inadequate. Recently it has been

shown that the delamination theory of wear (14) can be used effectively

to predict the behavior of these coatings. This paper reviews some of our previously published (15-17) results on the role of metallic

coatings

.

A Brief Description of the Delamination Theory of Wear

According to the delamination theory of wear which was recently

proposed by Suh (14), low speed sliding wear of metals occurs by

delamination of sheet-like wear particles from the sliding contact,

by the following mechanisms:

(1) plastic deformation and fracture of the original

asperities (i.e. original machining marks)

(2) plastic deformation of the subsurface caused by the

traction existing at the contact area

(3) void and crack formation in the deformed subsurface

(4) subsurface void growth and crack extension

(5) generation of long subsurface cracks nearly parallel

to the surface

(6) crack extension to the surface to generate wear sheets.
The rate of delamination has been shown to be dependent upon such

metallurgical variables as the type, size, and concentration of second

phase particles, inclusions and voids (18). Previous investigations

(18-21) have shovm that delamination occurs in a wide range of different

materials, from well annealed copper to fully hardened AISI A340 steel.

In the original paper Suh (14) postulated the existance of a soft,

low dislocation density zone at the outer surface of the wear track.

It was believed that the dislocation image force coupled with the clean

surface immediately behind the slider would allow dislocations very

close to the surface to escape. Therefore, it was concluded that this

layer could deform continuously without much workhardening and subsequent wear by delamination. For any metal, there would be some critical

depth below which the image force would be too small to extract a significant number of dislocations.

A corollary of this argument is that if a composite surface is

created by plating a thin layer of a softer metal (thinner than the

softer metal's critical depth) on a harder substrate, large plastic

deformation and delamination of the substrate may be prevented. This

is based on the fact that dislocations are not stable in very thin

layers of metals having a low flow stress, and most of them could be

eliminated by the image forces. Therefore, the thin and soft plating

could deform without workhardening and wear. The soft layer also

reduces the frictional force which will delay substrate delamination.

Experimental Procedure

Wear tests were carried out with a cylinder-on-cylinder geometry.

The specimens were rotated at a surface speed of 180 cm/min and the

stationary pins were pushed against the specimens by a normal load of

2.25 kg. The wear tests were carried out dry, both in air and in argon

gas. Some tests were also conducted under boundary lubrication in air.

Gulflex 39 or a pure mineral oil (Nujol) was gravity-fed to the contact

at a steady rate. All tests were performed at room temp, for 30 min.

The substrate materials used had a variety of hardnesses: AISI

1018 (84 kg/mm^), AISI 1095 (170 kg/mm^) and AISI 4140 (270, 370 and

460 kg/mm^ — obtained by different heat treatments) . In all cases the

sliders were made of the same material as the specimen, heat treated,

and plated in the same fashion.

The plating thickness varied from

.05 to 10 ym. The platings tested were gold, gold over a nickel flash,

cadmium, nickel, and silver. Some of

the gold plated specimens were plated

with a flash of gold first and were

then heated at 500°C for 2 hrs. in

vacuum to obtain a diffused bonding

between the plate and the substrate.

Then the required thickness of gold was plated on the samples.

Fig, 1 Comparison between wear

tracks of AISI klkO steel (17)

(a) unplated

(b) 1.0 ym Ni plated

85
Selected wear tracks were observed with the scanning electron

microscope. Some of the specimens were subsequently sectioned parallel

to the wear track, metallographically polished and etched to reveal the

structure below the wear tracks. The subsurface damage was then examined with the scanning electron microscope.

Experimental Results and Discussion

A. Effect of Plate Thickness

The effectiveness of plating in wear reduction is dramatically

shown in Fig. 1 for unplated and 1 ym nickel plated AISI 4140 steel

(17) . The large difference in the wear track size between the unplated

(Fig. la) and the plated sample (Fig. lb) is clearly observed. The wear rate data of these two samples and other specimens with thicker nickel

plates is presented in Fig. 2. The wear rate of the 1 ym nickel plated

sample is three orders of magnitude lower than the unplated specimen.

Fig. 2 also shows that as the nickel plate thickness is increased, wear rate increases. Similar dependence of the wear rate on the plate thickness is also shown in Fig. 3 for cadmium plated AISI 1020 steel (16).

The dependence of wear on the plate thickness can be explained in

terms of the delamination theory of wear. In thin layers some of the

dislocations can be eliminated by the action of image forces on the

dislocations. Therefore dislocation accumulation and strain hardening

does not occur, and the layers can deform substantially without wear by

delamination. However, if the plate is thick, dislocation accumulation

and strain hardening could take place within the plated layer and cause wear by crack formation and delamination of the plated layer. The

existance of cracks in a thick plate (25 ym) of gold on AISI 1020 steel

1000

1000

« 100-

E9

O

_ 100

E

I I I I I Mil I II 1 1 nil TTTn

O—OIn Argon Gas —

In Air

0.1 1.0 10

Initial Ni plote thickness (//.m)

0.10-01

I

I I I I I Hi I I I I I III

0.1 10 10 Thickness of Cadmiunn Plate ( p m )

Fig. 3 The effect of Cd plate Fig. 2 The effect of Ni plate

thickness on wear rate in argon (17) thickness on wear rate (16)

86
Fig. A Crack formation in

25 ym thick Au plate, after

wear testing (17)

Fig. 5 Wear of initial thick plates

vs. time (17)

is observed in Fig. 4 (17). The thick layers wear off by delamination

until the thickness is reduced to a smaller thickness in which the dislocations are presumably unstable. At this point, the wear rate is

reduced. The transient behavior of thick coatings is shown in Fig. 5 for an initial 10 ym nickel plate on AISI 4140 steel (17)

.

B . Surface and Subsurface Examination

The wear tracks of the unplated samples appeared to be very rough

when compared with that of 0.1 ym cadmium plated steel tested in argon

Sliding Direction

Fig. 6 Wear tracks of

AISI 1020 steel tested

in argon (16)

(a) unplated

(b) 0.1 ym Cd plated

(b)

0

0

Time

20

(min)

30

87
Fig. 7 Subsurface

deformation of AISI 1020

steel tested in argon (16)

(a) unplated

'Surlcce (b) 0.1 urn Cd plated

(b)

(16), as shown in Figs. 6a and 6b, respectively. The cratered appearance of the unplated steel indicates that many wear sheets have been

separated from the wear track. On the other hand, the wear track of

the plated sample is much smoother, with parallel furrows in the sliding

direction. Only a few indications of delamination were observed on the

0.1 ym Cd plated steel specimens.

Figure 7 compares the subsurface damage and deformation of the

unplated steel (Fig. 7a) and the 0.1 ym Cd plated steel (Fig. 7b) tested

in argon (16) . The plated sample has undergone less deformation and

contains a smaller number of subsurface voids and cracks. The depth of

subsurface deformation in the unplated sample is 30 - 35 ym while the

deformation depth in the plated steel is only 25 - 30 ym. The fact that

the plated samples have some voids and cracks in the steel substrate

indicates that the cadmium layer will eventually be removed when the

steel subsurface delaminates. An extended test in argon with a 0.1 ym

Cd plated specimen exhibited subsurface delamination after 86 m of

sliding. The unplated steel specimen tested for the same distance of

sliding had a weight loss greater than the Cd plated specimen by a factor of 5000.

C . Friction Coefficients

The coefficient of friction of the samples plated with the optimum

thickness of cadmium (0.1 ym) and nickel (1.0 ym) were lower than the

unplated samples. In the cadmium plating tests (16) the friction co- efficient decreased from 0.85 to 0.25; whereas it decreased from 0.63

to 0.A5 for nickel plating tests (17). This reduction of frictional

force by plating an optimum thickness of material is partially responsible for lower subsurface deformation and retarded delamination, since

88
Table 1 Experimental results on the wear resistance of 1 ym plates on

steel Wear testec in arson. 2.25Ke normal load. 54m slid

Coating

Substrate and^Hardness

(Kg/mm )

Wear Rate

—8 (mg/cm X 10 )

Coefficient of Friction

Cd AISI 1018 (84) 3.6 0.35

rA 1.8 0.25

Cd AlAO (270) 3.6 0.35

Cd 4140 (370) 3.6 0.25

Ag AISI 1095 (170) 1.8 0.33

Au

(diffused) AISI 1095 (170) 1.8 0.85

Au

(Nl underlayer)

1095 (170) -1 .8 0.9

Nl AISI 1019 (84) liamedlate -

Nl 1095 (170) ..

Nl II

Nl 4140 (370)

Nl 4140 (460) 1.8 0.45

coefficient is a very strong function of friction coefficient (20,22).

D . The Effect of Environment

The wear results in Fig. 3 indicate that a much smaller wear reduction was achieved by cadmium plating when the tests were carried out in

air rather than argon. However, the lowest wear rate is still obtained

with the 0.1 ym cadmium plating. This poor performance in air is caused

by oxidation and abrasion of the plate by the hard oxide particles.

A series of tests were conducted under boundary lubrication to minimize

oxidation (16). It was found that wear reduction with 0.1 ym cadmium

plated steel could only be obtained with the pure mineral oil (Nujol)

and not with the Gulf lex 39 lubricant. The poor performance of the

latter oil was caused by the formation of new compounds on the wear track by chemical reaction.

E. The Hardness Match Between Plate and Substrate

The wear test results for 30 min (54 m sliding) for all coating

materials and steel substrates with various hardnesses is summarized

in Table 1 for the tests in argon (17). The 1 ym plate of gold exhibited low wear rate in air but other filates, i.e. nickel, silver and

cadmium, did not last because of oxidation and abrasion. Table 1 indicates that nickel platings on steels softer than 460 kg/mm^ failed

immediately whereas cadmium platings showed low wear for all substrates

tested. This experimentally observed phenomenon is in fact consistent

with the predictions of the delamination theory of wear, that the thin

plating must be softer than the substrate for major wear reduction.

If the plated material is harder than the substrate, dislocations can

89
accumulate below the plate and cause early failure of the specimen by

substrate delamination . This fact was observed for most nickel plates

on steel since nickel was harder than those substrates.

Soft metallic coatings have been used for wear reductions previously by others (1-11). These investigators assume that the soft

metal layer melts during sliding and acts as a lubricant. This explanation can only be correct when the sliding speeds and the normal loads are high enough to cause a large flash temperature at the contact.

However, the results on the coatings presented here for low speeds and

loads and high melting point materials such as nickel (m.p. 1455°C)

,

indicate that the so called "lubrication" mechanism is in general not

correct. The role of thin metallic coatings can be explained better

in terms of the delamination theory of wear.

Conclusions

(1) The delamination theory of wear provides a theoretical basis

for reducing wear through the development of a coated metal surface.

(2) For major wear reduction the coating material must be softer

than the substrate material.

(3) There exists a most effective thickness of soft coating for wear resistance. This thickness is less than 1 ym for steel plated

with cadmium, silver, gold and nickel.

(4) Cadmium, silver and nickel plates are effective in wear reductions only under conditions where the formation of hard oxide

layers or corrosion products are inhibited.

Acknowledgements

This research was supported by the Advanced Research Projects

Agency of the Department of Defense and was monitored by ONR under

contract No. N00014-67-A-0204-0080. The authors wish to express their

appreciation to Dr. Edward Van Reuth and Lt . Richard Miller.

References

1. Y. Tsuya and R. Takagi, Wear, 7 (1964) 131-143.

2. R. Takagi and T. Liu, ASLE Trans., 10 (1967) 115-123.

3. R. Takagi and T. Liu, ASLE Trans., 11 (1968) 64-71.

4. A. J. Soloman and M. Antler, Plating, Aug. (1970) 812-816.

5. T. J. Kuczkowski and D. H. Buckley, NASA TN D-2721 (1965).

6. H. E. Evans and T. W. Flatley, NASA TN D-1339 (1962).

7. H. E. Evans and T. W. Flatley, NASA TN D-2101 (1963).

8. T. W. Flatley, NASA TN D-2304 (1965).

9. P. E. Brown, ASLE Trans., 13 (1970) 225-239.

10. T. L. Rindings, AEDC-TR-75-1 (1965).

11

.

R. E. Lee, Jr., AEDC-TR-65-1 9 (1965).

90
12. F. P. Bowden and D. Tabor, "The Friction and Lubrication of Solids",

Clarendon Press, Oxford (1954).

13. E. Rabinowicz, ASLE Trans., 10 (1967) 1-9.

14. N. P. Suh, Wear, 25 (1973) 111-124.

15. E. P. Abrahamson, II, S. Jahanmir, N. P. Suh and D. A. Colling,

Proc. Int. Conf . on Production, Tokyo, Japan, (1974) 408-413.

16. S. Jahanmir, N. P. Suh and E. P. Abrahamson, II, Wear, 32 (1975)

33-49.

17. S. Jahanmir, E. P. Abrahamson and N. P. Suh, Wear, (1976) to be

submitted for publication.

18. N. P. Suh, S. Jahanmir, E. P. Abrahamson, II and A. P. L. Turner,

J. Lub. Tech., Trans. ASME, 96 (1974) 631-637.

19. S. Jahanmir, N. P. Suh and E. P. Abrahamson, II, Wear, 28 (1974)

235-249.

20. N. P. Suh, Proc. Battelle Materials Science Colloquium, on "Fundamental Aspects of Structural Alloy Design", to be published

by Plenum Press, 1976.

21. S. Jahanmir, E. P. Abrahamson, II and N. P. Suh, Proceedings

Third North American Metalworking Research Conference, Carnegie

Press, (1975) 854-864.

22. E. Rabinowicz, "Friction, Wear and Lubrication", Center for Advanced

Engineering Study, MIT, (1974).

91
DISCUSSION

L. Leonard, Franklin Institute : It seems to me that there is an alternate explanation for your data. You said that when the coating

is too thick the maximum stress is within the coating. Then the low

strength of the coating itself causes it to wear relatively rapidly.

When the coating is thin, why isn't the lubricity or the coefficient

of friction the major factor in wear?

S. Jahanmir : We are saying that the wear reduction or the wear retardation occurs because there is a low friction coefficient which

leads to a low friction force on the surface which, in turn, causes a low amount of subsurface deformation.

L. Leonard : Have you looked at any of the pieces of wear sheet that

have come off which supposedly have low dislocation densities in a transmission electron microscope?

S. Jahanmir : It is a very difficult process to observe the

dislocation density of these thin films because the dislocations

escape through very thin films.

L. Leonard : Yes, but if the dislocation density is high, this would

counteract your argument.

S. Jahanmir : We have not found any direct evidence for the role of

dislocations in the process of plating.

A. W. Ruff, National Bureau of Standards : Do you have any data on the

variation of stress or some stress connected property with distance

from the sliding surface in the bulk materials that you worked with?

S. Jahanmir : No, we do not have any definite results for that. One

of the graduate students working in the group is using a finite

element method to find the stresses and strains below the surface and,

in fact, he is going to consider how the stress patterns or the

displacement patterns change when there is a soft coating on the

surface.

A, W. Ruff : Some work that I am doing now on iron surfaces in sliding

is producing some results on the stress and strain distribution below

the surface using electron channeling techniques. For iron, I find

that the stress mounts steadily as one approaches the wearing surface.

There is no decrease in residual stress and I think probably that same technique might be useful in monitoring the first 1 to 10 microns of

material. We are hoping to get some data on some fee materials such

as nickel or aluminum to compare to some of the hardness data that were reported. Hardness measurements in thin films near free surfaces

are sometimes difficult to do.

92
R. M. Thomson, National Bureau of Standards : One of the very

important parameters in an experiment of this kind is the local

temperature and, of course, this parameter is not directly measurable

and must be inferred. Have you considered the effect of temperature

in the whole process?

S. Jahanmir : All of our tests have been done at very low speeds (1.8

meters per minute) which will not cause a large temperature rise at

the surface.

R. M. Thomson : In micron layers you could expect it.

S. Jahanmir : The flash temperature right at the contact depends on

the sliding speed. If the speed is very low the flash temperature is

not going to be large enough to affect the properties of the surface.

Of course it would depend on the melting point of the material in

question because the properties of each material differ. There is

work in progress to look into the effect of speed on this whole

process. Changes in the speed produce changes in temperature.

95

SESSION III

SOLID

FILM

LUBRICANTS

H.Ravner

Naval Research Laboratory

(substituting for W.A.GIaeser)

THE SEARCH FOR FACTORS TO PREVENT MECHANICAL FAILURES DUE TO CORROSION

WITH SOLID FILM LUBRICANTS

M. K. G«bel M. B. Peterson

Naval Air Development Center

Warminster, Pennsylvania l897^

Wear Sciences, Inc.

Scotia, New York 12502

The failure of mechanical components of contemporary flight

vehicles has frenuently been attributed to the corrosion characteristics of solid film lubricants. An ongoing program at the Naval Air

Development Center is Investigating how some parameters affect corrosion between solid film lubricants and various aircraft alloys. This

paper will summarize some of the findings of this study.

Keywords: Corrosion; electro-chemical; lubricants; salt-spray

I . INTRODUCTION

Solid film lubricants have been in usage only about 25 years and

it has only been in the last 15 years that they were found to cause

corrosion. The controversy still exists as to whether these materials

accelerate corrosion. In the early years of solid film lubricant technology, the general assumption was that the adhesion and thickness of

the lubricant were the only determining factors for corrosion protection.

Originally, these materials were formulated for usage on both parts of

a two-component system, so that the corrosive effect on bare metal with

this type of lubrication was not considered. However, In 1959, a study was Instigated at our naval laboratory on the basis of field information

which indicated that corrosion was Induced when a dry- film-lubricated

metal surface is In contact with an anodlsed aluminum surface^

A slight step forward was then made in the upgrading of the

corrosion-resistant properties of these lubricants. However, due to

insufficient knowledge of the mechanism of corrosion involving solid

film lubricants and metals, this was found to be inadequate. Evidence

appeared indicating graphite powder caused galvanic corrosion (2) and

M0S2 in powder pellet form set up a galvanic cell with Al and Mg(3).

As yet, there appears to be no data in the literature on electrochemical tests of resin-bonded M0S2 and graphite with metals. It would be

interesting to determine If the resin binder changes the electrochCTiical nature of the powder and if applied pressure Influences this

effect.

97
During 1961 - 1965 period, another research center worked on

factors Influencing corrosion protection provided by solid film lubricant coatlngs^^^. This work Increased the corrosion protection provided with solid film lubrication (specification MIL-L-46010(MR) )

.

For Army weapons' application, protection from the environmental combination of salt-spray and stack-gas emission is not necessary. It is,

however, for naval aircraft applications.

Early in 1969, an investigation was conducted at our naval laboratory on the corrosion protection of solid film lubricants on steel

in an environment with the combined effects of moisture, sea water

and sulfurous acid (a product produced during combustion of fuel oil)

for future upgrading of solid film lubricant specifications. A

corrosion test was devised for ultra-thin preservative compounds (5,6)_

This was found, with a few changes, to adapt easily to solid film

lubricant corrosion testing.

In 1971, the authors' Interest and investigation Into parameters

affecting corrosion between aircraft alloys and solid film lubricants

began as a result of findings (8) in a survey of these materials at

the Naval Rework Facilities. A fastener corrosion problem was found

at one facility (Figure I). The

only solution to the problem at

that time resulted in high

expenditures of money and manhours. It seemed advisable not

only to upgrade the corrosion

resistance of these lubricants

but also to determine the causes

of corrosion when they were used.

