I

AlllDS T73EMD PUBLICATIONS

u.o. C5B=7TRTMENT O

National Institute of Standards and Technology

(formerly National Bureau of Standards)

NIST Special Publication 744

Formation of Lubricating Films at

Elevated Temperatures From the

Gas Phase

E. E. Klaus, J. L. Duda, and S. K. Naidu

LOO

N2 PLUS

LUBRICANT VAPOR

PRE-HEAT DEPOSITION

OVEN OVEN

DOE ECUT

Tribology Program

R. G. Munro and S. M. Hsu,

Editors and Project Leaders

NIST Special Publication 744

Formation of Lubricating Films at

Elevated Temperatures From the

Gas Phase

E. E. Klaus, J. L. Duda, and S. K. Naidu

Department of Chemical Engineering

The Pennsylvania State University

University Park, PA 16802

R. G. Munro and S. M. Hsu, Editors and Project Leaders

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Gaithersburg, MD 20899

for

Department of Energy

Office of Energy Conversion

and Utilization Technology

Washington, DC 20585

NOTE: As of 23 August 1988, the National Bureau of Standards (NBS) became the National Institute of Standards and Technology (NIST) when President

Reagan signed into law the Omnibus Trade and

Competitiveness Act.

September 1988

U.S. Department of Commerce

C. William Verity, Secretary

National Institute of Standards and Technology

(formerly National Bureau of Standards)

Ernest Ambler, Director
Library of Congress

Catalog Card Number: 88-600554

National Institute of Standards

and Technology

Special Publication 744

97 pages (Sept. 1988)

CODEN: XNBSAV

U.S. Government Printing Office

Washington: 1988

For sale by the Superintendent

of Documents,

U.S. Government Printing Office,

Washington, DC 20402
CONTENTS

Page

Preface v

Executive Siimmary vii

I . Introduction 1

II. Experimental Equipment and Test Procedures 2

A. Vapor Deposition Apparatus 2

B. Dynamic Wear Testing With a Four-Ball Wear Tester 7

III. Vapor Deposition Studies 8

A. Tricresyl Phosphate (TCP) 8

Effect of Substrate and Temperature 8 Effect of Lubricant Vapor Concentration 13

Effect of Carrier Gas Concentration 16

SEM and EDAX Analysis 21

B. Diphenyl Dltertiarybutylpheny 1 Phosphate (GT) 37

C. Polyphenyl Ether (PPE) 42

D. Trimethylolpropane Triheptanoate (TMPTH) 49

E. Comparisons of Deposition Rates of Phosphate Esters

and Non- Phosphate Hydrocarbons 64

F. Deposition Studies With Evaporating Liquid

Delivery System 65

IV. Wear Studies of Vapor-Deposited Lubricant Films 69

A. Phosphate Esters (TCP, TBP, and GT) 69

B. Non-Phosphate Hydrocarbons (PPE and TMPTH) 75

V. Conclusions 83

IV. References 85

Appendix A. Cooperative Efforts Based on High-Temperature

Lubrication Studies at The Pennsylvania State

University 87

Appendix B. Publications, Theses and Presentations Based on HighTemperature Lubrication Studies at The Pennsylvania

State University 88

iii

PREFACE

The U.S. government and industrial enterprises pay a high cost, on the

order of $100 billion annually, as a result of losses to friction and wear. These losses are incurred in many forms including energy expended by

friction, direct and indirect losses due to discarded materials, and lost

productivity. Efforts to understand and reduce these losses require an interdisciplinary research effort involving chemistry, physics, materials

science, mechanical engineering, and a wide range of chemical and

materials property characterization capabilities. These capabilities are blended in the rapidly evolving field of tribology, the science of friction

and wear and the control of those phenomena through lubrication.