Thus the present program was initiated with the following

three objectives:

I. Static corrosion studies

in a S02/salt spray environment

of the corrosion resistance of

solid film lubricants in con- junction with various inhibitors,

metal pretreatment, etc. , on aircraft alloys.

2. Dynamic testing under 207,, 407o, 607. and 807. relative humidity

with electron diffraction analysis of the debris.

3. Electro-chemical investigation of the galvanic effects of

dissimilar metals in combination with solid film lubricants.

These studies are described in the following paragraphs.

Figure I. Corrosion Noted on A-3 Stress Wing Spar

98
2. STATIC CORROSION STUDIES

Since many Navy planes on aircraft carriers are often stored on

the flight deck, they are constantly exposed to an environment of

stack gases and salt spray. The analysis of stack soot deposits

taken from some carriers are shown in Table I (^). These results

Indicate the presence of acidic sulfur compounds. Therefore, a sul furous-acld salt spray

environment was selected

for this phase of the study.

The apparatus used is shown

in Figure 2, The lubricant

is applied to one side of

the metal disk and Inserted

in the turntable above a S02-salt spray mist,

rotating for a cycle con- sisting of two hour spraying

and two hour drying at

ambient temperatures of

77OF. The criteria for

failure was the appearance

of any evidence of corrosion.

Using this test method the

following systems were

investigated:

1. Conventional formulations of solid film

lubricants on aircraft

alloys and the effect of

burnished surfaces.

TABLE I. STACK SOOT ANALYSIS

Carrier SO -4

1 2.7 217.

2 2.8 33

3 2.4 11

4 4.0 11

1 *anit

Fig. 2. SO^/Salt Spray

Apparatus

2. Lubricating

solids, individual and

combined, in a binder on

steel.

3. Formulations with different types of inhibitors on steel and

aluminum.

4, Different surface pretreatments of steel and aluminum prior

to lubricant application.

Initially the steel specimens were 1020 steel but when this

became unobtainable, 1010 was substituted since the metallurgy of the

two give similar corrosion characteristics. This was verified by

duplicate tests with various lubricants on both 1020 and 1010 and

obtaining correlating results. In the initial testing of conventional

99
lubricants (results shown in reference (7)), it was found that none

containing graphite passed this test. One film having no graphite in

it, however, also exhibited rapid corrosion characteristics. Also

presented in reference (7) are data indicating that M0S2 in a phenolic

binder resulted in corrosion sooner than was observed with an equal

amount of graphite in a phenolic binder. These observations demonstrate

that to provide a corrosion preventive solid film lubricant, synergistic

effects of the solid contents of the film must be considered and that

removing graphite does not result in a corrosion resistant film. For

example, a film with M0S2 and Sb203 in a phenolic binder will cause

rapid corrosion on 1020 steel in the S02/salt spray environment when

no corrosion inhibitor is present.

In reference (7) the results of adding various types of inhibitors

showed, of the Inhibitors studied, the acid acceptor and oxidizing

passivator to be the most effective corrosion inhibitor for this type

of lubricant on steel. When the solid film lubricant was applied to

aluminum, only the organic hydrophobic inhibitor had any effect on the

corrosion resistance.

Since the surfaces of these films become burnished when in actual

use, it was considered necessary to study this effect on the corrosion

protection properties of these materials. It is obvious from Table II

that when burnished, these lubricants lose their corrosion protection

by an enormous amount. However, it has been determined that even the

burnished commercial lubricant protected the steel surface longer than

an uncoated surface which shows severe corrosion in about ten minutes.

TABLE II. EFFECTS OF BURNISHING ON CORROSION

TEST METHOD - SO 2 /SALT SPRAY CORROSION TEST

Lubricant Mg Ti(4V)

ALLOYS

Ti(6V) Al 1010

* MoSg, Sb^Og. phenolic-vinyl resin 3 7 >12 9 4

MoS^, Sb^O^, phenolic-vinyl resin (b) 1 1 0 ** M0S2, inh. epoxy- phenolic resin 3 6 >12 3 6

M0S2, inh. epoxy- phenolic resin (b) 0 1 3 2 * M0S2, inh, epoxy-phenollc resin 0 6 0 1 5 M0S2, inh. epoxy-phenolic resin (b) 0 2 1 2 MoS , Sb203, Inh. phenolic 0 12 >12 3 4

MoSj, Sb^O^, inh. phenolic (b) 0 3 0

M0S2, thermo-plastlc binder >12 6

MoS 2, thermo-plastlc binder (b) 4

* heat-cured

** air-dried

(b) burnished

100
Table III demonstrates the results obtained on steel and aluminum

with the various types of pretreatments. The phosphating of the steel

and anodizing the aluminum definitely affords these metals better cor- rosion protection. However, the iron-manganese phosphate did not

enhance the corrosion protection of the solid film lubricant as much

as would have been expected.

TABLE III. CORROSION RESULTS OF PRETREATED lOlO STEEL AND ALUMINUM

(Cycles to Failure)

Lubricant

None

* M0S2, Sb203, inh.

modified resin

**MoS2, graphite,

phenolic

* M0S2 Sb203,

phenolic-vinyl

resin

Plain

1010

0

8

Sand

Blast

(1010)

100 OVB

0

3

Vapor

Blast

(1010)

0

3

Phosphate

(1010)

FeMn Zn

3

9 7

Plain

Al

Anodized

Al

11

* Heat -cured ** Air-dried

3. EFFECT OF HUMIDITY ON LIFE AND CORROSION

The corrosion behavior of solid film lubricants under stress and

motion were investigated via a humidity-controlled wear configuration

(LFW-l) , using a stationary block (4130 steel) on a rotating steel ring

(lubricant coated 4620 steel) . A miniature hygrometer indicator gave

the temperature and relative humidity readings. To effect various

humidities glycerin/water mixtures, which should not affect the debris

chemically, were employed. The speed of the rotating steel ring was

72 1 2 rpm under a load of 630 pounds. The tests were run under 20,

40, 60 and 80% t relative humidity. The lubricant was applied to a phosphated steel ring by normal spraying procedure.

The results indicated that humidity has little effect on the wear

life of an improved corrosion-resistant film. This confirms previous

studies where it has been noted that increased humidity had little

effect of the wear life of solid film lubricants.

i 101
The wear debris generated from these Investigations were examined

by electron diffraction analysis in order to determine the corrosion

mechanism of solid film lubricants in conjunction with humidity, stress

and motion. Identification was made by "best fit" between the ASTM

powder diffraction film and electron diffraction data. The composition

of the wear particles appeared varied although the same set of test

conditions on one lubricant was employed. Other patterns were observed

but determination of the other compounds present was not possible by

the above method. Some of the electron diffraction analysis results of

the debris are listed in Table IV.

TABLE IV. RESULTS OF ELECTRON

DIFFRACTION ANALYSIS OF DEBRIS

W.iOj.F.jO,

Tr.20,.r«,O,.MDS2.F.)

.?.jO),r.,(ij.c.Nosi.c

SCHEMATIC OF ELECTRO CHEMICAL

TEST APPARATUS

VT«« 0) (D

6

mOWK tISISItNCI

- IV. sit s«ii solution

These results are based on the

average of three diffraction

patterns per sample. The most

significant fact drawn from this

table is that the wear debris of

the most corrosion protective

lubricant was Fe304 which is less

abrasive than Fe203.

4, ELECTRO- CHEMICAL INVESTIGATION

There rages quite a controversy about the effects of solid

film lubricants when used with

dissimilar metals, especially if

graphite is present. For this

reason, the electro-chemical

nature of the solid film lubricated steel surface and aluminum

couple was Investigated. A

simple experimental apparatus

(Figure III) consisting of a cell with 37. synthetic sea salt

solution, a 5" strip recorder, vacuum tube voltmeter and a resistance box were used to

measure the potential and current

density of our couples.

The values given In the

tables are usually those obtained

during a four-hour test period in

which steady state occurred. However, a few specimens were run overnight and one for a week, but

the potentials were not appreciably different. The area of the

aluminum specimens were set at

twice the steel area since aircraft applications usually consist

of a larger aluminum area than steel.

102
Baseline data was obtained of various combinations of alumlntjm and

steel alloys commonly used on aircraft. Table V tabulates the results

(average of three tests) vhlch correlate with the order of metals and

alloys In the galvanic series In sea water. The results indicate the

corrosion potential between mild steel and aluminum to be less than a stainless steel /aluminum couple. Next, the lubricants with different

corrosion protection properties were studied. They are the same lubricants used to determine effect of humidity In the second task area. These results are listed In Table VI. The current gives an Indication

of the rate of corrosion and so Lubricant A, which demonstrated the

best corrosion protection In the S02/salt spray test, also exhibited an extremely low corrosion rate. The lower current can be caused by an Inhibitor acting at the pores or by a more homogeneous porefree film.

The latter Is most likely since Inhibition of the pores should give the

same Increase In potential difference for both steels. Lubricants B and

C show very little difference In corrosion rate although B Is slightly

better than C In S02/salt spray corrosion test.

TABLE V. ELECTRO-CHEmCAL RESULTS OF DISSIMILAR METALS

(ma/cm^)

(mV) (mA) Current Density

Metal to Metal Potential Current Cathode

2024-T3 vs 304 -842 O.ll 0.004

7075 -T6 vs 304 -940 O.IO 0.003

2024-T3 vs 4130 -397 0.04 0.001

7075-T6 vs 4130 -509 0.09 0.003

TABLE VI.

(ma/cm ) (mV) (mA) Current Den; Metal to Metal + Lubricant Potential Current Cathode

2024-TS vs 304 + C -987 0.06 0.002

7075-T6 vs 304 + C -1105 0.08 0.003

2024-T3 vs 304 + B -1000 0.03 O.OOl

7075-T6 vs 304 + B -lllO 0.04 O.OOl

2024-T3 vs 304 + A -898 0.0004 0.00002

7075-T6 vs 304 + A -1017 0.0005 0.00002

2024-T3 vs 4130 + C -767 0.06 0.002

7075-T6 vs 4130 + C -733 0.06 0.002

2024-T3 vs 4130 + B -545 0.05 0.002

7075-T6 vs 4130 + B -614 0.06 0.002

2024-T3 vs 4130 + A -568 0.0004 O.OOOOl

7075-T6 vs 4130 + A -700 0.0002 0.00001

Lubricants: A - MoS2f Sb203, Inhibitor and modified resin

B - MoS2> graphite, modified resin

C - M0S2, graphite, resin

105
5. CONCLUSION

It has been found that burnishing the surface of a solid film lubricant causes rapid loss of corrosion protection. The reason for this

could be that the M0S2 particles are pressed closer together and become

more of the conductor. If this is true, it would mean that a lubricant

does not have to contain graphite to cause galvanic corrosion when involved in a dissimilar metal couple. Pretreatments were found to have

less effect than expected on corrosion prevention. Humidity was found

to have no effect on wear life or corrosion rate. A new method has

shown promise in studying solid film lubricants electrochemically.

To find more answers, future investigations will be conducted to

find the effect of new binders, inhibitors, porosity, conductivity,

lubricating solids on corrosion protection by this type of material.

6. ACKNOWLEDGEMENTS

Many people have been instrumental in obtaining results and helping

with test procedure. The authors wish to acknowledge the help of

Dr. David Berman and Dr. M. Campbell for their technical contribution on

the electro-chemical test; Mr. John Danovich for the electron diffraction analysis; and Messrs. A. D'Angiolini, John Manning, and Robert

McCartney for laboratory data.

REFERENCES

(1) Cerini, J. P., "Results of a Laboratory Study to Determine the Corrosive Properties of Dry Film Lubricants," Report No. NAMC-AM.-

AE42173, Part 1 (1959).

(2) Bicht, A. E,, "Graphite Corrosion, Its Cause and Cure," Report No.

69-WRNE-125 (1969).

(3) Sonnino, Carlo B., and Kollmansberger , R. B, , "The Effect of MoSg

Lubricating Pigments on the Corrosion Resistance of Al and Mg

Alloys," Source Unknown.

(4) Murphy, G. P. , and Meade, F. S. , "Factors Influencing Corrosion Protection Provided by Solid Film Lubricant Coatings," U.S. Army Weapons

Command, RIA Research and Engineering Division (1965).

(5) Knight, W. E., "The Development of Ultra-Thin Film Preservative

Compounds," Report No. NAEC-AML-2090 (1964).

(6) Lion, E. J. , "Development of an Environmental Wet Test Chamber for Simulating Carrier Deck and Other Corrosive Environments," Report No. NAECAML-2436 (1966).

(7) Gabel, M. K. , "Corrosion and Solid Film Lubricants," ASLE Preprint

No. 73AM- 3C-4 and Lubrication Engineering , Vol. 29, No. 10, pp 442.

(8) Peterson, M. B. and Finkin, E. F. , "Application of New and Improved

Solid Lubricant Material and Processes to Naval Aircraft," Report

No. MTI 71TR48, July 14, 1971 (pp 78).

(9) Brown, S., Shaffer, I., and Goldberg, S. "Optimum Corrosion Protective

Finishing Systems for Aluminum Alloys Used in Naval Aircraft," Technical Paper, Houston Trl-Service Conference on Corrosion, 1972.

10k
DISCUSSION

A. J. Babecki, NASA, Goddard Space Flight Center : There has been a lot

of test work done to show that M0S2 is a good lubricant in vacuum and

graphite is a good lubricant in non-vacuum conditions. Why is M0S2

used in non-vacuum conditions when graphite should really be used?

M. K. Gabel : The Air Force and the Army are banning the use of any

lubricant containing graphite because there are problems with what they

call galvanic corrosion. I feel they are going a little too far in

banning all of these lubricants. However, I have found that the new

films that do not contain graphite seem to have as good an endurance

life as those that do contain graphite. They also provide adequate

corrosion protection.

A. J. Babecki : Maybe the lubrication and wear life is being provided

by something else. What is the binder in these films?

j M. K. Gabel : Most of the commercial lubricants that I studied have

thermosetting binders. The formulations that I made in the laboratory

contained phenolic resin type binders which are also thermosetting.

G. E. DeLong, Naval Air Systems Command Representative, Pacific: Most

of the materials for inhibiting corrosion assume that a material is

going to go into solution in the presence of moisture, which also assumes by definition that the inhibitor is a consumatDle part of that

particular formulation. In the case of a burnished material, it would

be rather difficult for this to occur. Is it a valid assumption, with

dry film lubricants, that the inhibitors only work by going into

solution, and therefore the dry film lubricant is time limited in its

application in a damp environment?

M. K. Gabel : I don't think that all of the inhibitor would go into

solution. It is distributed evenly throughout the film and as long as the film is there, some inhibitor is probably still present.

105
RUST-INHIBITED NONREACTIVE PERFLUORINATED POLYMER GREASES

Joseph Messina

U.S. Army, Frankford Arsenal

Pitman-Dunn Research Laboratories

Philadelphia, Pennsylvania 19137

ABSTRACT

Perf luoroalkylpolyether fluids thickened with polytetrafluoroethylene

were studied in connection with the development of rust-inhibited chemically inert greases for liquid-fueled rocket engines. It was found

that 1.0 to 3.0 wt. percent of a physically and chemically modified

organophilic dimethyloctadecylbenzyl ammonium bentonite + sodium nitrite imparts very effective rust-preventive properties to perfluoro

polymer grease mixtures. Data are given which show that the rust-inhibited greases are nonreactive on contact with conventional fuels and

oxidizers, exhibit lubricating properties comparable to soap-thickened

greases with a significant improvement in extreme pressure properties,

and are nonreactive at high impact energies in the presence of LOX.

The results of this work are applicable to all liquid-fueled rocket

engines for missiles and space vehicles.

KEYWORDS: Chemisorption; Corrosion preventives; Inhibitor additives;

LOX compatible greases; Nonreactive lubricants; Rust inhibited

perfluoro greases; Surface chemistry lubrication.

INTRODUCTION

Perf luorinated alkylpolyether fluids prepared either by photooxidation

of perf luoroolef ins at -30C (1,2) or by the anionic polymerization at

temperatures below -27. 5C of hexaf luoropropylene epoxide in the presence of solvents using cesium fluoride (3) have recently been made

commercially available. These fluids have considerably lower vapor

pressures than the perfluorotrialkylamines which had previously been

utilized as components of liquid-fueled rocket engine lubricants (4,5).

A recent study of the physical and chemical properties of the alkylpolyether fluids thickened with polytetraf luoroethylene have indicated

that the resultant grease mixtures are suitable as inert lubricants for

rocket engines powered by liquid propellants such as ethyl alcohol

aniline, hydrocarbon fuels (JP-4, JP-5, RP-1) , diethylenetriamine

(DETA) , unsymmetrical dimethylhydrazine (UDMH) , hydrazine, hydrogen

peroxide, inhibited red fuming nitric acid (IRFNA) , nitrogen tetroxide

and liquid oxygen (LOX) (6) . It was found that the polytetraf luoroethylene-perfluoroalkylpolyether grease mixtures exhibited effective

106
lubricating properties, thermal, hydrolytic and oxidative stabilities,

wide temperature range, extreme pressure properties, nonreactivity with

fuels and oxidizers and shear stability. These lubricants are now ex- tensively used on crew compartment (APOLLO) and launch components

(Saturn boosters) of manned and unmanned space vehicles (7). While

these uses indicate significant progress in the development of chemically inert lubricants for liquid-fueled rocket engines, it has been

observed that the polytetrafluoroethylene-perf luoroalkylpolyether

grease mixtures do not provide effective protection of ferrous alloys

against rust. Using ASTM D1743-64, it was found that lubricated tapered roller bearings, SAE 4620 or SAE 8720 with 1010 mild steel roller

retainer, rusted badly after fourteen days exposure at 100 percent

relative humidity. A typical illustration is given in Fig. 1. -

Further, recent tests (8) conducted on a 440C stainless steel R-4

bearing rotating at 3,000 rpm at 5-psi pure oxygen at 70 percent relative humidity lubricated with perf luoroalkylpolyether-polytetraf luoroathylene grease was found to be inoperable after approximately 1,000

hours due to extreme pitting and rusting. (This bearing is a component

of the fan proposed for use for a minimum of 5,000 hours on the astronauts' orbital workshop.)

To overcome this deficiency, work was initiated at the author's laboratory toward the modification of the perfluoro polymer greases with

commercially and experimentally available rust inhibitors. Initial

exploratory tests (ASTM D 1743-64) indicated that adequate rust prevention could not be achieved through the addition of up to 3.0 wt. percent of rust inhibitors such as sorbitan mono-oleate, sorbitan trioleate, barium petroleum sulfonate, fatty amido phosphate and barium

dinonylnaphthalene sulfonate. Also, contact compatibility tests with

fuels and oxidizers showed evidence of reactivity, and, as a result,

such inhibitors could not be considered satisfactory for liquid-fueled

systems. In the work described here, a commercially available modified

bentonite consisting of dimethyloctadecylbenzyl ammonium bentonite with

sodium nitrite (9) was found to have rust inhibiting properties with

perf luoroalkylpolyether-polytetraf luoroethylene grease mixtures. This

paper describes the preparation and properties of a series of rust-inhibited nonreactive perfluoro polymer greases for liquid-fueled rocket

motors

.

EXPERIMENTAL

Grease Preparation

Materials. The thickener was a low molecular weight tetraf luoroethylene poljnuer having the following properties: softening point 321.1 C;

mol. wt. 20,000-30,000; particle size 100 percent less than 30 microns.

The product was supplied as a 7.5 percent suspension in trichlorotrif luoroethane

.

107
108
The fluids used were f luorinated alkylpolyethers (Table 1) . The preparations and properties of these fluids were described previously

(1,2,3,7,10).

The additive was a chemically and physically modified organophilic

bentonite (dimethyloctadecylbenzyl ammonium bentonite + sodium nitrite)

(9).

Composition. The compositions of the greases are given in Table 2.

All greases were prepared to NLGI Number 2 Grade (265-295 worked pene-

(9).

Compositi

All greas

tration)

.

Table 1. PERFLUOROALKYLPOLYETHER FLUIDS

Viscosity,

cSt. at Pour Density

Fluid 37.78 C Point, C at 23.8 C

PD-1023 96. 3^ -26.1 1.91^

PD-1024 153.0^ -29.0 1.91^

PD-1025 424.0^ -17.7 1.92

PD-1026 18.0^ -56.2 1.86

PD-1027 85.0^ -42.2 1.89

PD-1028 270. 0^ -34.5 1.90

PD-1029 495. 0^ -28.8 1.91

aRef. (1,2,10). bRef. (3,7). ""at 15. 5C.

Table 2. GREASE COMPOSITION (WT., PERCENT)

Perfluoroalkyl- Rust

Grease polyether Thickener^ Inhibitor'

PD-1030 85.1(96.3 cSt)^ 13.9 1.0

PD-1031 85.3(153.0 cSt)^ 13.7 1.0

PD-1032 84.7(424.0 cSt)^ 14.3 1.0

PD-1033 85.0(18.0 cSt)^ 14.0 1.0

PD-1034 84.7(85.0 cSt)b 12.3 3.0

PD-1035 82.3(270.0 cSt)^ 15.5 2.2

PD-1036 87.4(495.0 cSt)^ 11.6 1.0

^Ref. (1,2,10); '^Ref. (3,7); ^^polytetrafluoroethylene, molwt,

20,000-30,000, (7); ^^dimethyloctadecylbenzyl ammonium bentonite +

sodium nitrite (9).

109
Dispersion Procedure. Each grease mixture was prepared as follows:

The dispersion of PTFE in trichlorotrifluoroethane was heated on a steam bath until ^^^50 percent of the solvent evaporated. Approximately

75 percent of the required quantity of base oil was then added, the

mixture was stirred and heating continued until all of the trichlorotrifluoroethane had evaporated. (The absence of trichlorotrif luoroethane was determined by gas chromatography using the following technique. A sample of the grease mixture was extracted using CP.

benzene. Gas chromatography was used to show (absence of a retention

peak after 3.3 minutes) that all the trichlorotrifluoroethane had

evaporated (20-ft carbowax 20M column at 50 C, helium flow, 10 ml per

minute)

.

The remainder of the base oil was then added while stirring, and stirring continued until a homogenous grease-like product was obtained.

The mixture was cooled to room temperature. The rust inhibitor was

then added, thoroughly mixed into the grease, the mixing being completed by passing the grease twice through a colloid mill with a stator-torotor clearance set at 0.001 inch. The homogenized mixture was permitted to remain at room temperature for 24 hours prior to use. Greases were prepared in 200-g batches (Table 2).

Rust Preparation

Initial studies were directed toward establishing the minimum quantity

of rust inhibitor required for each grease to pass the rust test using

the tapered roller bearing described in ASTM D1743-64. (This test was used since it is currently specified in numerous specifications, e.g.,

MIL-G-23827, MIL-G-25013, MIL-G-21164 and MIL-G-81322) . The minimum

quantity of rust inhibitor required for rust prevention is given in

Table 2. The same test was run on grease samples prepared without the

inhibitor and also on uninhibited commercial greases (7,10). Figures

1 and 2 are typical of the test results; all bearings lubricated with

greases containing the rust inhibitor showed no corrosion; all samples

without the inhibitor rusted badly. This was found to be so whether

the viscosity of the fluid used to prepare the test grease was as low

as 18.0 est or as high as 495.0 cSt at 37.78 C (Table 2).

Reactivity with Fuel and Oxidizers and Metals at High Shear

Contact Tests. Contact compatibility tests were run at 25 ± 1.0 C.

One gram each of the material being tested (or 1 ml if fluid) was placed in a 5 ml graduated glass-stoppered cylinder and 1 ml of fuel or oxidizer added. Visual observations preceded by shaking were made

after five minutes and 1, 24, 48 and 72 hours. The fuels and oxidizers

used were EtOH, JP-4, UDMH, DETA, CgH5NH2, N2H4 , H2O2 , IRFNA and N2O4.

The tests using N2O4 were run in closed pressure glass jars 1x5

inches. The results (Tables 3,4) show that the low molecular weight

polytetrafluoroethylene thickener was nonreactive and insoluble with

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112
all of the test fuels and oxidizers. The dimethyloctadecylbenzyl

ammonium bentonite + sodium nitrite rust inhibitor swelled but showed

no reactivity. The data in Tables 3 and 4 indicate no reactivity of

the test materials with the fuels and oxidizers. Although some of the

fuels and oxidizers were somewhat soluble in the perfluoroalkylpolyether fluids, the degree of solubility was less on the greases made up

with the same base oils. Only a slight solubility (<5 percent by volume) of the grease was noted. Evidently the structure of the grease

dispersion is such that the fluid is not readily accessible for solution.

Impact Tests. Liquid oxygen impact compatibility tests were run at an impact level of 72.3 foot-pounds (11). The thickener, rust inhibitor,

fluids and greases were considered nonreactive with LOX if they withstood 20 separate impact trials without reaction (flashes, explosions

or other indications of sensitivity) . Since none of the materials

tested were found to be reactive with LOX, within the limits mentioned

in Table 5, it appears that the thickener, the rust inhibitor, the

fluorinated polymer fluids and greases made from these products may be

useful for rocket motor systems which use LOX as the oxidizer. It is

significant to note that the modified organophilic bentonite was not

sensitive when tested alone CPD-1016) or as part of the grease mixtures

(PD-1031, PD-1034 and PD-1035) when subjected to high-impact energy

levels. PD numbers in text and tables are in-house code designations

for experimental materials used by the author's laboratories.

Metals at High Shear. It has been reported that explosive reactivity

occurs when aluminum surfaces are coated with polytetraf luoroethylene

(12) or poljmiers of chlorotrif luoroethylene (13-16) when subjected to

mutual shear at high loads. To determine whether similar reactivity is

associated with the products described here, tests were run on the

fluids and greases using proposed ASTM D-2 Method (17). Seven tenths

± 0.1 of a ml of materials was placed in a cylindrical hole (1/2 in dia

X 1/2 in deep) in a block of 2024-T4 aluminum. A dowel of 2024-T4

aluminum (1/4 in diameter, rounded end (0.250 ± 0.001 inch radius tip)

X 3.0 inches long) was rotated into the block at 1,760 rev per min

under a load of 1,000 psi for one minute. The dowel mating surface in

the specimen block was made using a fluted carbide-tipped ball-end mill

0.500 ± 0.001 in in diameter with 0.250 ± 0.001 in radius tip finished

to 8 to 16 microinch rms. The load is weighted to provide a 1,000 psi

at the dowel and specimen block mating surfaces. The data is given in

Table 6. All of the lubricants tested were reactive to some degree; none appeared to be as sensitive as the polychlorotrifluoroethylene

fluid (13-16) . The addition of the rust inhibitor did not increase the

reactivity of the lubricant with aluminum at the 1,000 psi load. However, since the data is important in the potential use of the lubricants for metallic connectors and thread fasteners in rocket motor

systems, it would appear that as a precautionary measure this apparent

deficiency of the lubricants should be checked on any batch contemplated for use.

113
Table 5. IMPACT COMPATIBILITY

no reaction in 20 trials Ref. (11).

Table 6. REACTIVITY OF LUBRICAl^TS WITH ALUMINTOl AT HIGH SHEAR^

Explosive

Lubricant Reactions/No.

of Trials

Perfluoroalkylpolyether , (96.3 cSt)^ 4/6

Perf luoroalkylpolyether , (96.3 cSt) + thickener.... 1/6

Perfluoroalkylpolyether, (96.3 cSt) + thickener + rust inhibitor 1/6

Perfluoroalkylpolyether, (85.0 cSt)'^ 2/6

Perfluoroalkylpolyether, (85.0 cSt) + thickener.... 1/6

Perfluoroalkylpolyether, (85.0 cSt) + thickener + rust inhibitor 1/6

Perfluoroalkylpolyether, (270.0 cStf 2/6

Perfluoroalkylpolyether, (270.0 cSt) + thickener... 1/6

Perfluoroalkylpolyether, (270.0 cSt) + thickener + rust inhibitor 1/6

Polychlorotrif luoroethylene oil, 6.5 cSt at 37.78 C. 6/6

^Load, 1000 psi at 1760 rpm, dowel and block 2424-T4 Al, Ref. (17);

Ref. (1,2,10); ^Ref. (3,7).

111+
Lubricant Properties

Antiwear characteristics were determined on the four-ball wear tester,

using ASTM D 2266-64T. Wear scar diameters were measured on the three

stationary balls after one hour at 1200 rpm at 75 C with 10-kg and 40-

kg loads using a travelling microscope at 40 X. The values are the

average of the readings taken parallel and normal to the scuff marks.

The data (Table 7) show that the addition of the rust inhibitor to

each thickened oil lowers the scar diameter to slight degree.

Extreme Pressure (EP) properties of the test greases were determined

using the four-ball method described in ASTM D 2596-69. It was found

that the rust-inhibited perfluoro poljmier greases did not weld under an applied load of 800 kg.

Comparisons were made with similar greases containing no rust inhibitor.

The data (Table 7) show a marked improvement in EP properties due to the

addition of the rust inhibitor. Further, the EP characteristics of the

inhibited or uninhibited polytetrafluoroethylene-perf luoro-alkylpolyether greases are far superior to the currently used EP MIL SPEC

greases, e.g., MIL-G-23827, MIL-G-21164, or MIL-G-81322.

It is also of interest to note that the scar diameters on the balls in

the EP tests using ASTM D 2596-69 which did not weld at 800-kg loads

were all below 4.0 mm. These data (no weld at 800-kg load or scar diameters below 4.0 mm) clearly suggest that the modified organophilic

bentonite plays a major role in enhancing the EP properties of the test

greases

.

Other Lubricant Properties. The data (Table 8) show that the inhibited

perfluoro polymer greases exhibit high dropping points, low water washout characteristics, low vapor pressures, and good mechanical stabilities. It is of interest to note that no adverse changes took place in

these properties due to the addition of the rust inhibitor.

Limited tests were also conducted to determine the effect of the rust

inhibitor on low temperature torque. It was found (Table 9) that the

starting torques of the inhibited greases were higher than the uninhibited greases. It should be noted, however, that the torque values of

the inhibited perfluoro polymer greases compared favorably with the

products meeting MIL-G-27617 "Grease, Aircraft, Fuel and Oil Resistant."

The latter specifies a 7,000 g-cm starting torque at -22.0 C.

SUMMARY OF RESULTS

The primary purpose of the work was to develop rust-inhibited nonreactive perfluoro polymer greases for liquid-fueled rocket motors. Based

on the test data given, it must be concluded that the modified ammonium

bentonite imparts effective rust-prevention properties to perfluoroalkylpolyethers fluids thickened with poly tetraf luoroethylene . It is

115
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116
Table 9. TORQUES (G-CM) ON PERFLUORO GREASES^

Grease

Inhibited Uninhibited

Starting Running Starting Running

PD--1031 (1. 0)b.. 5074^^ 1770^ 3540C 1925C

PD--1034 (3. 0)b.. 4307^ . 1032^ 4071^1 2478^

PD--1035 (2. 2)b. . 8053^ 2212^ 5768^ 1667^

^ASTM D 1478-63; ^rust inhibitor, wt , %; ^test temperature, -22 C;

*^test temperature, -40 C.

further concluded that the rust inhibitor is nonreactive (alone or in

grease mixtures) on co»itact with the fuels and oxidizers (Tables 3,4)

and under impact with LOX at high energy levels (Table 5) . The lubricating properties of the inhibited polytetrafluoroethylene-perf luoroalkylpolyether greases are comparable to soap-thickened greases with a marked improvement in extreme pressure properties (Tables 7-9) . The

lubricants reported here are applicable to all liquid-fueled rocket

engines for missiles and space vehicles.

ACKNOWLEDGMENTS

The author wishes to express his appreciation to F. Key, George C.

Marshall Space Flight Center, for conducting the LOX impact compatibility tests.

REFERENCES

(1) Sianesi, D., Pasetti, A., and Corti, C, "Photo Oxidation of

Perfluoro-olef ins ; Polyperoxides and Polyethers of Hexafluoropropenes," Die Makromolekulare Chemie , 86^, 308-311 (1965).

(2) Sianesi, D., "Perf luorinated Polyethers Synthesis and Properties

of a New Class of Inert Fluids," La Chimica e L'industria, 50 , 206-214 (1968).

(3) Gumprecht, William H. , "PR-143-A New Class of High Temperature

Fluids," ASLE Transactions, 9_, 24-30 (1966).

(4) Messina, J. , "Greases Nonreactive with Missile Fuels and Oxidizers," NLGI Spokesman, 2_7, 177-182 (1963).

(5) Messina, J., and Gisser, H. , "Grease-Type Lubricants Compatible

with Missile Fuels and Oxidizers," Ind. Eng. Chem. Prod. Res. and

Dev. 2, 209-212 (1963).

(6) Messina, J. , "Perf luorinated Lubricants for Liquid-Fueled Rocket

Motor Systems," Lub. Eng. 23_, 459-463 (1967).

(7) Skehan, John T. , "The Development of Fluorinated Greases for Aerospace, Military and Industrial Applications," presented at the

NLGI 37th Annual Meeting, Kansas City, Mo., Oct. 1969.

(8) Private Communication, Demorest, K. , NASA, Huntsville, Alabama.

(9) Private Communication, House, R. F., National Lead Co., Baroid

Chemicals, Inc., Houston, Texas.

117
(10) Bray Oil Company, Los Angeles, California, Technical Bulletins,

"Grease Rocket Propellant Compatible," dated November 10, 1968

and "Grease Rocket Propellant", dated November 15, 1968.

(11) Specification 106, "Compatibility Testing, Liquid Oxygen Systems

and Materials," Marshall Space Flight Center, Huntsville, Alabama.

(12) Morris, G. , "Impact Testing of Non-metallic Materials in Liquid

Oxygen," Report ER 1016-1, Martin Co., Baltimore, Md. , March 1958.

(13) Ehrenfeld, R. L. , "Polychlorotrifluoroethylene Oils as Industrial

Lubricants," Ind. Eng. Chem. 52, 65-66A (1960).

(14) Gunderson, R.C., and Hart, A. W. , "Synthetic Lubricants,"

Reinhold Publishing Corp., New York City, 1962, p. 255.

(15) Eisman, B. J., Jr., "Reactions of Chlorof luorohydrocarbons with

Metals," ASHRAE Transactions, 69, 371-379 (1963).

(16) Hyler, W.S., Structural Materials Engineering Dept., BATTELLE

Columbus Laboratories, Columbus, Ohio, Letter Report dated May 1,

1968.

(17) Proposed Method of Test for "Explosive Reactivity of Lubricants

With Aerospace Alloys under High Shear," ASTM Committee D-2 on Petroleum Products and Lubricants, Special Preprint, May 1970,

pp. 11-13.

118
I DISCUSSION

A. J. Babecki , NASA, Goddard Space Flight Center : We use the Krytox

oils and greases in a lot of spacecraft applications, such as in

instrument bearings, because it has such good lubricating properties,

not only with the conventional bearing steels, but also with the

various gear materials such as aluminum and austenitic stainless steel.

I am a little perturbed by your statement that it causes corrosion. We

have had 52100 and 440C steel samples partly submerged in Krytox 143AB

for almost two years now and we don't see any evidence of corrosion.

Under what conditions does the Krytox fluid cause corrosion and,

secondly, since it is supposed to be an inert material, is it the fluid

or is it moisture in the fluid that might cause the corrosion?

J. Messina : The fact that you haven't experienced corrosion is

puzzling to me. Unless there is an inhibitor in the system, there will

be a tendency for water to seep into the metal. Our laboratory tests

indicate that Krytox grease will not keep moisture out.

J. E. Stern, NASA, Goddard Space Flight Center : In Al Babecki 's tests,

the Krytox has probably been dehydrated. The bearing has been

submerged in a significant quantity of fluid which precludes attack by

oxygen or moisture in the air. Do you have any experience with the

corrosiveness of the Krytoxes or the Montecateni fluids in a vacuum environment?

J. Messina : No, we have done no work in that area.

N. Glassman, David Taylor Naval Ship Research and Development Center : What inhibitor are you using?

J. Messina : It's montmoril lonite clay which is encapsulated by means

of the dimethyloctadecyl benzyl ammonium compound and then to which

sodium nitrite is added.

N. Glassman : Is the sodium nitrite a solid in this particular case?

J. Messina : Yes, so is the other component.

N. Glassman : What are the operating conditions of the bearings when

this grease is used?

J. Messina : The bearings were run at 3,000 rpm under very low loads.

119
THE ROLE OF COATINGS IN THE PREVENTION

OF EROSION DAMAGE

A. P. Thiruvengadam

and

A. A. Hochrein, Jr.

Daedalean Associates Incorporated

15110 Frederick Road

Woodbine, Maryland 21797

ABSTRACT

Among the various methods available for protection

against cavitation erosion, elastomeric coatings offer

great potential. However, adhesion failures, tearing, and

substrate mismatching are some of the problems to be solved

for a successful application of protecting coatings. Basic

studies in understanding this problem includes stress-wave

interaction analysis, controlled experiments with a vibratory cavitation erosion apparatus and the generation of high

strain data with the help of a Hopkinson pressure bar apparatus. It is hoped that such a coordinated approach would

lead to the design and development of successful coating

systems

.

Keywords: Adhesion; cavitation; coatings; design charts;

elastomers; erosion; Hopkinson pressure bar apparatus; impedances; mismatching; stress wave interactions; substrates;

tearing; vibratory apparatus.

Cavitation erosion is a serious problem in high speed

flow systems including high performance ships. It is characterized by the formation of vapor bubbles at low pressure

regions. This is called inception of cavitation. Following

the inception process, the cavitation bubbles grow to their

maximum sizes in the low pressure regions and collapse violently when they are carried into high pressure regions.

If these bubbles collapse on or near the solid surface, the

collapse pressures may be strong enough to erode the material. Currently there are two theories of bubble collapse;

both have some experimental and theoretical support. One

is the spherical collapse mechanism in which the bubble is

assumed to collapse symmetrically maintaining its spherical

shape throughout the collapse time. The collapse energy

is transmitted back to the material in the form of a short

120
range shock wave causing damage. The other mechanism assumes that the bubble collapses unsymmetr ical ly producing

a high speed jet which impinges on the solid surface causing material erosion.