In 1980, the U.S. Department of Energy (DOE) began a new program, the

Energy Conversion and Utilization Technologies (ECUT) program, that

included tribology as one of its principal activities. The ECUT program as

a whole was designed to improve the rate at which technological advances

made in basic research are transferred to industry or other end-use

applications. In developing this new program, DOE had perceived that

there was a significant conceptual barrier between basic science and its

end-use applications. Basic science is exploratory and often deals with

highly constrained systems. Its objective is to understand fundamental

principles. End-use applications, in contrast, are devoted to specific

products and have economic gain as their primary objective. The emphasis

on different objectives caused communications between the two activities to

be cumbersome and slow. The ECUT program was established to reduce this

barrier by providing an interface between the two objectives in the form of

a new concept of "directed basic research." Within this concept,

developments in basic science would be utilized and supplemented by

additional research, conducted selectively, with the objective of

developing the new technology into a form that could be readily adapted to

specific industries.

Within the context of directed basic research, the ECUT Tribology

program was established to address the critical research issues relating to

reducing the enormous energy and economic losses promulgated by friction

and wear processes . The initial guidance for the program was provided by

two studies, conducted by the American Society of Mechanical Engineers,

entitled "Strategy for Energy Conservation through Tribology" and published

in two parts (1977 and 1981). These works were followed by several

DOE-sponsored studies that investigated and analyzed the specific

tribological losses in 11 different industries. It was estimated that

about three quadrillion BTU's of energy could be saved within these

industries with modest, feasible reductions in friction and wear. Most of

those reducible losses were concentrated in the transportation industry and

focused on engine technology. In that area, it was quickly found that the

dominant critical issue was lubrication.

V
Lubricants serve several purposes in engines . By forming a medium

between moving surfaces, such as between piston rings and cylinder liners,

friction between the moving parts can be reduced and efficiency enhanced.

The lubricating film also reduces the damage to the surfaces and thus

enhances the durability and wear life of the components. Further, a liquid

lubricant can remove wear debris from the tribocontact and thereby prevent

or reduce the three-body type of abrasion on the surfaces. All of these

functions are important to the successful operation of engines.

Advances in engine technology are mostly directed towards a more efficient use of the energy developed in the engine cycle. A prime target

for improving efficiency is the reduction of the energy loss that occurs by

way of thermal diffusion to the engine coolants and other engine parts.

Approximately one- third of the energy released in combustion is lost in

this manner. Designs for low heat rejection engines that would reduce this

energy loss, therefore, could achieve significant gains in efficiency.

As with conventional engines, a critical issue for these new engine

designs is lubrication. By losing less energy through diffusion, the low

heat rejection engines must operate at higher temperatures than occur in

conventional engines. Temperatures in the range 300°C to 600°C are expected in the near-term designs, and in the more distant future

temperatures as high as 1200°C are envisioned. Such temperatures are beyond the useful range of conventional lubricants. Indeed, conventional

lubricants have upper operating limits of only about 250°C. Consequently,

the technology base for lubrication must be expanded dramatically if the

advantages of the new engine designs are to be realized in practice. The

ECUT Tribology program is addressing this challenge on several fronts. It

is clear that the small incremental refinem.ents of lubricants traditionally

produced by the trial -and-error approach will not be adequate for

developing the new generation of high temperature lubricants. Thus,

the ECUT Tribology program is conducting fundamental studies of lubricating

mechanisms, particularly as they apply to the high temperatures of low heat

rejection engines. New measurement capabilities and methodologies are being developed to enable laboratory studies of mechanisms to be conducted

and to provide a means for testing and screening potential high temperature

basestocks and additives. Data acquired from these efforts are being used

to gain insights into the role of tribochemical interactions that occur between lubricants and substrates. These data are also forming the basis

for assessing the progress made towards achieving high temperature

lubricants

.

vi
New concepts in lubrication and lubricant delivery systems are also

being investigated. The following report discusses one of the more exciting alternatives to using liquid or solid lubricants: gas phase

lubrication. This new approach is being explored, in part, under ECUT

sponsorship via an NBS subcontract to Pennsylvania State University. This

report represents the results of the first phase of that research effort.