It has been observed both in laboratory experiments as

well as in service that these bubbles erode even the strongest material in relatively short durations depending upon

the intensity of bubble collapse. In a rotating disk

screening test, common propeller materials such as manganese

bronze, manganese -nickel bronze and nickel -aluminum bronze

would erode at a rate of about one sixteenth of an inch in

72 hours. This corresponds to about 8 inches per year rate

of erosion.

The material, at the same intensity of erosion, would

erode one eighth of an inch in only 3 hours. Epoxy glasses

possess at least two orders of magnitude less erosion resistance compared to other structural materials. Reinforcements would improve the resistance slightly. It is essential to develop suitable protection techniques for avoiding

cavitation erosion problems in high speed systems. Elastomeric coatings offer good potential as protective coatings.

An epoxy-glass disc coated with a neoprene coating exposed

to cavitation erosion performed well for more than 72 hours;

whereas it would have been eroded completely in less than

3 hours.

However, there are a few problems associated with the

application of protective coatings to the substrate. For

example, the response of a viscoelastic coat ing -adhes ive

substrate system to the dynamic bubble collapse pressures

is a fairly complex phenomenon. Several coat ing - substrate

parameters enter into the analysis. Depending upon the

specific bubble collapse intensities and coating systems,

several failure modes are possible.

If the relaxation times are smaller than the bubble

collapse durations and if the bubbles collapse violently

enough, the coatings erode in a brittle manner. On the

other hand, the coatings can fail by tearing. Even before

tearing and/or erosion is initiated, the adhesive or the

substrate might fail. Adhesion failure, crushing, spalling

and scabbing have been observed in cavitation erosion as

well as in rain erosion. In order to understand the conditions under which these failures are initiated, an elementary wave propagation model has been developed. With

the help of this model, the stress-wave interactions may

be calculated. Depending upon the complexity of the coat121
ing system, one may use a computer for calculating the

stresses within the mult i - layered system.

From such calculations, design charts have been developed for selecting the suitable coatings and adhesives for

specific substrates and intensities of cavitation. In

spite of the elementary nature of this theoretical model

and of its assumptions and limitations, it offers some

practical results that can be used by materials' engineers

developing coating systems for erosive environments. A

typical application of the analysis was demonstrated through

the use of the vibratory cavitation erosion apparatus. A

test specimen is vibrated at 20 KHz producing a stream of

cavitation bubbles in a beaker of water. The coated substrates are exposed to this stream of bubbles. The intensity of erosion may be controlled through the amplitude of

vibration and the test specimen distance.

One of the coating systems tested failed in brittle

mode within 30 minutes. With the guidance of the analysis,

another coating system was selected specifically for this

application which lasted more than 32 hours. Judicious

combination of theory and experiments led to a significant

improvement in performance from 30 minutes to 32 hours.

The same coating that performed well for 32 hours would

fail in less than 3 hours if it were used on the wrong substrate as a result of wave reflections caused by impedance

mismatching. Further research is needed to establish such

practical approaches.

122
DISCUSSION

H. Ravner, Naval Research Laboratory : Did you use only water as a substrate?

A. P. Thiruvenqadam : Yes, we used sea water and laboratory tap water.

P. Weinberg, Naval Air Systems Cormand : What is the effect of moisture

on the polymer protective coatings on the composites?

A. P. Thiruvengadam : Just exposure to salt water may reduce the

adhesion strength by 5 to 10 percent. Exposure to cavitation will

further reduce the adhesion strength. These results are based on laboratory measurements.

123
SOLID FILM LUBRICANT TO PREVENT FRETTING

DAMAGE IN TITANIUM ENGINE COMPONENTS

B. D. McConnell

Air Force Materials Laboratory

Wright -Patter son Air Force Base, Ohio ^5^55

ABSTRACT

An air dryina solid film lubricant originally developed for applications in the field has been found to be an excellent coating for prevention of fretting damage in titanium-titanium mated parts . This

film, designated AFSL-41 , is composed of molybdenum disulfide (M0S2)

and antimony trioxide (Sb203) carried in a methyl phenyl silicone

binder. The film air dries without heat through the use of a curing

agent, an amino-alkyl alkoxy-silane . A brief description of the formulation is presented along with laboratory evaluation data comparing

its anti-fretting performance with other candidate materials. The performance on titanium led to its consideration as a candidate anti fretting coating for titanium components. The various screening tests used

to determine anti fretting properties are described along with results

which led to AFSL-41 being selected for full scale engine tests. Current use of AFSL-41 by two gas turbine engine manufacturers on a production basis is discussed and the types of applications (over 100 in one engine) are described.

Key words: Curing agent; fretting; silicone resin; solid lubricants;

titanium; wear.

Introduction

The development of an air drying solid film lubricant for application in the field has been described previously (Reference 1, 2, & 3). Early in the development it was found that this formulation exhibited

excellent performance on titanium substrates. This led to its evaluation as an anti-fretting coating for titanium. The purpose of this

paper is to briefly describe the formulation, its evaluation and use

as an anti-fretting coating for gas turbine engine components.

Formulation

The composition of the film was based on a silicone resin system

developed for air drying paints by Stout (Reference 4) . This system

possessed good thermal stability (over 500°F) and the ability to cure without heat. The solid lubricant materials selected for the film was

an optimum mixture of one part M0S2 and one part Sb203 by weight. This

mixture was used in a lubricant to binder volume ratio of 70:30. The

12k
resin binder is a mixture of 90% by weight polymethyl phenyl-siloxane

and 10% gama amino-propyltriethoxy silane. The amino-silane serves as the cxiring agent to cross link the siloxane resin. The film can be applied by brushing, dipping, or spraying and is effective in film thick- nesses of 0.0002 to 0.0005 inches. The film air dries to the touch in

5-10 minutes and cures ready for use in six hours.

Evaluation

The laboratory screening tests used in developing and optimizing

the formulation have been discussed in detail previously and will not

be presented here (Reference 1) . Initial fretting testing of AFSL-41

was conducted by an engine manufacturer in a bench fretting machine

described in detail elsewhere (Reference 5) . The contact is depicted

graphically in Figure 1 and consists of a spherical tipped specimen (A)

0.5 inch in diameter with a ground spherical tip of 4.0 inch radius.

This serves as one half the specimen combination and it rubs in a reciprocating motion against the

flat, horizontal surface of a stationary specimen or platen (B) . The

platen is gripped in a holder (C)

which is, in turn, clamped onto a heavy work table , which may be adjusted vertically for position. The

spherical specimen is held rigid at one end of the holder rod (E) which

is connected to a vibration exciter

capable of 10 to 2000 cps vibration

frequencies. The vibrator exciter

of this arm is equipped with a flat

spring, flexible joint which permits

slight vertical movement of the arm

for leveling purposes. A guided

push rod assembly (H) aligned directly above the test sphere and platen

combination allows the normal load

to be varied over a wide range . The

horizontal reciprocating motion of

the spherical specimen on the platen

surface produces a wear scar, the cross-sectional area of which, taken

at the plane of maximum wear penetration relative to the unfretted surface is a reliable measure of material removal and thus of relative

fretting wear. Figure 2 compares the fretting wear results of AFSL-41

using two different manufacturers silicone resins to that of unlubricated titanium and a silver film. These results were encouraging and

prompted fiirther tests more closely simulating the actual application.

These tests were carried out on a coated compressor blade dovetail

section of an actual compressor wheel. The blade was siibjected to

Figure I Fretting Test Geometry

125
40

30

<

iiJ q:<

-I<z

o

O

lu

V)

I w

wo

q:K 2 20

a: ?

< —

^ ,0

<

<

to

> o

o

o

b

roO

O

o

Fretting Parameters- Frequency 200 CPS

Amplitude 7 mils Normal Load I 5 lbs Duration I 5 min Test Temperature 7 5 °F

d2

c

Figure 2 Fretting Wear Test Results

vibrational testing under a stress of + 3700 psig in the 2nd bending

mode. The condition ot the coated dovetail section after 12,000,000

cycles is the criteria for these tests. Table I compares the performance of AFSL-41 to other coatings evaluated in this test.

Table I Compressor Blade Fretting Test

Material

AFSL-41 on Blade only

Blade Silver Plated Over Nickel

Flash

Blade Silver Plated Only

Flame Sprayed Cu-Ni-In

Graphite Varnish on Blade

Graphite Varnish on Blade and Slot

Cycles Fretting Resistance

12 X 10^ Excellent

12 X 10^ Good

12 X 10^ Fair

12 X 10^ Excellent

4 X 10^ Poor

4 X 10^ Poor

The final evaluation was conducted in full-scale engine tests with side

by side comparison with silver coated compressor blades. A number of

first and second stage compressor blades were coated with AFSL-41 and

assembled into the wheel. The first engine test ran for 187 hours be126
fore shutdown for other causes. Upon teardown, inspection of the

blades showed them to be in excellent condition and at least comparabl

to the silver coated blades. The only evidence of change was some ini

tial burnishing of the bonded film and considerably longer operation

could be expected. The blades were reinstalled in the engine and subjected to a second full scale engine test. This test ran for 202

hours giving almost 400 hours of total time on the coating. The

blades were removed, examined, and found to be in excellent condition.

The coating was intact and was well burnished. There was no evidence

of film damage which indicated the loading was uniform. It was con- cluded that AFSL-41 was comparable to silver in preventing fretting

damage to the dovetail surfaces of the compressor blades.

Another engine manufacturer became interested in AFSL-41 for its

anti-fretting properties on titanium alloys. This manufacturer con- ducted fretting-fatigue tests on a combined bearing/vibratory load

apparatus, shown schematically in Figure 3.

Base

Direction of Magnetic Motion of Tip

Moferin! \ Electromagnet

HT^Vt

Figure 3 Bearing/Vibratory Load Tester

The test specimen is a bar which is clamped rigidly on one end and pas

ses between two load bearing pins through which the test load is applied by means of a hydraulic ram. The free end of the test bar is

vibrated with an electromagnet at a rate of 3000. to 5000 cycles per

minute. The coating to be evaluated is applied normally to the test

bar although both bar and bearing pins may be coated if desired. The

normal test time is approximately 30 hours. Tests were conducted at

600°F and 50KSI bearing stress. Two titanium alloys, Ti-8-1-1 and

Ti-6-2-4-6 were used in the evaluations. Many candidate coatings have

been evaluated in this test and selected results are compared in

Table II. Most of the coatings were ^ble to attain 10^ cycles before

failure and in some cases achieved 10 cycles at the lower alternating

stress levels.

The conclusions drawn from these tests allowed selection of coatings for further testing and eventual full scale engine testing. A

127
Table II Fretting Fatigue Tests - Ti-6-2-4-6 pins, 6Q0°F, 50KSI

Bearing Stress

Alternating Cycles to

Alloy Coating Stress, KSI Failure

Ti-8-1-1 Fel-Pro C-200 60 1 .47

5 55 1.8 X 10^

45 3.5 X 10^

35 7 . 56 X 10^

Ti-8-1-1 Graphite Varnish 45 2.94 X 10^

30 5 .

1

X 10^

25 7 .

3

X 10^

20 DNF*

20 DNF

Ti-8-1-1 AFSL-41 27.5 6.0 X 10^

^ .J DNF

Ti-6-2-4-6 Fel-Pro C-200 >j \j 1.3 X 10^

27.5 DNF

35 J . o V 1 0^

L J_ \J iL. 'rt \J rir oIj fix jU5n 3.26

5

X 10

30 2.1 X 10^

25 DNF

25 3.34 X 10^

Ti-6-2-4-6 Cu-Ni-In 35 5.3

4

X 10^

25 2.8 X 10

*Did not fail

prime consideration in the final selection of coatings for production

use was cost of the materials and cost of processing. Some of the

coatings performed better in the above tests than AFSL-41 but due to

higher costs of these coatings AFSL-41 was specified in those applications where it would meet the hardware requirements as determined by

final engine testing.

Conclusions

Two gas turbine engine manufacturers have selected AFSL-41 for use

on a production basis in their respective engines. Both have issued

specifications covering the material and its use in their engine. The

TF-41 engine utilizes AFSL-41 in a compressor wheel seal plate application while the F-lOO engine now has over 100 different applications

128
ranging from compressor blade roots, disks and hubs, shaft couplings,

and ball socket seats to entire bearing assemblies. Further tests are being conducted to find other applications where AFSL-41 can be effectively utilized.

References

1. Benzing, R. J. and McConnell, B. D., "Wear Behavior of an Air

Drying Methyl Phenyl Silicone Bonded Molybdeniam Disulfide

Lubricating Film" AFML-TR-70-179 , October 1970.

2. Benzing, R. J. and McConnell, B. D. and Clow, W. L. , "A Methyl

Phenyl Polysiloxane Bonded Lubricating Film - On the Synthesis

and Chemical Curing" in ASLE Proceedings - International Conference

on Solid Lubrication 1971, pages 103-115.

3. Benzing, R. J. and McConnell, B. D., "Wear Behavior of a Phenyl

Methyl Silicone Bonded Solid Film Lubricant" in ASLE Proceedings - International Conference on Solid Lubrication 1971, pages 116-128.

4. Stout, R. L., "Air Drying, High Temperature Resistant, Silicone

Protective Coatings," AFML-TR-67-433 . April 1968.

5. Bolin, E. L., and Hanik, D. K. , "Evaluation of Anti-Fretting

(Solid Lubricant) Coatings for Titanium Alloy Components" in

Proceedings of Conference on Solid Lubricants, Kansas City,

Missouri, September 9-11, 1969.

129
DISCUSSION

A. W. Ruff, National Bureau of Standards : The behavior of titanium

varies considerably with environment. Can you give any details as to

the temperature, humidity, etc., in the vicinity of the fretting

contact both in service and laboratory conditions?

B. D. McConnel

1

: There were no precautions taken to exclude any

moisture or provide any special environment for the laboratory tests.

G. Hurt, Detroit Diesel, Allison Division : Almost all of these engines

currently use the technique of air blown seals which means that the

engine is filled with atmospheric air. The engine that Bob McConnell

is talking about is used by the Navy and the Air Force and is exposed

to a salt water environment. The engine does have to be protected and

lubricated with this solid film lubricant kind of material. The

temperature in the front end is not high--about 100 to 120 F maximum--but further back in the compressor the temperature can get up

to 200 to 300 degrees.

A. J. babecki , NASA, Goddard Space Flight Center : Were the coating

thicknesses the same regardless of the technique employed? It seems to me that the coating thickness would affect the performance of the

material; that is, if the silicone rubber bonded coating was much

thicker than the silver, I would think that that in itself would tend

to make the silicone much better.

b. D. McConnell : I can't recall the thickness of the electroplated

silver. Most of the other bonded films were in the 0.0002-0.0005 in

thickness range.

A. J. Babecki : This may have use in other applications where we are not concerned about fretting, but perhaps with sliding friction or something where there is large amplitude motion. Has there been any

testing to determine the quality of the adhesion of the silicone rubber

bonded coatings?

B. D. McConnel 1 : This is not a silicone rubber. This is a methyl

phenyl silicone which is a fluid. We did not perform any adhesion

tests, as such. We ran various formulations under some fairly high

loads. How long a film will run under some of these conditions is a fair indication of how well it will adhere. We found that, although

this formulation is air drying without any heat curing, the curing

agent cross-links the polymer or the resin. The adhesion is equal to

or, in many cases, better than a lot of the high temperature polyimide

type resins or the epoxy resins. We ran these at temperatures up to

800 F and we got much better high temperature performance.

130
A. J. Babecki : Would the performance of the coating be similar to or

j better than MLR-2 or the MLF-5?

B. U. McConnel

1

: Equal to in most cases.

G. Hurt : We had blade failures due to fretting. These failures were

eliminated by applying a multilayer coating to the fretting areas. We

also use your coating in other locations to prevent galvanic corrosion

in the compressor of the engine.

G. E. DeLong, Naval Air Systems Command Representative, Pacific : You

said something about 800 F for a turbine blade. The temperature would

be a lot higher than that for a turbine blade.

B. D. McConnel

1

: I said that we had evaluated the films in laboratory

bench tests up to 800 F and found that this film, although it air drys

without any heat cure, has performance equal to or better than many of

the heat cured films such as the polyimides, the epoxies and other

types of binders.

151
A STUDY OF SOLID LUBRICATED GEARS AT ELEVATED TEMPERATURES

A. J. Haltner, D. K. Snediker*, and K. E. Demorest**

General Electric Company, Space Division

Philadelphia Pennsylvania I9IOI |

1 *Battelle Memorial Institute I Columbus, Ohio U32OI '

I **National Aeronautics and Space Administration

George C. Marshall Space Center

Huntsville, Alabama 55812

j ABSTRACT: \ An experimental study has been made of the performance of advanced, ' state-of-the-art solid lubricants applied to spur gears expected to | operate to high temperatures in reusable space vehicles (space shuttle). | Tests of Inconel 71*8 gears covered the range 25-482<^C in air, and the

j lubricants studied included M0S2 in several binders (glass, metal matrix, [ polyimide, and polyphenylene) and mixed fluoride compositions. All of | the combinations demonstrated acceptable wear over portions of the | temperature range, but the glass bonded films showed the most promise— [ particularly at higher temperatures.

Keywords: Fluoride Films; Gears; Molybdenum Disulfide; Solid Lubricant^

j

Wear 1

INTRODUCTION: | The design of lubrication systems for space shuttle machine elements pre-j

sents a unique set of problems due to the mission profile of this class I of vehicle. The shuttle will be launched in a manner similar to today's

j spacecraft, and will spend some period of time in an orbital environj ment. It must then reenter the earth's atmosphere in a configuration , that will permit it to maneuver and land as a conventional, though un- '. powered, aircraft. |

I

j The shuttle will thus be subjected to a series of environmental condi- i tions which, taken individually, can be addressed by current technoloj gies, but in combination will make lubrication by conventional means I improbable. In particular, several mission-critical mechanisms will be

| located in areas on the shuttle that can be exposed to extremely high 1 temperatures during reentry. Very high temperature regions will include ; wing leading edges and the area of the nose ablator. It is clear that f special design provisions must be made for nearby mechanisms such as control-surface and landing-gear-door hinges and actuators.

An additional requirement is that the shuttle operate with a high degree 1

132
of reliability, and be reusable with a minimum of maintenance.

It is particularly important to note that space shuttle machine elements

must operate in an environmental regime where considerations of material

selection and application are inseparable. It will no longer be possible to design a machine element, then select an appropriate material.

On the contrary, the design configuration will be def ined to a large

degree by the materials available; and because the final design is dependent upon satisfactorily matching such factors as strength, friction/

wear characteristics, oxidation resistance, and thermal properties, the

choices available to today's designer are severely limited. (For many

candidate materials, the necessary information does not exist in the

literature and must be generated.) Space shuttle environments require

that machine elements and their lubricants be treated as integral parts

of a lubrication system , and not be dealt with separately.

It is obvious that reusable space vehicles place unique demands on lubricant systems in terms of environment, life, and reliability.

Machine elements such as gears will be repeatedly subjected to an environment which is unique in spacecraft applications: exposure to an oxidizing atmosphere over a wide temperature range. In the past decade,

a significant amount of research has been devoted to the development of

advanced solid lubricants for use at elevated temperatures. (For a re- view of the current literature, see Reference 1). While these lubricant systems are, in many cases, highly developed, there is little

information on the application to actual machine elements. Thus, the

designer of machine elements and lubricant systems for reusable space

vehicles is hindered by the lack of knowledge of the limitations of

these advanced lubricants when applied to typical machine elements such

as gears.

PROGRAM OBJECTIVES:

The purpose of this experimental program was to provide the designer of

reusable space vehicles with the data necessary to design critical

machine elements and to carry out valid trade-offs in terms of life,

weight, reliability, maintainability, power requirements, etc. The

air has been to provide data sufficiently general to be useful in the

design of machine elements that may not yet be defined, while providing

sufficient depth to allow direct use by the designer.

A considerable amount of research and development effort has gone into

the areas of high-temperature lubricants and machine-element design.

(See, for example. References 2, 3, and 4.) While these prior studies

have provided a good baseline, or point of departure, for the present

program, they represent the materials state-of-the-art of the mid1960' s. Also, much of the earlier work was not addressed to the unique

characteristics of the shuttle environment — in particular, that certain machine elements must function both at elevated (i.e., reentry)

and ambient temperatures, and under pressures ranging from orbital

vacuum to sea lev^ atmospheric.

153
The objectives of this research were to identify and develop lubrication

systems for machine elements operating in space shuttle environments,

and to characterize those systems to the degree necessary to provide

vehicle designers with valid trade-off data, particularly with respect

to limiting conditions of wear -life, friction, and material interfaces.

Several advanced solid lubricant systems were evaluated for performance

and life when applied to spur gears operated at various temperature

levels varying from room ambient to 500° C. Endurance, wear debrisquantity and morphology and failure mode were determined. Based on the

selection of the most promising candidate lubricants, a test matrix for

gears was set-up and carried out. The gears were fabricated from Inconel 718, chosen for its hot strength and compatibility with some of the

solid lubricants. To carry out the gear test program, a new gear

tester of four-square design was designed and constructed with high

temperature capability. New techniques were devised for determining

lubricant film failure in gears.

THE FOUR-SQUARE GEAR TESTER:

The major problem in the design of a gear test machine is in the method

of loading the gear teeth with a well-characterized, constant load. In

general, there are two methods of accomplishing the task; (1) loading

gears against a djmamic load or (2) against themselves. The foursquare gear test machine, which was used for the majority of tests on

this program, represents the second concept. This configuration offers

the advantages of low driving torque, flexibility, simplicity, and

capability of running two sets of gears at once. The test machine program requirements and design goals are summarized in Table 1.

TABLE 1. GEAR TEST MACHINE REQUIREMENTS

Test Requirements Design Goals

Gear Sample Diameter .0508 meters .0508 - .1016

Meters

Tooth Load 5.65 NT-M 1.13-113 NT-M

Speed 100 rpm 0-300 rpm

Temperature Range 210 - 4820C 21° - 9270c

Number of Test Specimens 1 mesh at a time

Failure Detection Yes Yes

The test machine functions in the following manner (see Figure 1). After installation of the four gears, two on either end, the system is

locked to prevent the rotation of the gears using a pin. Then the

spring side of the adjustable coupling is released, adjusted to a torque

corresponding to the desired tooth load, and relocked to maintain the

spring torsion. The system is then released by removing the locking pin

I3U
and the required tooth load is placed on the gear teeth in both meshes.

The greatest advantage of this system stems from the fact that the

spring torque cancels itself through the system and thus the drive

motor must work against friction forces only.

Figure 2 shows the layout of the four square tester developed for this

program. The gears are attached to a stub shaft and heat dam made from

Inconel 718. This material was chosen because of its high temperature

strength and corrosion properties. The stub shaft is fastened with

eighth-inch spiral pins to a 316 stainless steel drive shaft which is

hollowed to provide a heat dam for additional resistance to the flow

of heat from the gear face to the shaft bearing.

The shaft support bearings, packed with Andok-C grease, are aircrafttype full complement, thin section ball bearings capable of withstanding 2240 NT radial loads. They are fitted with a face seal on the

specimen (hot) side to prevent the lubricant from bleeding and creeping

into the hot zone. Each set of shaft bearings is supported by a carbon

steel trunnion block. These blocks can be heated to relieve thermal

stresses due to radial gradients across the bearing, should these

stresses become excessive.

One set of half -shafts is joined by a Thomas Coupling which makes use

of stainless steel flex members to provide for angular and parallel

misalignment. The coupling also provides a small preload to position

the shafts in their trunnion blocks and allow for thermal expansion.

The torsion spring is made from a chrome-vanadium steel alloy. This

spring provides the torque load, a light bearing preload, and acts as a flexible coupling. While many four square systems use' a torsion bar,

this design makes use of a coil spring because of space requirements

and the flatter spring rate characteristic of such springs, making control of torque simpler and more reliable.

The adjustable coupling is made from 316 stainless and it serves to

lock the gears and provide a means of deflecting the spring by hanging

weights from its grooved member . The two members of the coupling are

free to rotate relative to each other after the loosening of locking

screws. After deflecting the spring, the parts are locked together and

that torque is maintained on the test gears.

The gears on the test machine are covered with a transite insulation

box that acts as a heat shield to contain radiant energy and provides a housing for the quartz lamps and their reflectors. The temperature of

the lamps is controlled with variable transformers.

A failure detection system for lubricant films on the gears is based on electrical conductivity across the gear mesh. To achieve this purpose,

the trunnion blocks and the adjustable coupling are electrically iso155
lated with Kapton sheets and with Teflon sleeves around their fasteners.

Shaft stresses due to mechanical and thermal loading are within recom- mended values for the chosen materials. The heat dams are designed to

provide a 380°C gradient when the gear boxes reach 500°C. This is important in maintaining very low radial thermal gradients across the

support bearings. Such gradients can bring about closure of the bearing radial play and greatly increased ball loads. Piece parts such as

the sprocket, spring, and coupling have safety factors greater than 2. Design goals in general exceeded the test requirements in order to allow for unexpected problems and potential future applications to more

severe test conditions.

Figure 2 shows a drawing of the involute, 24-pitch spur gear test speclr

men used in these tests. All gears lubricated with bonded films are fabricated from Inconel 718, a material chosen for its high temperature strength and compatibility with the AFSL-Class lubricants.

The gear test conditions are as follows:

f, 2 Stress 684 x 10 N /m , Hertzian mean stress

Speed 100 rpm

Atmosphere Air, -40 percent R.H.

TEST PROCEDURES:

The gear test specimens were coated with the various lubricant films by

the lubricant suppliers according to their procedures (Reference 5).

Prior to coating, the gears had been serialized and the run-out maxima

marked and indexed with respect to red-line plots. After coating, the

gears were inspected visually and stored until use.

The following procedure was followed for all gear tests.

(1) The appropriate pair of gears was mounted on the machine with the

run-out maxima properly aligned.

(2) The thermocouple was placed in the shaft of the driven gear.

(3) The mesh was heated to the test temperature. Typically, this took

from 5 minutes for 149°C tests to 30 minutes for 4820C tests.

(4) Upon reaching temperature, the test gears were loaded. This was accomplished by deflecting the torsional spring to an equivalent

of 40 pounds force by deadweight loading a lever arm attached to

the shaft. Once loaded, the coupling was locked and the weights

removed. A line was drawn across the coupling interface to aid

in detecting any slippage (and hence load relaxation) at the

interface.

(5) The wear debris pan was put in place and the electrical conductivity circuitry checked.

(6) The machine was turned on and the 100 + 5 rpm speed checked with

a chronometric tachometer.

156
(7) During the course of the test, up to three critical parameters

were monitored as evidence of failure:

(a) Electrical conductivity

(b) Wear debris-generation rate and morphology

(c ) Visual inspection during periodic slow-downs.

FAILURE CRITERIA, THE ELECTRICAL CONDUCTIVITY METHOD:

A major problem in the evaluation of lubricants on gears is the determination of the point of lubricant failure. Solid film lubricant

failure and the onset of substrate wear cannot be easily detected by

monitoring performance parameters such as torque or by acoustic means. Substrate wear may be measured by the use of master gears or pins and

micrometers; however, these methods require that the test be stopped

and the specimens cooled to room temperature. The visual inspection

of the gears during a brief slow-down is effective but does not lend

itself to long-term, unattended operation.

A new method, based upon the electrical conductivity across the mesh,

has been developed. This method makes use of the fact that dry lubricant films form insulating or semi-conducting films. If a very low

potential, approximately 100 mv, is imposed across a gear mesh coated

with an intact film, the circuit resistance remains high and virtually

no current flows. When the film wears, allowing metal-to-metal contact,

the circuit resistance drops to allow a large increase in current flow.

Figure 3 shows a diagram of the circuit used for these tests. The

four-square gear test machine was designed such that each mesh could

be electrically isolated. Electrical contact to the machine shafting

was made by slip rings. In a typical test, oscillograph traces show

a transient run-in condition followed by a period of smooth operation.

(Figure 4) When a thin spot in the coating develops, a spike appears

1/revolution (Trace D). Single spikes have been correlated visually

with a bare spot on the tooth predicted by the position of the spike

in the time domain.

More spikes appear as other teeth degrade, until several dead shorts

per revolution are observed. Failure is defined as two or more total

shorts accompanied by a generally high level of noise. The test is

then continued for 5 to 10 minutes to make certain that lubricant fail- ure has taken place. (AFSL-28 at 482°C repaired itself from a seemingly terminal condition twice before finally failing several hours later.)

Gear lubricant failure detection by this method has two drawbacks.

First, the thin or bare spot responsible for the short can be extremely

subtle and localized. This technique must be accompanied by careful

post-test visual inspection to ensure that a general failure has oc- curred rather than just an anomalous isolated event on a single tooth

resulting from factors not related to lubricant performance. Furthermore, the damage is often difficult to detect and characterize. A

157
second drawback is that MLR-2 and MLR-66 both conduct electricity at

temperatures >350°C and, hence, the method may not be applied.

The conductivity method for detecting the failure of dry film lubricants on gears is extremely promising. By determining when the lubricant itself has failed, the method allows the testing of lubricant

properties uncomplicated by differences in the wear resistance of the

substrate. Thus, much test time is saved and data are obtained that are directly related to the lubricant itself.

TEST RESULTS:

Inconel 718 gears were used lubricated according to the test matrix

given in Table 2.

TABLE 2. GEAR TEST MATRIX

Temperature

Lubr icant Substrate 70°F 300°F 600°F 900°F

Clevite 300

MLR-2

MLR-40

NPI-A25

AFSL-28

AFSL-29

Inconel 718

Load = 100 ksi mean Hertz stress; speed = 100 rpm.

Wherever possible in these tests, the failure point of the lubricant

film was established by the electrical conductivity technique previously discussed.

Table 3 summarizes the results of the gear tests. Figure 5 shows the

comparative times-to-f allure for the various lubricants at the three

temperature levels.

DISCUSSION:

Gears coated with MLR-2 and MLR-66 at 482° and 316°C, showed a distinct

gray appearance at the elevated temperature indicating widespread

oxidation. This oxidation was accompanied by a rapid breakdown of the

lubricant film. At the higher temperature, AFSL-28 showed a "healing"

property not noted in the other lubricants at any temperature. This

system made two complete recoveries from what appeared to be a condition of terminal degradation: multiple electrical shorts on several

teeth and increased acoustic noise. In general, failure was a rather

long process comprising -25 percent of the life of a gear. It usually

began with a short on one tooth that was often due to degradation on

158
TABLE 3. RESULTS OF GEAR TESTS (100 RPM in Air-684 x 10^

N /m2 - Mean Hertzian Contact Stress)

Mesh Time to

Temp Failure Failure

Lubricant Lubricant Description (°C) (min) Criterion

MLR2

MLR-66

AFSL-28

AFSL-29

0.55 MoS^, 0.45 Sb20^,

Polylmlde binder

(Ref. 5)

MoS^/Sb^O , Polyphenylene binder

(Ref. 6)

Fused calcium and

Barium fluoride

mixture, aluminum

phosphate (Ref. 7)

Fused calcium, barium

and magnesium fluoride

mixture (Ref. 7)

Vltrolube MoS„, C + glass

binder

482 13 Visual

316 195 Conduc t iv Ity

149 975 Conductivity

25 42 Conductivity

482 15 Visual

316 302 Conduc t ivi ty

149 1322 Conductivity

25 66 Conductivity

482 150 Conductivity

316 78 Conductivity

149 43 Conductivity

25 19 Conductivity

482 216 Conductivity

316 93 Conductivity

149 56 Conductivity

25 14 Conductivity

482 189 Visual

316 428 Visual

149 752 Visual

25 882 Visual

a tooth face that was visible. As the test continued, other failure

spots usually remote from the original appeared. Failure did not

generally propagate from tooth-to-tooth, as one might expect if failure were due to a geometric anomaly such as excessive run-out.

MLR-66 consistently produced copious quantities of wear debris, at all

test temperatures. AFSL-28 and AFSL-29 both produced very little, even

at failure. In fact, none of the tests was accompanied by any change

in debris generation rate at failure. Only MLR-66 at 316°C showed any

evidence of peeling. In this case, peeling took place early in the

139
test and seemed to involve excess lubricant.

The Vitrolube coating appears to yield somewhat better wear results

over the temperature range than the other four coatings tested on this

program. The coating had a superior wear life at room temperature, and

though there was a progressive deterioration in wear life as the test

temperature was raised, the wear life at high temperature was at least

comparable to the wear life for the fused fluoride films. The wear debris was powdery and particularly voluminous at room temperature with

about three or four times as much debris volume as was observed at

other temperatures. The wear debris at 149 and 316°C was also powdery

but at 482°C, the debris was in the form of flakes. The qualitative

wear observations would at first appear to be inconsistent with the

data on coating life. However, it is possible that the wear mechanism

is changing as the temperature rises. At the lower temperatures, the

debris is formed at a much higher degree of subdivision which would

make it appear to be formed in greater quantities. It is possibly the

agglomeration of very small particles. At elevated temperature, the wear particles resemble the physical state of the material in the lubricant coating. Although some quantity of metal was observed in the

debris at 25°C and 149°C, it was more prominent at 316°C and a significant constituent at 482°C .

Failure of the lubricant coating in all of the tests was observed to

proceed according to the sequence.

(1) Appearance of evidence of wear at or near the pitch line. This

resembles a score mark. Since, ideally, pure rolling occurs at

the pitch line, it is difficult to associate this with adhesive

or abrasive wear phenomena. It is possible that particles are leaving the coating as a result of cyclic stress (fatigue). At room temperature, the phenomenon is evident after 40,000-50,000

stress cycles.

(2) The occurrence of a dull coloration on the teeth (possibly related

to metal oxide formation).

(3) The appearance of bare metal spots on individual teeth.

CONCLUSION:

These fairly limited tests indicate that none of the lubricating films

studied^are comgletely satisfactory over the entire temperature range

from 25 to ^82 C. The maximum cycle life observed at any temperature

was about 10 . The Vitrolube was apparently the best performer at room temperature and at the highest temperature its wear life was com- parable to the fused fluoride films. On this basis, it is the most

promising candidate gear lubricant studied. It is quite possible that

these results could be significantly influenced by more careful control

of such variables as film processing, substrate physical and chemical

properties, film thickness, and a more elaborate process of film runin and conditioning.

iko
This work was sponsored by the National Aeronautics and Space Administration, Marshall Space Flight Center, Alabama, 35812, under Contract

NAS 8-26282.

' REFERENCES

:

(1) Snediker, D. K. , Sturgeon, L. F. , and Haltner, A. J., "The

Development and Evaluation of Lubricating Systems for Reusable

Space Vehicles, "Summary Report, covering work performed from

July 1, 1970, to June 30, 1971, on Contract NAS 8-26282, April 5,

1972.

(2) Bamberger, E. N. , "Development of Unlubricated High Temperature

Bearings", General Electric Co., Flight Propulsion Laboratory,

TIS R63FPD50, January, 1963.

(3) Armstrong, C. S., "Air Frame Bearings for Advanced Vehicles",

AFFDL-TR-69-66, August 15, 1969.

(4) Matt, R. J., Muratore, J. B. , and Zupkus, C. J., "Investigation

of Air-Frame Bearing Materials for Aerospace Vehicles", Proceedings of the USAF-SWRI Aerospace Bearing Conference P.M. Ku, Editor,

September, 1964, p. 108.

(5) Campbell, M. E. , Solid Lubricants, A Survey , Technology Utilization Office, NASA, Washington, D.C., 1972.

ll+l

SPUR GEAR DATA NUMBER OF TEETH 48 DIAMETRAL PITCH 24 PITCH DIAMETER 2.00 DDC KKt-OOUKt. CCIIDC A Mm C ( f^l [ I 1 C O 1 AINul—t. V^U 1 I t.r<> 20° TOOTH THICKNESS ARC .05545 BASE CIRCLE DIAMETER 1.8793852 FORM DIAMETER 1.9367 ADDENDUM .0417 ROOT DIAMETER 1.896

1.884 OUTSIDE DIAMETER 2.083 2.080 MASTER GEAR TOOTH THICKNESS .06545 TESTING RADIUS .9999

.9969 TOTAL COMPOSITE ERROR .0030 TOOTH-TO-TOOTH COMPOSITE ERROR .0015 MEASURING PIN DIAMETER .0720 MEASUREMENT OVER (2) PINS 2. 101

1

2.0982 AGMA QUALITY NUMBER 88

I .0005

+ B.0005I //

.3750 . .3755

.220 DIA THRU

I 4 HOLES LOCATED -250 -*J AS SHOWN

-.03X45 (TYP)

SECTION A-A

NOTE:

1. BREAK SHARP CORNERS .005

2. REMOVE BURRS .015

FIGURE 2. DRAWING OF INCONEL 718 AND CLEVITE 300 TEST GEAR

11+3
I-l'l-l

~90 MV

lO'^ AMPS

VOLTAGE

DIVIDER

PICOAMMETER

DIGITAL VOLTMETER

FIGURE 3. CIRCUIT DIAGRAM FOR CONDUCTIVITY MEASUREMENTS

Ikh
11 niN.

23 MIN.

0.0 HIN.

OJ MIN.

31 MIH.

33 MIN.

39 I1IN.

W MIN.

56 MIN.

1 REVULUTIOU or

FIGURE 4. PLOTS OF THE ELECTRICAL CONDUCTIVITY ACROSS A SOLID

LUBRICATED GEAR MESH AS A FUNCTION OF ELAPSED TEST

TIME

1^5
FIGURE 5. TIME TO FAILURE AS A FUNCTION OF TEMPERATURE

146
DISCUSSION

J. E. Stern, NASA, Goddard Space Flight Center : Did you take the gears

apart to examine them?

D. K. Snediker : No, we had a hole in the top of the heat shield. We

looked down through the hole and rotated the two axes of the foursquare machine. We watched the teeth go by and the positioning of the

quartz lamp was such that we got a very good definition of the surface

geometry, with shadowing of any irregularities. It turned out to be a pretty good method of visual inspection.