The accomplishments to date are very encouraging and currently are being

extended in a joint PSU-NBS study of gas phase lubrication of advanced

structural ceramics at high temperatures.

R. G. Munro

S. M. Hsu

DOE/ECUT Project Leaders

NBS Contract Officers

vii
EXECUTIVE SUMMARY

Conventional liquid lubricants, when subjected to temperatures of

250°C and above for extended time in an air atmosphere, degrade rapidly to

make large amounts of solid sludge and deposits. Based on boundary

lubrication of bearings, these same lubricants, when subjected to

temperatures from 250°C to the melting point of the bearing metal, produce

in the micro- to milli-second residence time in the bearing contact enough

"friction polymer" to result in good lubrication. This report describes

the use of these conventional liquid lubricants delivered in a homogeneous

vapor phase where the carrier gas is nitrogen, air, or mixtures of these

two gases. The lubricants studied include alkyl and aryl phosphate esters,

organic acid esters, polyphenyl ethers, and mineral oil.

A static deposition test was conducted in a tubular ceramic oven where

the lubricant-containing carrier gas contacted the metal substrate

resulting in a deposit resulting from a chemical reaction of the lubricant

vapor on the surface of the metal. The lubricant was introduced into the

carrier gas in an evaporation chamber where the pre -heated gas passed over

a fiberglass packing wet with the desired amount of liquid lubricant. The

temperature in the evaporation chamber was kept somewhat above that

required to maintain the desired concentration of the liquid in the carrier

gas. This temperature was determined by the vapor-pressure temperature

curve for the lubricant. Reactions of the lubricant in the gas phase

during delivery were minimized until contact was made with the solid

surface of the target metal or the bearing. Even under oxidizing

conditions , it has been shown with hydrocarbon vapors and oxygen that the

formation of complex gaseous or solid reaction products take place

primarily in the liquid phase or on a solid surface but not in the gas

phase

.

A second target for the gas phase lubricant was a dynamic four-ball

wear tester utilizing M50 tool-steel balls. These tests were conducted at

371 °C which was limited by the steel machine elements in the wear tester.

The static deposition tests were run over the range of 500 to 850°C using

stainless steel, iron, copper, and nickel as the metal targets with quartz

as a non-reactive control specimen.

The specimen coatings applied by the vapor-delivered lubrication

techniques are very uniform as measured by radio tracer techniques or the use of an SEM with a microprobe attachment. Photomicrographs taken with

the SEM also show uniform coatings. For convenience, the coatings are measured quantitatively with a seven-place electrobalance or a six-place

microbalance . The two-oven system used for static coating consists of one oven which is continuously swept with nitrogen gas for pre-heating or high- temperature soak of the coated specimen. The second oven contains a constant flow of lubricant vapor in the carrier gas at the desired test

temperature. The specimens to be coated can be transferred from one oven

to the other in less than a second. In this manner, coating times of as small as a few seconds can be achieved while specimens to be coated or after coating can be stored under nitrogen at the desired test temperature

viii
for as long as desired. Coatings are given in micrograms per square

centimeter which can be translated approximately into molecular layers of

coating thickness by multiplying the /ig/cm^ by 10. Coatings of up to

several hundred molecular layers (50 /ig/cm^ ) adhere well to the wire

specimen. When the coating thickness reaches a value of several thousand

molecular layers, the deposit develops sufficient thickness and integrity

so that differences in properties, such as coefficient of thermal

expansion, between the wire and the thick coating causes the coating to

crack on cooling to ambient temperatures. The coatings formed appear to be

solid at the temperature of application. The effect of the metal substrate

on the coating rate can best be demonstrated by a tabulation of the coating

achieved during the first 1 minute of deposition by tricresyl phosphate

(TCP), diphenyl-ditertiarybutyl phenyl phosphate (DPDTPP) , poly phenyl

ether (PPE) , and trimethylol propane triheptanoate (TMPTH) . These coatings

were all applied at 700°C from a nitrogen carrier gas containing between

1.0 and 1.75 mol percent of the lubricant.