J. E. Stern : You couldn't then identify the cause of recovery?

U. K. Snediker : No, we could not, although I suspect it was probably

due to a reburnishing of the lubricant. In all of these gears, there

was a lot of solid lubricant powder, a wear debris, that was moved from

the area around the pitch line and the areas on the adendum and

dedendum immediately adjacent to the pitch line down into the dedendum

and up onto the adendum of the tooth. Therefore, there was plenty of

lubricant around all of the time, and there was the shower of debris

raining down in most cases. There may be a favorable relationship

between the oxidation products of Inconel 718 in air and this

particular lubricant system that produced a good lubricating film.

J. E. Stern : Did you make any chemical analyses of the wear debris?

D. K. Snediker : No.

A. J. Babecki , NASA, Goddard Space Flight Center : In an LFW-1 tester

you can accurately determine what the Hertzian contact stress is. but

in the case of the gear tester, I do not see how you can really

determine this stress because of the out-of-roundness of the gears. I imagine that there is a pretty high Hertzian pressure. Is that

correct?

D. K. Snediker : I think we are talking about less than 200,000 psi mean Hertzian contact stress.

A. J. Babecki : Would that be somewhat variable as the gear rotated?

D. K. Snediker : For this grade gear, it would be somewhat, but not

terribly, variable.

A. J. babecki : As wear progresses, is the loading on the gears somehow

continuously adjusted to take care of this wear so that there is a constant Hertzian stress or does the stress diminish as the wear progresses?

1U7
D. K. Snedi'ker : One of the reasons we wanted to stop the test before

we began to wear the tooth itself was to avoid this problem. We did

not wear the teeth. After the initial run-in period, which lasted only

a couple of minutes, there was a relatively thin equilibrium film and I do not think the conformity of the contact changed much. There may

have been a 10 to 20 percent change, but we tried to avoid the problem

by testing the lubricant film rather than the gear.

A. J. babecki : From your experience, would you put more credence in

the LFW-1 test data than in the actual gear data for evaluating coating

performance?

D. K. Snediker : I usually trust real machine element data provided it

is carefully developed and the variables are controlled more than test

machine data. With solid lubricants, every case is different and one must be very careful about generalizing. The list of lubricants we evaluated resulted from some generalizing on the basis of test machine

data and some of them turned out to be perfectly dreadful. The Pel-Pro

C-200 that Bobby McConnell used performed pretty well in a journal

bearing for something that can be purchased at the hardware store. We

tried it on a gear and it did not work.

1U8
SESSION IV

I

.'i

CHEMICAL

COATINGS

Chairman: Lt.R. S.Miller

Office Of Naval Research

VAPOR DEPOSITION OF WEAR- RESISTANT COATINGS

Wilbur J. Wilson, Melvin F. Browning,

and John M. Blocher, Jr.

Battelle Memorial Institute

Colimbus, Ohio U52OI

Vapor deposition processes by which a variety of wear and environment

resistant coatings are applied are described. Criteria for selecting

coating/substrate systems are discussed in the light of economic and

technological changes.

Several examples of vapor-deposited, wear-resistant coatings are given

in addition to specific examples of commercial applications.

Ke5rwords: Chemical; coating; deposition; vapor; wear

Broadly speaking, the term vapor deposition describes any process by

which a solid is formed, the materials having arrived at a surface as a

gas. Thus, the "pack cementation" processes by which parts are chromized or silicided are properly termed, vapor deposition, since

coating materials move from particles within the pack to the parts by

vapor transport. Similarly, certain atmosphere heat-treating processes

are included since the atmosphere contributes to the product as in

nitriding and carburizing of steel parts.

Physical vapor deposition (PVD) has expanded rapidly in recent years as dramatic improvements in vacuum technology have been made. Thus, PVD

has moved beyond the classical vacuum coating by vaporization of metals

to include reactive sputtering, ion plating, etc. However, it is with

chemical vapor deposition that we wish to deal here.

Chemical vapor deposition (CVD), like vacuum coating and electrodeposition, is a molecular forming process; that is, the deposit is

built up atom by atom or molecule by molecule. The starting material

in the case of CVD is a gaseous compound which is caused to react

chemically at the substrate to form the deposited material. The substrate is usually heated to activate the process, although sometimes it

may be cooler than the source temperature, thus establishing a new

equilibrium that causes the gas mixture to give up some of the desired

product.

CVD is applicable to the formation of a wide variety of materials - metals, alloys, semiconductors, carbides, oxides, nitrides, sulfides,

silicides, and borides. These are deposited by the appropriate chemical

151
reaction of gases or the vapors of volatile compounds such as fluorides,

chlorides, bromides, iodides, hydrides, hydrocarbons, organometallics,

and carbonyls.

PROPERTIES AND CHARACTERISTICS

Table 1 lists some of the useful chemical reactions. The required substrate temperatures cover a wide range, depending upon the chemical

reaction used, and the desired structure of the deposited materials,

which is a sensitive function of temperature. The greatest flexibility

in the choice of reactions lies in the range of 800 C and above. The

requirement in many cases for processing at elevated temperature,

coupled with the use of gases or vapors which can be corrosive, poisonous, inflammable, or air-sensitive are disadvantages of the process

which tend to limit its use. Fortunately, CVD has a number of advantages, in that it:

(1) Provides superior throwing power - unique

adaptability to coating in crevices or the

inside bores of small tubing

(2) Has unique adaptability to coating particles

in a gas-f luidized bed

(3) Provides dense coatings of some materials

not readily obtained by other methods

(4) Is adaptable to the vapor forming of

dif f icult-to-fabricate materials

(5) Provides for control of coating structure - ranges from epitaxial growth, through large

grained, small grained, and amorphous, to

powder

(6) Can yield products of high purity or controlled composition; feed materials

readily purified; doping readily accom- plished and controlled; alloying feasible

in some cases

(7) Can be carried out in simple equipment if

operation at atmospheric pressure is

feasible.

The limitations of CVD relate primarily to the availability of reactions

that will proceed at temperatures compatible with the nature of the substrate. Not only must the substrate be chemically inert toward the

coating atmosphere at the required temperature, but for coats thicker

152
than in the micron range, the coefficients of thermal expansion of the

coating and substrate must be reasonably closely matched. Otherwise,

the stress resulting from differential contraction of the coating and

substrate on cooling from the coating temperature may well be sufficient

to rupture brittle coatings. If possible, the choice of configuration

should leave the coating in compression to minimize the effect of

differential thermal expansion.

Generalizations concerning CVD processes are dangerous, in that each

reaction has its own characteristics. To successfully apply CVD

processes other than those that have been developed to the "cookbook"

stage, one must have an uncommonly good understanding of the reaction

chemistry, the compatibility of the materials involved, and the mass

transport characteristics of the gas stream.

For example, a coating may be satisfactory on the upstream surfaces of

the substrate, while eddies at a downstream surface may provide enough

dwell time of the reactant gases to result in "snowing out" or powder,

either of the coating material or of some intermediate solid product.

Thus one generally seeks to:

(1) Maximize chemical compatibility

(2) Minimize thermal expansion differentials

(3) Maximize uniformity of exposure of

substrate to the reactant gases.

APPLICATIONS

To present a better understanding of the state-of-the-art of CVD and the

potential for future applications, several examples will be given of

instances in which modern technology actually depends on CVD.

Titanium carbide coatings on cemented carbide tool inserts, on stamping

and punching dies, and on thread guides, etc., are having a significant

impact on industry. Wilhelm Ruppert, while at the Metallgesellschaf

t

was the first to recognize and apply the superior wear resistance of CVD

Tic coatings on metal working tools . Although work on the TiC

coating of cemented carbides was started in 1958 and proposed in a 1960

patent , it was not until 1968 that sintered hard carbide tools with

TiC coatings were applied on an industrial scale, as reviewed by

Hinterman, Gass, and Lindstrom (7).

Figure 1 compares the wear resistance of TiC coated and uncoated

cemented carbide tool inserts after machining 600 in ^ to a depth of

0.2 inch at 0.018 in /rev from AISI 4340 steel at a surface speed of

500 ft/min. The wear resistance in this case is attributed to the

153
decrease in friction between the hot chip and the tool surface just

behind the cutting edge. Such inserts are coated by reaction of

titanium tetrachloride with methane in a hydrogen atmosphere at about

1000 C in a retort similar to that shown in Figure 2.

Figure 3 shows several thread guides coated by the same process.

Figure 4 shows a wide variety of parts coated by CVD. This photo is by

courtesy of the Laboratoire Suisse de Recherches Hologeres (Research

Laboratory of the Swiss Watchmaking Industry) (8)

Through the years CVD work at Battelle has most often been oriented

toward achieving environmental protection of parts. That is, offering

improved resistance to heat, corrosion/erosion, radiation damage, etc.

However, certain recent work is of interest in relation to wear studies.

In this connection, hardness of candidate coating materials is of prime

interest. Table 2 is a compilation of hardness data for selected

materials. Table 3 gives the chemical reactions by which six of these

materials are deposited. The choice of coating materials is limited by

stability of a particular substrate in the coating atmosphere at the

required temperature. In addition, suitable match of coating to substrate thermal expansion is required to retain an integral coating upon

cool down and subsequent thermal cycling. Limited accommodation to

thermal expansion mismatch may be made by limiting the thickness of the

coating to a few micrometers.

On an experimental basis coatings are applied in two ways. Figure 5

illustrates the application of a coating by heating the substrate by

induction in a stream of reactant gases.

Figure 6 shows a graphite tube coated with about 80 mils of silicon

carbide in such a system.

Figure 7 illustrates the coating of parts while immersed in a bed of

fluidizing particles.

Figure 8 shows steel parts coated by the fluidized bed process.

Figure 9, 10, 11, and 12 are photomicrographs of typical coatings in

metallographic cross section.

FUTURE

Turning to a brief consideration of cost, it is apparent that CVD

processing is quite expensive for small numbers of pieces because of the

necessity for custom design of equipment and processing procedures.

Nevertheless, the processes are already in commercial use for highly

specialized parts, as well as for large numbers of small parts, such as

I5U
coated carbide tool inserts. The CVD processing industry has grown

slowly but at this stage continued growth seems assured

(1) As specific procedures are devised and

rendered more reliable

(2) As the unique advantages of CVD become

more widely known

(3) As industrial economics place increasing

premium on extended servicibility

.

REFERENCES

(1) A. Munster and W. Ruppert, Z. Elektrochem. , Vol. 57 (1953), p. 564.

(2) W. Ruppert, Metalloberf lache. Vol. 14 (1960), p. 193.

(3) H. Wiegand and W. Ruppert, Metalloberf lache. Vol. 14 (1960),

p. 229.

(4) H. Wiegand and W. Ruppert, Metalloberflache. Vol. 15 (1961), p. 1.

(5) J. M. Blocher, Jr., and J. C. Withers, Eds., "Chemical Vapor

Deposition". Proceedings of Second International Conference, Los

Angeles, May, 1970. Princeton, N.J.: The Electrochemical Society,

Inc., 1970, p. 443-459.

(6) W. Ruppert, U.S. Patent 2,962,388, 1960.

(7) F. A. Glaski, Ed., Proceedings of the Third International

Conference on Chemical Vapor Deposition, Salt Lake City, April,

1972. Hinsdale, 111.: American Nuclear Society, 1972, p. 352-368.

(8) Brochure of Laboratoire Suisse de Recherches Horlogeres, Rue

Brequet 2, Neuchatel 7, Switzerland.

155
TABLE 1. REPRESENTATIVE CVD REACTIONS

Ni^^^U^S) 100,250 "c +4C0(g)

WF^(g) + 3H2(g) 300,800 C + ^^'^^S)

^V^^ 900-2300 C ^^^^ + 2H2(g)

4BCl3(g) + CH^(g) + 4H2(g) 1000-1500 C V^^^ ^ 12HCl(g)

2AlCl3(g) + acO^Cg) + 3H2(g) 700-150O C ^^2^3^^^ 3C0(g) + 6HCl(g)

TABLE 2. COMPARISON OF MICROHARDNESS VALUES

Material

Hardness, Hg

Knoop, 100 g load

Steel, AISI A-6 «:650

CryC3 1700

WC 1700-2400

Tie 2000-2750

SiC 2200-2950

B 2900-3300

W2C 3000

AI2O3 3000

TiB2 3000

B4C 3800-5000

156
TABLE 3. SELECTED CVD REACTIONS BY WHICH WEAR

RESISTANT MATERIALS ARE DEPOSITED

^^^^4 + ^2«6 700-900 C ^^^2 + ^"^^ ^ «2

»2

2AICI3 + 3H^0 ^QQ_^3QQ A1^03 + 6HC1

4BCI3 + CH^ + 4H2

,000-1500 C V + ^2HC1

»2 . TMi4 + ^«4TTooTTM + ^«^i

»2

^^^V^3 1100-1300 C +

3"^^

Cr(CgH^2^2 450-500 C^ *"^7^3 c-iroene fragments

157
FIGURE 1. COMPARISON OF WEAR OF COATED (ABOVE)

AND UNCOATED CEMENTED CARBIDE TOOL

INSERTS (Courtesy Kennemetal, Inc.)

158
FIGURE 2. CLOSED RETORT FOR TiC COATING: 1-FURNACE,

2-TUBE, 3- PARTS TO BE COATED, 4-COLD FLANGE

AND SUPPORT, 5- POWER SUPPLY, 6-GAS FLOW

CONTROLS, 7-TiCl, VAPORIZER, 8-RADIATION

SHIELD, 9-GAS PIPING

159
FIGURE 3. TiC-COATED THREAD GUIDES (Courtesy

W. Ruppert, Ed. Dorrenburg Sbhne)

160
FIGURE 4. VARIOUS CVD APPLICATIONS (Courtesy of

Laboratoire Suisse de Recherches

Hologeres)

161
Rotating drive and holder for mandrel

I— Quartz chamber

2- l/2-in. ODx 24 in. long

^ Induction coils

ooooooooooooooooo

Purge gas, \\. Graphite mandrel

^Slots

Reactant inlet + CHjSiClj

ooooooooooooooooo

Reaction products outlet

Schematic of Coating Chamber

Unit in Operation 2090VD

FIGURE 5. COATING UNIT USED IN VAPOR FORMING

SILICON CARBIDE CLOSED-END TUBES

16?
FIGURE 6. PHOTOGRAPH OF GRAPHITE ROD

COATED WITH ^0 MILS OF

SILICON CARBIDE

165
Scavenger

To hood

Oil bubbler

22 mm OD Vycor

coating chamber

Thermocouple

Split furnace, 2 zone

3/16 - inch support rod

Thermocouple

Specimen

23° cone

1/2 mm capillary

Titanium tetrachloride

7. SCHEMATIC OF APPARATUS FOR COATING EXTRUSION

PARTS WITH TITANIUM BORIDE IN A FLUIDIZED BED

161+
2276VD

FIGURE 8. STEEL PARTS COATED WITH TITANIUM

DIBORIDE BY FLUIDIZED BED PROCESS

lOOOX

FIGURE 9.

2287VD

PHOTOMICROGRAPH OF VAPOR DEPOSITED

ALUMINA COATING ON EXTRUSION PARTS

165
lOOOX 2285VD

FIGURE 10. PHOTCMICROGRA.PH OF VAPOR DEPOSITED

CHROMIUM CARBIDE OVERCOATED WITH

ALUMINA ON EXTRUSION PARTS

lOOOX 2286VD

FIGURE 11. PHOTOMICROGRAPH OF VAPOR DEPOSITED

TITANIUM DIBORIDE ON EXTRUSION PARTS

166
FIGURE 12. PHOTOMICROGRAPH OF VAPOR FORMED

CLOSED END SILICON CARBIDE TUBE

WITH GRAPHITE MANDREL PARTIALLY

REMOVED

167
DISCUSSION

R. S. Miller, Office of Naval Research ; Is the compatibility of

crystal structures between the substrate and the overcoat important?

W. J. Wilson : It is a very important consideration. The nature of the

substrate in this particular coating operation is probably even more

critical than in other types of coating operations. However, if we can

use the fluidized bed approach, we ordinarily get frequent renucleation

by the impingement of particles on the substrate. Therefore, the

crystal nature of the substrate is not so important. If an extended

surface is not bathed in a bombardment of particles, then the substrate

is very important. This sometimes results in very large crystals which

is normally a grave disadvantage.

P. Weinberg, Naval Air Systems Command : Can this technique be used to

coat the rollers in bearings with substances such as silicon carbide

and maintain satisfactory accuracy and tolerances?

W. J. Wilson : Yes, the fluidized bed technique is ordinarily used to

put this material on a symmetrical part such as a bearing. The control

of thickness is good from our frame of reference. If better control of

thickness is necessary, a bearing of this kind can be ground to a very

close tolerance, but the cost could be great. Generalizations are dangerous until one determines the thickness of coating that is

required and the nature of the substrate. If a ceramic substrate can

be used for the bearing and if the proper sphericity can be obtained,

then a bearing of this kind would be feasible. It depends on the

individual design.

A. J. Babecki, NASA, Goddard Space Flight Center : It appeared in the

micrographs that you showed that there was a good metallurgical bond

between the coating and the substrate. I assume from that that there

is some kind of diffusion into the substrate as occurs in nitriding.

Is that true?

W. J. Wilson : Generally not-- in some cases in vapor deposition from a boron atmosphere, borides may produce a diffusion bond. In most cases,

reaction of the substrate with the material which is deposited, or reaction with the atmosphere before the deposition, creates a layer

which separates the substrate from the deposit. The bonding

characteristics of this layer depend upon its nature. Sometimes the

bond can be improved with heat treating. More often than not, the

layer is a powdery material which ruins the whole operation.

168
A. J. Babecki : If you want to use one of these hard, wear-resistant

coatings, would it be necessary for the substrate to be hard enough to

sustain the load applied to it?

W. J. Wilson : Yes, it is of no value to put a brittle, hard coating on

the surface of a metal and then expose it to conditions in which the

substrate deforms. The coating simply pops off.

R. S. Miller : Can the process be made into a continuous operation?

W. J. Wilson : In some cases, for example the coating of wire, it has

been made into a continuous operation. The process is ordinarily a batch operation.

R. S. Miller : What is the general range of exposure times for your

process?

W. J. Wilson : The rates of deposition vary greatly. If rapidly moving

vapors impinge on the surface, the rates of deposition can approach 20

mils an hour. On the other hand, a relatively stagnant atmosphere

surrounding the surface will lead to a very slow deposition rate to get

an acceptable product. This condition occurs with diffusion in the gas

phase, which is the limiting step.

169
BARRIER FILMS FOR MINIATURE BEARINGS

Marianne K. Bernett and Harold Ravner

Chemistry Division

Naval Research Laboratory

Washington, D. C. 20375

Abstract

Miniature bearings (less than 30 mm O.D.) are employed in large

numbers by the Navy in synchro-servo motors, guidance systems and

general instrument use. The most frequent cause of malfunction of these

mechanisms is bearing lubricant failure; in the majority of instances,

failure is induced by migration of the lubricant away from the balls and

races, both during storage and service. As a result of long-term basic

studies at NRL on wetting and spreading of liquids, a low-energy, non- wetting "barrier film" was developed, which when applied to bearing race

surfaces, effectively prevented migration of the lubricant. The increasing use of these films has resulted in major reductions in replacement costs and maintenance, and has significantly increased the reliability of the mechanisms involved.