Lubricant

TCP DPDTPP PPE TMPTH

Substrate (a)

Iron 290 430 19 127

Copper 210 280 6 1 Stainless Steel 110 45 0 8

Nickel 15 10 16 24

Quartz 15 10 2 1

(a) Coating thickness in the first minute is given in )ug/cm^ To convert

this value to molecular layers the multiplication factor is

approximately 10.

These data show that the deposited lubrication coatings are strongly

influenced by the nature of the surface in contact with the gaseous

lubricant. The phosphate esters generally show a low energy path

decomposition in the presence of transition metals. The PPE and TMPTH with

more of a hydrocarbon- like structure appear to be more sensitive to metals

that provide hydrogenation or dehydrogenation catalysts to the system.

Long time tests show that the initial catalytic effect of the metal

surface will diminish with increasing thickness of the coating. Finally,

at some coating thickness the catalytic effect will disappear and a common

coating rate similar to that on a non-catalytic surface like quartz will be

achieved. The rate of disappearance of this catalytic effect appears to be

related to a diffusion- like process which differs with both lubricant types

and metal substrate.

The deposition rate for a given lubricant will increase with

increasing lubricant in the vapor phase over a lubricant concentration

change of 0.1 to 2.0 mol percent. The rate of deposit will increase

substantially for a given concentration of lubricant over the temperature

range of 600 to 850°C. The magnitudes of these concentration and

temperature changes are shown in detail in the body of this report.

Ix
In addition to the static deposition tests, dynamic wear tests in a four-ball wear tester have also been conducted on vapor -delivered

lubrication. In this case, M50 tool-steel balls have been used for their

hot hardness characteristics. The wear specimens and the metallurgy of the

tester limited the temperature of this system to 371 °C. The standard

duration of the test was 30 minutes, and the lubricant was delivered to the

four-ball wear tester below the level of the sliding contact in the ball

pot using the same vapor -de livery system used for the static tests. In all

cases, the amount of lubricant in the carrier gas was below the vapor

pressure of the lubricant component for the delivery temperature which in

turn was well below the 371 °C test temperature. This delivery system

precludes the possibility of lubricant condensation on the balls in the

bearing area. An extrapolation of the deposit rate of lubricant on the

balls, by chemical reaction on the ball surface, indicates that the

maximum rate of lubricant application is of the order of a few molecular

layers per second. All of the lubricant types illustrated in the static

deposit mode were also run at 371 "C in the four-ball wear tester. The

results in the wear tester are excellent. For example, it can be shown

that TCP at 0.5 mol percent in nitrogen shows wear properties over a load

range of 5 to 50 kg that are better than those shown by a well-refined

mineral oil at 75°C in the liquid form over the same load range. A study

of wear versus TCP concentration, at 10 kg load and 371°C, shows a minimum

wear rate at 0.5% TCP in nitrogen with higher wear rates at higher TCP

concentrations. In the latter cases, the wear scars show evidence of too

much film formation at the higher concentrations leading to three-body wear

from the wear debris trapped in the lubricant coating. These studies show

significant wear protection below the 0.5% optimum concentration which

suggests that significant coating rates occur at concentrations as low as 0.1% TCP. It appears that the optimal wear condition occurs when the rate

of film formation is equal to the rate of film removal.

Vapor-phase PPE lubrication at 1.0% concentration produced less wear

in the four-ball wear tester than did liquid-phase PPE lubrication tested

at the same load at 75°C

These results show that vapor-phase lubricants at low deposition rates can be effective lubricants in dynamic wear tests at loadings typical of

those found in many operational bearing systems. Specific data on the

four-ball wear tester run at 371 °C are shown in the body of the report.