Keywords: Barrier Films; Bearings, Fluorinated Polymers; Lubricant

Migration; Lubricants; Spreading Prevention

Synchro-servo motors and other rotating devices are important

components of computers, guidance controls, navigation instruments, fire

control systems, etc., and reliable performance of the miniature bearings

used in these systems is imperative. There are a variety of failure

modes associated with these bearings, including corrosion (rust, fretting), lubricant degradation (oxidation, catalysis, etc.), friction and wear (boundary film failure), and non-wetting of the surfaces. It was

shown, however, that the principal cause of unsatisfactory service and

storage life of these devices is the migration of lubricant away from

the bearings (1). The high costs associated with these failures prompted a Naval Research Laboratory study which has resulted in the development,

outlined here, of a novel oil-retaining technique (2,3). Use of this

technique has permitted a five to ten-fold life increase of treated

bearings.

The tendency of a liquid to spread on a solid surface depends on

the relative surface energies of the solid and the liquid; a useful

index of wettability of the solid by liquids is the "critical surface

tension of wetting", y > The value of y is obtained by plotting

170
the cosine of the contact angle (9) of each member of a homologous

series of liquids on a smooth solid surface against the surface tension

of those liquids. The intercept of the resulting straight line with

cos 0=1 (9=0°) is referred to as y (Fig. !)• For example, if y of a

Figure 1: Wettability by various liquids

of cast film of polymer

i?— =«2j

- C00CH2C^F^^ ^

solid surface is 20 dynes/cm, generally any liquid having a surface

tension less than 20 dynes/cm will spread on that surface, whereas any

liquid with a surface tension above 20 dynes /cm will not spread. Most

lubricants have surface tensions between 20 and 60 dynes/cm, and common bearing materials (metals, ceramic, etc.) have y 's much higher than

100; lubricants thus tend to readily spread and migrate away from

bearing surfaces. However, if a solid surface or film with a y^ lower

than 20 dynes/cm surrounds the lubricated area ^ it will effectively

prevent the oil from spreading past that area.

The lowest y values thus far found are exhibited by solids whose

outermost surface layer is composed of closely-packed perf luoromethyl

groups (4). A methacrylic polymer containing fully fluorinated side

chains which satisfy these requirements was found highly suitable as a non-wetting solid (5) because of its exceptionally low y of 10.5

dynes/cm (Fig. 1). When a thin film of a dilute solution of this

f luoropolymer is applied to bearing races, and the solvent permitted to

evaporate, a solid adherent barrier coating, approximately 1 micron

171
thick, is formed (2,3). Figure 2 demonstrates the ability of such

circular barrier films to retain drops of various lubricating oils

(including silicones, diesters, fluorinated diesters, and petroleum

fractions) for over 10 years at ambient temperature within the confines

of the area circumscribed on a stainless steel plate.

The effectiveness of barrier films formed as described above is

dependent on their location, continuity, adherence to the metal substrate

,

and stability in contact with lubricants over prolonged periods under

varied conditions of temperature and humidity. Exposure of barrier

films surrounding drops of various oils to 100% R.H. and 50°C does not

significantly alter their wetting characteristics, and subsequent drying

restores their original properties; immersion in a variety of organic

polar and nonpolar liquids for prolonged periods at ambient temperature

shows no deleterious effects (6). The coatings are stable to 100°C in

contact with various oils; at these higher temperatures, however, the

performance of the coatings is somewhat dependent on the t3rpe of solvent

Figure 2: Drops of various lubricating oils on stainless steel

contained within barrier films after

10 years exposure at ambient temperatures .

172
used for the preparation of the barrier films solutions (7). When

deposited from a relatively volatile solvent, the film is not homogeneous or sufficiently adherent. Slow solvent evaporation is therefore

an important requirement for acceptable barrier coatings, and the most

useful solvents have a minimum boiling point of 100° C. Since the

polymer is highly fluorinated, only highly fluorinated solvents such as hexafluoroxylene and Freon TF are compatible with it; this is advantageous in that the barrier coatings are not attacked by conventional

solvents used in routine cleaning of equipment.

Use by the Navy of barrier films in conjunction with an improved

synthetic ester-based instrument oil (8) for prelubricated miniature

ball bearings, has resulted in reduced maintenance costs and smaller

stockpile requirements; it has been estimated that these procedures have

generated an estimated $A, 000, 000 in annual savings to the Navy. This

system is now available commercially and used by airlines for flight

safety improvement, by NASA for space guidance systems in satellites,

and by the computer industry. It is anticipated that even wider use of

the barrier film concept will occur as information about its value

becomes more available.

References

1. V. G. FitzSimmons, C. M. Murphy, J. B. Romans and C. R. Singleterry, "Barrier Films Increase Service Lives of Prelubricated Miniature

Ball Bearings", J. Am. Soc. Lubr. Eng. 24, 35 (1968).

2. M. K. Bernett and W. A. Zisman, "Prevention of Liquid Spreading

or Creeping", Adv. Chem. Series 43, 332, Am. Chem. Soc. (1964).

3. M. K. Bernett and W. A. Zisman, "Lubrication System for Fine

Mechanisms", U. S. Patent 3,225,866 (Dec. 1965).

4. W. A. Zisman, "Relation of the Equilibrium Contact Angle to

Liquid and Solid Constitution", Adv. Chem. Series 43^, 1, Am. Chem. Soc.

(1964).

5. M. K. Bernett and W. A. Zisman, "Wetting Properties of Acrylic

and Methacrylic Polymers Containing Fluorinated Side Chains", J. Phys.

Chem. 66, 1207 (1972).

6. M. K. Bernett and W. A. Zisman, "Hydrophobic and Oleophobic

Fluoropolymer Coatings of Extremely Low Surface Energy - Properties and

Applications", NRL Report 6039 (1964).

7. M. K. Bernett, "Solvent Effect on Wetting Properties of Highly

Fluorinated Polymers", I & EC Prod. Res. Develop. 13, 250 (1974).

8. C. H. Blachly, V. G. FitzSimmons and H. Ravner, "NRL-MB-20B:

An Improved Instrument Bail-Bearing Lubricant", NRL Report 7087 (1970).

175
DISCUSSION

A. J. Babecki, NASA, Goddard Space Flight Center : Can you tell me the

difference in surface energy between methacrylic polymers and Teflon?

M. K. Berneii, Naval Research Laboratory : The surface energies for the

barrier film and Teflon are about 10 dynes and 18 dynes, respectively.

Therefore the difference is 8 dynes.

A. J . Babecki : One of the problems we have in the spacecraft business

is the contamination of mirrored surfaces by condensed outgassing of

polymeric or organic molecules. I was wondering if a very thin barrier

film on a mirrored surface would preclude that sort of condensation?

H. Ravner : I don't think it would preclude condensation on the

surface. If condensation occurred on the barrier film, it would

ball -up and rol] off.

A. J. babecki : We really don't get enough condensation to form liquid

droplets. It is a haze that forms on the surface of the mirror.

H. Ravner : The barrier film would reject the droplets in the sense

that they would not spread. They would remain as droplets balled up.

What would happen to ihem afterwards depends on the configuration of

the device.

A. J. Babecki : Have you done any work with reflectance measurements?

H. Ravner : No

G. H. Kitchen, Bell Telephone Laboratories : First, I want to tell you

how grateful we at the Bell System are for the years of satisfactory

performance we have had with this material. One of the things it has

enabled us to do is design around things that creep rapidly such as silicone fluid. I guess I am one of the people responsible for

changing from hexafl uoro xylene to Freon TF. The reason for the change

is that Freon TF is much faster drying than the xylene-hex. We must

mass produce these parts; we make about one million contacts a week

using this process. Although the xylene-hex is less volatile, its use was questionable with OSHA.

H. Ravner : I appreciate your comment. A too volatile a solvent cannot

be used for barrier coatings because the film itself is not physically

a good film. It does not adhere well, it is not coherent, it wrinkles

up, etc. It can be cut back with a less volatile solvent.

17^
J. E. Stern, NASA, Goddard Space Flight Center : You have used a barrier film with many fluids and lubricants. What has been the

reaction of the barrier film to fluorinated lubricants like Krytoxes?

H. Ravner : Barrier films are marginal with the Krytox materials and

other fluorinated materials. These are the only materials in which

there was some question as to usability.

J. E. Stern : How do you determine whether a treated bearing does not

have a film on the raceways or on the balls?

H. Ravner : The bearings must turn with very little applied torque. So

they are examined on a more or less routine basis to make sure that,

after they are lubricated, they are, in fact, wetted by the lubricant

and there is no debris which would foul up their use once they are installed.

G. Hurt, Detroit Diesel, Allison Division : What is the largest size

bearing that can be coated?

H. Ravner : A combination of bearing size and speed are the limiting

factors in the use of the barrier film. Miniature bearings are lubricated with a very small amount of lubricant, just a couple of

milligrams. It wets the surface completely. Because the diameter is

small, the centrifugal force that would tend to drive the lubricant

away is not as large as it would be for a larger bearing rotating at

the same speed. In a larger bearing, there would be liquid lubricant

rather than just the thin film applied to small bearings. You can't

have too large a bearing with too much lubricant rotating too rapidly.

That is why, for the most part, the Navy experience has been limited to

precision bearings which are 3/8 in in diameter.

R. G. Brown, David Taylor Naval Ship Research and Development Center : We have run tests that indicate that barrier films are temperature

dependent- -at least in the 200-250 F range.

H. Ravner : You're absolutely right. I should have indicated that

these are not high temperature films. Two hundred degrees C would be

too much for them. At about 150 C, they could probably last almost

indefinitely. The temperatures to which they are exposed in the

applications I was talking about are, for the most part, temperature

controlled.

R. G. Brown : In the larger bearing sizes, the bearing itself generated

enough heat to cause a rather short life.

175
p. Weinberg, Naval Air Systems Command : Maybe I can elaborate on one of the operational procedures. After they are coated with lubricant,

all the bearings are checked for wetting.

G. H. Kitchen : Nancy Kitchen and Bud Russell of Bell Laboratories

reported at last year's ACS meeting on the effect of temperature on the

lifetimes of barrier films. At 160 C, the lifetime is measured in

days, at 120 C, months, and at 100 to 80 C, something on the order of 15

years. This barrier material is not a barrier against gravity flow of

fluids. It was never intended to be that. When a bearing is pumped up

and the fluid is heated so that the lubricant is thinned, and there are

a number of G's of centrifugal force developed, the lubricant will move over the barrier every time. So all you have to do is use common

sense, because it is an excellent material.

176
POTENTIAL OF SPUTTERED COATINGS FOR

IMPRO'VED GAS BEARING INSTRUMENT PERFORMANCE

Maj, L.L. Fehrenbacher , B. D. McConnell, and Capt. C. Pellerin

AIR FORCE MATERIALS LABORATORY, Wright-Patterson AFB , OH ^5^33

K. R. Mecklenburg

Midwest Research Institute

Kansas City, MO 6iillO

ABSTRACT

Sputtered coatings were applied to I-J+OO "beryllium substrates to

determine and eventually optimize their friction and wear performance

relative to their potential for gas bearing inertial instrument applications. Solid lubricant-carbide basecoat combinations were sputter

deposited under varying conditions of substrate bias and thicknesses.

The surface morphology and chemistry of the coatings were characterized

prior and after friction and wear evaluations using scanning electron

microscopy and scanning Auger microscopy techniques. The friction and

wear tests consisted of sliding a 1/8 inch TiC ball under load on the

coated disc specimens rotating at 60 RPM. MoS2-Sb20o solid lubricants

were deposited on TiC, WC and Bi^C sputtered undercoats at varying thicknesses and sputtering conditions. The solid lubricant combinations

exhibited consistent wear life behavior showing moderate and strong

dependency on the substrate bias and solid lubricant thickness, respectively, regardless of the underlying substrate. Correlations between

coating chemistry, morphology, sputtering conditions and friction and

wear performance are given.

Key words: Antimony trioxide; friction; molybdenum disulfide; spin

axis gas bearings; sputtered coatings; surface characterization;

wear life.

Introduction

The sputter deposition of solid lubricant materials to a wide

variety of sliding, oscillating and rolling machine elements is growing

rapidly as a result of the intrinsic advantages of this method of application. (Reference 1, 2 & 3). In comparison to conventional bonded or burnished lubricant materials, this technique offers improved adherence,

uniform thickness, higher density, better particle-to-particle bonding,

lower debris generation during wear and, of great practical significance,

the ability to readily coat curved and complex shaped surfaces. Traditionally, inertial instrument gas bearings have depended on the use of

hard, wear resistant materials (oxides, carbides, metals) or boundary

film lubricants such as beryllium stearates and chromium behenates to

maintain the structural integrity of gyroscope spin bearings surfaces.

Laborious machining, lapping, and application procedures, porous coatings,

adherence, debris generation and lubricant degradation are problems

177
common to these two approaches.

Scope

Sputtered coatings were applied to I-UOO beryllium substrates

(circular disc specimens) to determine and eventually optimize their

friction and wear performance relative to their potential for gas bearing

inertial instrument applications. Solid lubricant - carbide basecoat

combinations were sputter deposited under varying conditions of substrate

bias and at varying thicknesses. The surface morphology and chemistry

of the coatings were characterized prior to friction and wear evaluations

via scanning electron microscopy and scanning Auger microscopy. The

friction-wear test conditions consisted of sliding a 1/8 inch (3.2mm)

Tie ball under a 15 gram load on the coated disc' specimen that was rotating at 60 RPM. The sliding velocity was 1.25 in/sec (31-8mm/sec)

at an initial contact stress of 108,000 psi (tUU.O MN/m^). All frictional wear measurements were made in air under ambient conditions.

The common methods of determining wear, such as measuring wear

scar width and/ or depth, were considered insensitive due to the narrow

width and thinness of the initial coatings. Coating life was based on the time to reach a friction coefficient of 1.25 x the normal operating

coefficient, at which time the experiment was terminated. The onset of

failure based on this 1.25 value was characterized by a significant

increase in the variation of friction coefficient. The various coating

combinations, sputtering parameters, and thicknesses are shown in Table

1. The wear tracks of the coated discs were subjected to post test

chemistry and morphology evaluations. As reflected in Tables 1 and 2,

phase 1 results involved the evaluation of MoS2-Sb202 solid lubricant

top coating over I-UOO beryllium, tungsten carbide, titanium carbide

and reactive TiC undercoats for low and high substrate bias conditions

at two thicknesses (for each bias condition).

Phase 2 involved testing of M0S2 sputtered over TiC with low

substrate bias at three thicknesses. (All thicknesses are +_ 20^ the

reported values in Tables 1 and 2)

Results

The MoS2-Sb203 top coat combinations exhibited consistent wear life

behavior irrespective of the underlying substrate. In all cases, for

an identical bias condition, the thick solid lubricant coatings

exhibited better than 2 times the wear of the thin coatings. Similarly,

for a constant solid lubricant thickness, the low substrate bias

yielded wear lifes 1.5 to 2 times greater than the high bias coatings.

However, as shown in Table 1, the thickness of the MoS2-Sb202 coating

was more influential to the life of the coating than the degree of bias

of the substrate.

The dynamic friction coefficients correlated directly with substrate bias and wear life performance, yielding lower values with the

long wear lives of the low bias condition. However, the change in

friction coefficient with thickness of the MoS2-Sb20-^ coatings at a constant bias was insignificant for the various carbide base coated

samples and markedly lower for the thicker non-carbide coated (uncoated

Table 2) specimens.

178
TABLE I

WEAR PERFORMANCE OF SPUTTERED COATED

GAS BEARING SPECIMENS

Uncoated

WC

Tic

Phase 1

MoS2-Sb202 Lubricant

Base Coat Substrate Bias

High

Low

High

Lov

High

Low

Thin (5.0)

Wear time, min.

8.9

12.6

8.5

16.3

*Short Life (2 gm,

5 min)

20.8

Lubricant Coating

(Microinches

)

Thick (10.0)

Wear time, min.

22

k9

22.6

59.^

52.2

* Long Life (2

gm, 28 min)

No failure

Tie Reactive High

Low

12.8

28.7

26

51.

U

Tic (Mi croinches

)

h . 5 Low

9.0 Low

Phase 2

MoSg Lubricant

Thickness (Microinches)

1.0 ^.0 13.0

.8 7-5 18.3

1.1 1+ . 7 11 . ^

*Tested with cylinder instead of TiC ball

179
TABLE 2

DYNAMIC FRICTION OF SPUTTERED COATED

GAS BEARING SPECIMENS

Phase 1

MoS2-Sb203 Lubricant

Base Coat Substrate Bias Thin M0S2 Thick M0S2

Friction Coefficients

Start Low Finish Start Low Finish

Uncoated High Bias .187 .093 .200 .040 .029 .072

Low Bias .061 .027 .069 .016 .016 .048

WC High Bias .125 .112 .144 .128 .120 .163

Low Bias .093 .040 .104 .107 .085 .133

Tie High Bias CO. 14) (0.05) (.20) .213 .173 .218

Low Bias .147 .096 .149 (0.1) (.07) (0.1)

failure

Tie High Bias

Reactive

Low Bias

.181

.131

.149

.096

.203

.136

.128

.133

.100

.108

.143

.147

MoS

Phase 2 2 Lubricant

Thickness (Microinches)

Tie (Microinches)

1 5 15

Friction Coefficients

Start Finish Start Finish Start Finis:

4 . 5 Low .245 .693 .277 .501 .256 .395

9 Low .331 .693 .256 .400 .176 .363

180
Uncoated - 200 X WC - 200 X

Tie - 200 X reactive TiC - 200 X

Figure 1. Thick (lO y in^ ) MoS2-Sb202 d.c. sputtered on various

base coats under low substrate bias

l8l
The M0S2 sputtered coatings exhibited higher friction coefficients

and considerably shorter wear lives in air than the MoS2-Sb202 coated

samples. In contrast to the MoSg-Sb^OS coatings, however, was the

decrease in dynamic friction coefficients with increasing thickness of

M0S2 (Phase 2, Table 2).

Chemistry-Morphology Characterization

Although the chemistry surface morphology characterization of pre

and post wear tested specimens is incomplete, some observations relating

the friction-wear performance and the coating chemistry-surface

morphology are offered. The consistent friction and wear behavior of

MoS2-Sb202 discs for identical sputtering conditions irrespective of

the underlying substrate is not surprising in light of the similarity

in surface appearance as shown by the scanning electron micrographs

(SEMS) of Figures 1 and 2. The integrity and smoothness of sputtered

coatings in the wear groove area is typified by the SEM in Figure 3.

Of special interest is the highly phase separated appearance associated

with the low substrate bias condition. Although the precise chemical

composition of the lighter reflecting phase in Figure 1 is iinknown

,

energy dispersive x-ray and scanning Auger analyses have shown that the

particles contain primarily antimony.

The experimental scanning Auger data is too preliminary to offer

an explanation for the lower friction-longer life behavior of highly

phase separated low bias condition. The answer, undoubtedly, resides

in the slight variations in coating chemistry and adherence that result

from sputtering with high and low biased substrates. The marked de- crease in friction coefficient with increasing MoS2-Sb20 coating

thickness (See Table 2, Phase 2) for the uncoated specimens is probably

the result of the greater influence of surface asperities between the

Tic ball and the beryllium substrate for the veiy thin solid lubricant

coatings

The higher friction-shorter life and thickness dependence of

friction coefficient for the M0S2 sputtered discs relative to the

behavior of the MoS2-Sb202 specimens is probably indicative of the greater degenerative influence of moisture in the air on M0S2 (Reference k)

Future Work

Extensive scanning Auger analyses are currently underway to determine the relation between sputtering parameters , coating chemistrymorphology and friction-wear performance. Included is concentration

profiling of the individual chemical elements in and outside the wear

grooves to determine the mechanical and chemical nature of wear. Future friction-wear testing will include an evaluation of l) the

effect of atmosphere (vacuim testing) on the relative behavior of

MoS2-Sb202 and MoSg specimens; 2) the effect of thickness of TiC and

WC hard coating only (TiC and WC) ; 3) The influence of substrate bias

and thickness for the best solid lubricant-carbide coating combination

based on the results of l) and 2) above.

182
»

I

Tic 200 X reactive TiC 200 X

Figure 2. Thick (lO y ins) MoS2-Sb202 d.c. sputtered on various

base coats with high substrate bias.

185
Conclusions

The capability to reproducibly sputter deposit solid lutricant

metal carbide adherent films of high density, controlled thickxiess

and surface properties has been demonstrated. Further, the excellent

friction and wear performance of these sputtered coatings under severe sliding conditions in air indicates outstanding potential for spin

axis gas bearing applications, especially with the optimization of

sputtering deposition parameters.

Figure 3. Wear groove on thin (5 y ins) MoSg-Sb^O^ sputtered

coating after 20.8 minutes under slidxng friction

loading

18U
REFEEENCES

Spalvins, T. , "Bearing Endiirance Tests in Vacuum for Sputtered

Molybdenum Disiilfide Films," NASA-TM X-3193, Januaiy 1975.

Stupp , B.C., "Sputtering and Ion Plating as Industrial Processes,"

presented at the SAE Automobile Engineering Meeting, May 1U-I8,

1973, Detroit, Mich; Preprint No. 7305^7-

Gardos , M.N., "Quality Control of Sputtered M0S2 Films, presented

at the 30th Annual Meeting ASLE, May 5-8, 1975, Preprint No.

75AM 2D-2.

Winer, W.O: MoSg as a Lubricant: A Review of the Fundamental

Knowledge. Wear, Vol 10, I967 pp li22-l+52.

185
DISCUSSION

K. A. Taylor, C. S. Draper Laboratories, Inc. : Do you have any feel

for the substrate surface finish as related to the wear test results?

L. F. Fehrenbacher : We didn't vary the surface finish. Other

investigators have found that for sputter deposited coatings, wear

lives improve with the fineness of the surface finish on the substrate.

I'm referring particularly to I'iike Gardos' data. The surface finish on these substrates was in the 3 to 4 yin range. I think surface finish

would have an effect.

K. A. Taylor : I think it would in the case of the 1-400 where the

beryllium oxide content is high.

L. F. Fehrenbacher : We put a solid lubricant coating on the beryllium

itself and the friction coefficient did decrease as a function of the

thickness of the solid lubricant coating. When a carbide undercoat is

added to the beryllium substrate, a better surface finish diminishes

the influence of the lubricant thickness.

186
FERROGRAPHY OF POLYMER FORMATION UNDER LUBRICATED RUBBING

CONTACT --AN EXPLORATORY INVESTIGATION

A. A. Reda

Foxboro/Trans -Sonic s, Inc.

Biirlington, Massachusetts OI805

Ferrographic analysis of lubricant samples from a variety of

operational machines and test rigs has revealed the presence of non- metallic particles. Some of these particles have characteristic shapes

and are composed of polymers. For example, under relatively high load

nonmetallic particles shaped like rolling pins were formed when a lubricant per specification Mil-L-23699 was used, (2) The same particles

were produced when polyphenyl ether was used as a lubricant . C^)

It was speculated that under the conditions of the test the lubricant

forms a polymeric film at the rubbing surfaces and it is rolled in the

shape of rolling pins when it is pealed off, Furey(3) has shown that

polymer films are formed directly on rubbing surfaces and that these

films are a factor in reducing wear. Furthermore, it is well known

that different additive packages in lubricants meeting the same specifications lead to different wear rates under identical load

conditions. It was considered useful to determine if Ferrography could

provide further information on the phenomena and to assess if Ferrographic analysis could be used to study the performance of lubricants.

Experimental Procedure

Experiments were carried out utilizing a ball-on-cylinder wear machine

described elsewhere, (4) It consists, essentially, of a lathe adapted

to hold a ICQ mm diameter AISI 52100 steel sleeve between the head and

tail stock and a deadweight loading arm holding a 25,4 mm diameter AISI

52100 steel ball.

Tests were carried out on two different ester lubricants at five

different loads of 25, 80, 150, 250 and 400 lbs, and at a constant

speed of 92 rpm for three hours

,

The lubricants designated A and E manufactured to specification

Mil-L-23699 contained different additive packages, designed for normal

and high loads respectively,

Ferrograms were prepared from lubricants at the end of each test. The

wear scars on the balls were carefully examined by optical microscopy

immediately after each test and again after cleaning to remove the

lubricant. Wear was assessed by the diameter of the wear scar. (Table 1)

187
Table 1

Load (lb.)

SUMMARIZED WEAR RESULTS

Diam, (mm) of Wear Scar

Lubricant A Lubricant E

25

80

150

250

400

0.45 0.7

2.2 1.3

3.4 1.7

1.5

1.2

From the results, less wear was experienced at low load (25 lbs.) with

lubricant A than with lubricant E, .45 and .70 mm diameter wear scars were seen, respectively. At higher loads there was more wear with

lubricant A than with lubricant E, for example the wear scars seen at

400 lbs. load were 3.4 mm and 1.7 mm respectively.

Ferrosraphic Examination

At a load of 25 lbs. Ferrograms prepared from lubricant A and E showed

very few polymer particles. The particles contain small metallic

particles embedded in them (Fig. 1). As the load increased to 150 lbs. an increased number of the polymer particles were observed along with a few rolling pins on Ferrograms prepared from oil E, while in the case of

oil A the pol3mier particles were not present but a new type of polymer

was present. The polymer was present as a mud like deposit having the

appearance of a dry clay bed. This material is referred to as polymud.

As the load increased to 400 lbs. oil A produced increased amounts of

polymud while oil E produced more of the flat polymer particles along

with some deposit of pol3nniud.

Surface Examination

Examination of the wear scars produced under the various test con- ditions revealed, in the case of lubricant A, surfaces having a relatively rough appearance. However, lubricant E produced comparatively

smooth surfaces, even at high load 150 lbs., and revealed patches of

polymeric material (Fig. 2). The polymeric material could be easily

scored and removed with a cotton swab soaked in organic solvent.

C onclus ion

As the difference between the lubricants tested was the additive

package, this probably accounts for the different wear performances

observed. Apparently the mechanical properties of the friction polymer

are influenced by the additive package. In the case of oil E, under

heavy load the polymeric film remained intact and was strong enough to

be rolled up. Under the same loads, using oil A the poljrmeric products

of friction were of indefinite shape and precipitated as a mud.

188
Fig. 2 Optical photomicrograph of a wear scar (X1120) on the 52100

ball, with patches of polymeric film.

189
Acknowledgements

The author is indebted to Dr. E. C. Van Rueth and the Advanced

Research Project Agency, U. S, Dept. of Defense, for technical and

contract support under contract N000-14-74-C-0135. He also expresses

appreciation to Dr. R. Miller of the Office of Naval Research, who

administered the contract, to Mr, Douglas Scott and the National

Engineering Laboratory, East Kilbride and Mr. V. C. Westcott of

Foxboro/Trans-Sonics , Inc. for valuable assistance.

References

1, W. K, Jones, Ferrographic analysis of the wear debris from

boundary lubrication experiments with a five ring polyphenyl

ether , ASLE Trans (1974)

2, A. A, Reda, A note on the investigation of friction polymer

rolling pin formation , WEAR 32 (1975) 115

3, M, J. Furey, The formation of polymeric films directly on rubbing

surfaces to reduce wear, WEAR 26 (1973) 369

4, A, A. Reda, R, Bowen and V. C. Westoctt, Characteristics of

particles generated at the interface between sliding steel surfaces,

WEAR 34 (1975) 261

190
DISCUSSION

W. J. Kelly, Hughes Helicopters : There is a large difference between

lubricants A and E. How can both lubricants meet specifications?

A. A, Reda : There is no test to differentiate between them.

W. J. Kelly : We have found that the MIL-L-23699 oil changes color

considerably in the atmosphere. It starts out as a light pink color

and it will turn dark.

A. A. Reda : The oil that I used turned completely black in about six

months.

191
MECHANICAL FAILURES PREVENTION GROUP

23rd MEETING

The Role of Coatings in the Prevention of Mechanical Failures

List of Registrants

Morton Antler

Bell Telephone Labs

600 Mountain Avenue

Murray Hill, NJ 07974

Alfred J. Babecki

NASA/Goddard

1405 Colony Road

Oxon Hill, MD 20021

H. Berger

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

M. K. Bernett

Naval Research Lab

Washington, DC 20375

J. M. Blocher, Jr.

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

Richard L. Bourassa

Keyes Fibre Company

Operations Building

College Avenue

Waterville, ME 04901

R. Gregory Brown

Naval Ship R. & D. Center

Annapolis, MD 21402

Robert D. Brown

Southwest Research Institute

8500 Culebra Road

San Antonio, TX 78284

M. F. Browning

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

H. C. Burnett

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

William F. Byrnes

Naval Air Engineering Center

GSED, Code 92724

Lakehurst, NJ 08733

Stanley W. Cross

Pratt & Whitney Aircraft Product

Support SWBI

400 Main Street

East Hartford, CT 06108

C. Dayson

National Research Council of Canada

3904 West 4th Avenue

Vancouver, B. C. Canada

Evan J. Deemer (ASCR 337)

Naval Air Systems Command

Representative, Atlantic

NAS Norfolk

Norfolk, VA 23511

George E. DeLong

Naval Air Systems Command

Representative, Pacific

NAS North Island

San Diego, CA 92135

192
K. E. Demorest

NASA/Marshall Space Flight Ctr.

Huntsville, AL 35812

M. J. Devine

Naval Air Development Center

Aero Technology Dept. Code 30-7

Warminster, PA 18974

Major L. F. Fehrenbacher

USAF Materials Lab.

Wright-Patterson AFB, OH 45433

P. M. Fleming

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Mrs. Maj-Britt K. Gabel

Naval Air Development Center

Warminster, PA 18974

Robert M. Giannini

David W. Taylor Naval Ship R. & D. Center (Code 715)

Employee Development Division

Bethesda, MD 20084

R. G. Gil lis

Embassy of Australia

1601 Massachusetts Avenue, NW

Washington, DC 20036

W. A. Glaeser

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

Nathan Glassman

David W, Taylor Naval Ship R. & D. Center

Annapolis, MD 21402

Walter W. Gunkel

General Motors Corporation

Product Assurance, EAS

GM Technical Center

Warren, MI 48090

Peter Hal lick

Technical Assistant, AFS-803

Federal Aviation Administration

800 Independence Avenue, SW

Washington, DC 20591

A. J. Haltner

General Electric Space Division

P. 0. Box 8555

Philadelphia, PA 19101

Walter F. Hays, III

Westinghouse Electric Corporation

A-304

P. 0. Box 9175

Lester, PA 19113

Henry R. Hegner

GARD, Inc.

7449 North Natchez Avenue

Niles, IL 60648

A. A. Hochrein, Jr.

Daedalean Associates, Inc.

15110 Frederick Road

Woodbine, MD 21797

J. D. Hoffman

Director, IMR

National Bureau of Standards

Washington, DC 20234

E. Horowitz

Deputy Director, IMR

National Bureau of Standards

Washington, DC 20234''

Jack Hubacher

Value Engineering Laboratory

2550 Huntington Avenue

Alexandria, VA 22303

George Hurt

Detroit Diesel Allison Division

P. 0. Box 894

Indianapolis, IN 46201

193
Said Jahanmir

MIT

77 Massachusetts Avenue

Cambridge, MA 02172

C. E. Johnson

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

R. B. Johnson

Executive Officer, IMR

National Bureau of Standards

Washington, DC 20234

S. A. Karpe

David W. Taylor Naval Ship R. & D. Center

Annapolis, MD 21402

William J. Kelly

Hughes Helicopters

Centinela Avenue at Teale* Street

Culver City, CA 90230

George H. Kitchen

Bell Telephone Laboratories

600 Mountain Avenue

Murray Hill, NJ 07724

E. E. Klaus

Pennsylvania State University

133 Chemical Eng. Building

University Park, PA 16802

Werner H. Knecht

W. German Fed. Bureau of Mil.

Technol ogy

18 Am Kurpark

5420 Lahnstein, W. Germany

Ben A. Kornhauser

National Materials Advisory Board

National Academy of Sciences

2101 Constitution Avenue

Washington, DC 20418

J. Kruger

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Laurence Leonard

Franklin Institute

Philadelphia, PA 19103

F. F. Ling

Rensselaer Polytechnic Institute

Mechanics Division

Troy, NY 12181

Kenneth C. Ludema

University of Michigan

Mechanical Engineering Department

East Engineering Building

Ann Arbor, MI 48104

J. R. Manning

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

R. E. Maringer

Battel le Columbus Labs

505 King Street

Columbus, OH 43201

W. A. Martin

Ontario Hydro

800 Kipling Avenue

Toronto, Ontario

M8Z 5S4 Canada

B. D. McConnell

USAF Materials Lab.

Wright-Patterson AFB, OH 45433

William J. McNair

Federal Aviation Admin., AFS-200

800 Independence Avenue

Washington, DC 20591

19U
K. R. Mecklenburg

Midwest Research Institute

Kansas City, MO 64110

J. Messina

Frankford Arsenal

Philadelphia, PA 19137

Lt. Richard S. Miller

Office of Naval Research

Code 211

Arlington, VA 22217

R. Misialek

Naval Ship Eng. Center, Code 6762

Philadelphia Div., Naval Base

Philadelphia, PA 19112

Lt. Colonel Richard A. Mollicone

Air Force Materials Lab/NA

Wright-Patterson AFB, OH 45433

Joseph L. Morrossi

AMNRC

Watertown, MA 02172

R. W. Moss

Battel le. Northwest

231-Z Bldg. , 200-W Area

Richland, WA 99352

D. W. Moyer

Pure Carbon Company

St. Marys, PA 15857

F. Ogburn

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

E. Passaglia

Chief, Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Capt. C. Pellerin

Air Force Materials Lab.

Wright-Patterson AFB, OH 45433

Thomas L. Peters

All is Chalmers Corporation

Dept. 3322 GB, Box 512

Milwaukee, WI 53201

M. B. Peterson

Wear Sciences, Inc.

32 Sutherland Drive

Scotia, NY 12302

Bernard L. Poppert

Naval Air Systems Command

AIR-340E

Washington, DC 20361

Gordon W. Powell

Professor, Dept. of Met. Engr.

Ohio State University

490 Watts Hall

Columbus, OH 43201

E. Rabinowicz

Massachusetts Institute of Tech.

Department of Mechanical Engr.

Room 35-010

Cambridge, MA 02139

Harold Ravner

Naval Research Lab.

Washington, DC 20375

A. A. Reda

Foxboro/Trans-Sonics , Inc.

Wheeler Road

Burlington, MA 01803

A. W. Ruff

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

W. H. Safranek

Battelle Columbus Labs.

505 King Avenue

Columbus, OH 43201

195
Donald J. Sargent

E/M Lubricants, Inc.

P. 0. Box 2200, Hwy. 52 NW

West Lafayette, IN 47906

Fritz K. Sautter

Watervl iet Arsenal

Watervliet, NY 12189

Peter B. Senholzi

Naval Air Eng. Center

GSED/NAEC, Code 92724

Lakehurst, NJ 08733

John J. Sherlock

Pure Carbon Company, Inc.

441 Hall Avenue

St. Marys, PA 15857

T. R. Shives

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Charles 0. Smith

University of Detroit

College of Engineering

4001 W. McNichols

Detroit, MI 48221

Henry G. Smith

Engineering Consultant

201 S. Francisco Avenue #1

Redondo Beach, CA 90277

David Snediker

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

Charles L. Staugaitis

NASA/Goddard Space Flight Ctr.

Glen Dale Road, Code 755

Greenbelt, MD 20771

Jesse E. Stern

NASA/Goddard Space Flight Center

Greenbelt, MD 20783

N. P. Suh

Massachusetts Institute of Tech.

Cambridge, MA 02139

Kenneth A. Taylor

C. S. Draper Lab. , Inc.

68 Albany Street

Cambridge, MA 02139

William G. Thelen

David W. Taylor Naval Ship R. & D. Center

Employee Development Division

Code 715

Bethesda, MD 20084

Alagu P. Thiruvengadam

Daedalean Associates, Inc.

15110 Frederick Road

Woodbine, MD 21797

R. M. Thomson

Assistant for Scientific Affairs

National Bureau of Standards

Washington, DC 20234

M. Tobin

Westinghouse Electric Corporation

Astro Nuclear Lab.

P. 0. Box 10864

Pittsburgh, PA 15236

G. A. Uriano

Institute for Materials Research

National Bureau of Standards

Washington, DC 20234

J. B. Wachtman

Inorganic Material Division

National Bureau of Standards

Washington, DC 20234

196
Philip Weinberg

Naval Air Systems Command

AIR-52032E

Washington, DC 20361

W. A. Willard

Metallurgy Division

National Bureau of Standards

Washington, DC 20234

Wilbur J. Wilson

Battel le Columbus Labs

505 King Avenue

Columbus, OH 43201

William H. Womer

Naval Air Engineering Center

GSED, Code 92724

Lakehurst, NJ 08733
NBS-1 14A (REV. 7-73)

U .S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 452

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLF AND SUBTITLH

The Role of Coatings in the Prevention of Mechanical FailuresProceedings of the 23rd Meeting of the Mechanical Failures

Prevention Group

5. Publication Date September 1976

6. Performing Organization Codei

^i^yjmm^ editors

T. Robert Shives and William A. Willard

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS Institute for Materials Research, Wash., DC 20234; Office of

Naval Research, Arlington, VA 22217; Naval Air Development Ctr.,

Warminster, PA 18974; Frankford Arsenal, U.S. Army, Philadelphia,

PA 19137; Federal Aviation Administration, Wash., DC 20591

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

I Library of Congress Catalog Card Number: 76-608258. !

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant I bibliography or literature survey, mention it here.) These Proceedings consist of a group of eighteen submitted papers and discussions

from the 23rd Meeting of the Mechanical Failures Prevention Group which was held j at the National Bureau of Standards in Gaithersburg, Maryland on October 29-31, [ 1975. The central theme of the Proceedings is the Role of Coatings in the i Prevention of Mechanical Failures, with emphasis on polymer coatings, metal ! coatings, solid film lubricants, and chemical coatings.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

Barrier film; chemical coatings; failure prevention; mechanical failure; metal

coatings; plated coatings; polymer coatings; solid film lubricants; sputtered

coatings; wear

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