The study of overall lubricant deposit rate and stability at high

temperature suggests that adequate lubricant can be delivered at the

temperatures studied. The results with the four-ball wear tester show that

the amount of lubricant delivered is important in wear control. These data

show that a balance of deposition rate and removal rate in a dynamic system can be controlled. This control is necessary to minimize wear.

Wear tests at higher temperatures are needed to extend these results

from 371 °C to the temperature range where static tests have been evaluated.

At these higher temperatures, data from the static tests will improve our ability to demonstrate and to understand delivery rate versus removal rate

in dynamic testing.

X
I . INTRODUCTION

The purpose of this project was to investigate the feasibility of a new vapor phase lubricant-delivery system and its associated lubricants for

high- temperature applications. This work was initiated in response to the

need for advances in lubrication technology to achieve energy conservation

in the metal -fabricating and transportation industries. The Research

Committee on Lubrication of the ASME identified these two areas in an assessment conducted for the Department of Energy [1].

Better metal -forming capabilities to minimize machining and grinding

requires lubrication techniques and lubricants that can be used effectively

at temperatures approaching the melting points of the metals involved. In

transportation, one of the most productive areas for increasing energy

efficiencies is referred to as the adiabatic engine [2] . This trend means

that lubricants and lubricant delivery systems must be designed to

accommodate temperatures much higher than those used in conventional

high-performance engines. Problems are already evident in current

high-performance engines where lubricant life and, in some cases, engine

performance are limited by the amount of lubricant degradation at operating

conditions

.

Recent studies have begun to define the temperatures encountered in

conventional lubrication systems [3-9]. Temperatures range from those in

the bulk system (75-150°C) to boundary lubrication temperatures of 315 to

340°C. Conventional lubrication under these temperatures is delivered as a liquid saturated with air. Studies of lubrication chemistry over the

temperature range covered by bulk systems, elas tohydrodynamic bearings and

boundary lubrication systems show that oxidation reactions modified by

metal catalytic systems predominate [10-16]. An extension of these

reactions to higher temperatures results in excessive sludge and deposits.

Thus, successful lubrication at the high temperatures of interest (350-

1600°C) requires new concepts of lubrication and/or lubricant delivery

systems

.

The primary goal of this study is to investigate a new

high- temperature vapor phase lubricant delivery system and the associated

lubricant materials. In this system, a lubricating film is formed on a hot

metal surface from a homogeneous vapor phase exposed to the surface

.

Molecules from the vapor are adsorbed on the metal and react to form the

lubricant film. This process involves high- temperature thermal reactions

and thermal -oxidative combinations in the appropriate temperature region.

The vapor in these reactions consists of essentially two components: a carrier gas and the intended lubricant. The major ingredient of the

gas-phase mixture is the carrier gas, a chemically inert material such as nitrogen. In some cases, the thermal reactions will be enhanced by the

addition of oxygen to the carrier gas. The lubricant component is a volatile substance such as organic phosphate ester, polyphenyl ether,

organic acid esters, or a hydrocarbon.

1
II. EXPERIMENTAL EQUIPMENT AND TEST PROCEDURES

A. Vapor Deposition Apparatus

The apparatus used for producing the lubricant vapor and depositing it on a substrate surface is shown schematically in figure 1 . The equipment

consists of three sections: vaporization, deposition, and preheating. The

main criteria for the design and associated operating procedure are:

a) The lubricant should be vaporized quickly to eliminate reactions

at elevated temperatures in the liquid state.

b) The metal substrate should be preheated to avoid condensation and

liquid phase reactions.

c) The procedure should have the capability of making precise

measurements of the amount of deposit for precisely measured

periods of time.

d) The techniques should incorporate the means to control and vary

the following parameters: metal surface temperature, vapor

temperature, lubricant concentration, vapor flow rate, and

carrier gas composition.

The vaporization apparatus is shown in more detail in figure 2. It

consists of three stainless steel tubes (0.794 cm o.d.) fitted with a T

joint. The joint is filled with a glass packing to increase the surface

area available for lubricant vaporization. In this section lubricant vapor

is produced and then transported to the deposition section by a carrier

gas. The carrier is either an inert gas (nitrogen) or a reactant gas (air

or a mixture of air and nitrogen) . The carrier gas is fed through the

system at a rate of 200 ml/min. The vaporization apparatus is heated to

the required temperature using heating tape regulated by a variable

transformer. The temperature is selected to give the desired lubricant

vapor pressure, while keeping the temperature below the thermal

decomposition temperature 6f the lubricant being evaluated.

The deposition and preheating sections consist of two ovens. In the

preheating oven (fig. 1), the substrate to be coated is heated to the

deposition temperature under an inert atmosphere (nitrogen) . When the

substrate reaches the desired test temperature, it is guided into the

deposition oven. In the deposition oven, the substrate is exposed to the

lubricant vapor in the carrier gas. The flow rates of gases in both

preheating and deposition ovens is maintained at 200 ml/min. The gases

from both ovens exit through a vent located between the two ovens. The

deposition oven and preheating oven are connected by a stainless steel tube

that can be removed for substrate loading and unloading. A stainless steel

rod with a hook on one end is mounted inside the tube. This rod can be

slid across the preheating and deposition ovens. The substrates are wire

specimens of about 0.025 to 0.05 cm in diameter. A hook on the stainless

steel rod acts as a sample holder for the substrate. The deposition oven

2
is connected to the vaporization apparatus by a ceramic tube that is

cemented at the joint between the vaporization apparatus and the deposition

oven

.

The operating procedure for the vapor deposition tests consists of

three stages: cleaning and drying the substrate, preheating, and

deposition. The substrate is washed successively with naphtha and

tetrahydrofuran and then dried in a high velocity stream of air. The

substrate is then held in the preheating oven for 5 minutes to vaporize any

residual solvent. The substrate is then weighed using ^n electrobalance or

a six-place microbalance . Stainless steel, copper, and quartz substrates

are weighed using the electrobalance, whereas magnetic substrates such as iron and nickel are weighed using a six-place microbalance. The substrate

is then placed on the sample holder and preheated to the desired deposition

temperature under a nitrogen blanket in the preheating oven. After it

reaches the desired deposition temperature, the substrate holder is slid

across to the deposition oven, where the substrate contacts the lubricant

vapor for the duration of the test. After deposition is completed, the

substrate is pulled back to the preheating oven. The substrate is cooled

for 10 minutes under a steady flow of nitrogen and then weighed. The

weight gain of the substrate is reported as the amount of deposit formed.

It should be noted that while the substrate is being preheated in nitrogen,

oxygen impurities in nitrogen may cause oxide coatings to form on the

substrate surface. This weight gain due to formation of oxide coatings in

the preheating section is subtracted from the total weight gain reported.

After the completion of a deposition test, the deposit is examined

using the Scanning Electron Microscopy (SEM) and Energy Dispersive

Analytical X-Ray Spectroscopy (EDAX) . The SEM is used to examine the

surface of the substrates and the morphology of the vapor deposited films.

EDAX is used to determine the relative elemental composition of the

deposits

.

An earlier version of the vapor deposition apparatus, figure 3, was used with some of the lubricants discussed in this report. That

preliminary technique was replaced because the exact concentration of the

lubricant in the vapor was not known or controlled, and the liquid-phase

reactions could modify the lubricant which was kept as a liquid at elevated

temperature for the duration of the experiment.

The earlier apparatus consists of a ceramic tube 0.76 m long and 0.025 m i.d., with three sections, each heated by a separate tubular oven. In

the first section, a stainless steel boat or crucible (0.0095 m x 0.0064 m

x 0.076 m) filled with lubricant is heated to produce the desired vapor

pressure of the lubricant. In the second section, the temperature is

increased and the gas stream containing the lubricant vapor is brought into

contact with the specimen surface where the solid lubricant film is to be

formed. The last section is used to preheat the specimen while the boat is

being heated to the desired temperature. The direction of flow of the

carrier gas is controlled by the three-way valve. At the beginning of an experiment, the flow of carrier gas is directed away from the specimen to

be coated while the boat and specimen are heated to the desired

3
temperature. When the desired system temperature is reached, the direction

of the carrier gas is reversed to carry the lubricant vapor to the specimen

which is then positioned in the middle section. The specimen is then

exposed to the vaporized lubricant for a specified time during which a solid lubricant film is deposited on the specimen.

4
VEI^T

Figure 1. Apparatus for vapor phase lubrication of stationary sampl

and temperature profile in apparatus.

(A) Position of sample during preheating cycle.

(B) Position of sample during deposition.

5
Lubricant From Positive Olspl accr.enc Pur.p

1* iiMm

^ 1"T

1"

4

4"

Gas Stream

Packing, Ciass Helices Class Wool

Figure 2. Lubricant vaporization apparatus.

6
4^
B. Dynamic Wear Testing with a Four-Ball Wear Tester

A modified four-ball wear tester was used to study the anti-wear

characteristics of vapor deposited lubricating films. A schematic of the

test apparatus is shown in figure 4. The wear tester consisted of three

contacting balls held in place in a ball pot and a top rotating ball that

impinges onto the stationary balls with an adjustable load. The wear

tester was modified to accommodate a lubricant vapor delivery system. The

design allows for control of both the temperature and the environment

surrounding the ball pot and spindle. The lubricant vapor was produced in

a vaporization apparatus similar to that described earlier. Lubricant

vapors were carried to the ball pot by a carrier gas (nitrogen, air, or mixtures of nitrogen and air) at a rate of 200 ml/min. The ball pot was brought up to the desired test temperature, usually 371 °C, with a continuous purge of nitrogen gas. To initiate a test, liquid lubricant

from a syringe pump was fed into the vaporization column where it was vaporized and delivered into the preheated ball pot for 2 minutes. Next,

the wear tester was started using a set speed of 600 rpm. Then, the load was applied bringing the rotating upper ball bearing into contact with the

three stationary ball bearings. Loads from 5 to 60 kg were studied. Each

test was run for 30 min with a continuous supply of lubricant vapor. After

30 min, the load was removed and the balls were disassembled from the ball

pot. The wear scars on the lower three balls were measured using a calibrated microscope. The diameters of these scars were taken as a good

measure of the wear behavior of the lubricant. The wear scars were

dependent on the type of lubricant, load, speed, test time, and temperature

used in the tests.

Vapor

Steam

Inlet

Figure 4. Four-ball wear tester modified for

vapor delivered lubrication studies.

8
III. VAPOR DEPOSITION STUDIES

The vapor deposition apparatus described in previous sections was used

to study the vapor deposition behavior of a wide variety of lubricants.

The lubricating film deposition characteristics were found to be dependent

on lubricant type, substrate material, substrate temperature, lubricant

vapor concentration, and carrier gas composition. In this work, data were obtained for tricresyl phosphate (TCP) , tributyl phosphate (TSP) , diphenyl

ditertiarybutylphenyl phosphate (GT) , polyphenyl ether (PPE)

trimethylolpropane triheptanoate (TMPTH) , di-2-ethylhexyl sebacate (DEHS)

and super-refined mineral oil.

The trends observed in the formation of the vapor deposited

lubricating films are discussed in this section. The first discussion, on TCP vapor deposited films, is used to summarize the primary factors that

influence vapor deposition.

A. Tricresyl Phosphate (TCP)

Effect of Substrate and Temperature : Deposition data were obtained for TCP on iron, stainless steel, copper, nickel, aluminum, and quartz at 500, 600,

and 700°C in figures 5-7. The concentration of TCP in nitrogen carrier gas

for this series of tests was 1.55 wt . percent. It has been shown by a number of investigators that phosphate esters exhibit a low energy

decomposition path on certain transition metals such as iron. The data in

figures 5-7 show that the initial deposition rates are high for copper,

iron, and stainless steel. For iron and stainless steel these high rates

decrease with test time and approach a constant rate. As the temperature

increases, it takes less time to reach this constant deposition rate. At

700°C the same final deposition rates are achieved for iron and stainless

steel. The lower catalytic effect of stainless steel compared to iron can

be attributed to lower corrosion rates on stainless steel. The initial

high deposition rates and the subsequent decrease of these rates with test

time appear to be related to the availability of iron to react with TCP in

forming the coating. As the thickness of the coating increases, this

impedes the diffusion of iron molecules through the coating. Consequently,

as the thickness of the coating increases, less iron is available for

reaction and coating formation. Therefore, the rate of deposition

decreases. It is anticipated that a similar loss in deposition rate will

be observed for copper at longer times. Studies reported later in this

report indicate that for long times or thick coatings, the rate of

deposition becomes independent of the substrate and approaches the rate

characteristic of a non-catalytic surface such as quartz.

Deposition data for TCP vapor on stainless steel for tests of 1 to 60

seconds duration is shown in figure 8. These tests were conducted using

0.049 to 1.23% TCP vapor in a nitrogen carrier gas at 700° C. These data

clearly show the initial high deposition rate on stainless steel that

decreases with test time to approach a constant rate.

9
Figure 5. Deposit thickness versus time for various substrates

exposed to a vapor of 1.55% TCP In nitrogen stream

at 500*C.

0 5 10 15 20 25 30

Time (min.)

Figure 6. Deposit thickness versus time for various substrates

exposed to a vapor of 1.55% TCP In nitrogen at eOO'C.

10
Time (min.)

Figure 7. Deposit thickness versus time for various substrates

exposed to a vapor of 1.55% TCP in nitrogen stream

at TOCC.

11
Time (sec.)

Figure 8. Deposit thickness versus time for stainless steel wire at

700°C exposed to vapor concentrations of TCP of 0.0A9% to

1.23% in a nitrogen carrier gas. [1 yg/cm^ = 10 monolayers].

12
T

o

E

•H

u u u

o 0 0 O c o

o o o

<i

in

<3 O

o

to o

o

to o

inCM O

o

oto o

o

o

( uio/3rt) aisocloa

u

o

o

^i

•o • (U tn

cn to O

a

(1) (U

0)V4 5-1 5 O

C

cnu

(0

(U —tc

.1-1 CD

(A IW O

O

o

w

Q) in E

c o

o

On

0)U

13
Copper, nickel, aluminum, and quartz surfaces are compared at 500,

600, and 700°C with iron and stainless steel in figures 5-7. These data

suggest that copper has a significant catalytic effect on TCP vapor

deposition. The catalytic effect appears to persist at all three

temperatures. Nickel, quartz, and aluminum all show little measurable TCP

deposit after 30 minutes at 500°C. At 600°C these three materials show the same very low deposition rate over the 30-minute period. In all cases, the

deposition rate appears to be linear from zero time and there appears to be

no initial high rate. Aluminum melts between 600°C and 700°C and could not

be tested at 700°C. Nickel and quartz again show the same low linear

deposition rate at 700°C. Metal substrates that cause deposition rates

higher than on quartz can be considered to have a catalytic effect on deposit formation. On this basis, these data demonstrate the significant

catalytic effect of iron and copper and the lack of catalytic effect for

aluminum and nickel for TCP vapor deposition.

The effect of temperature on TCP vapor deposition is also illustrated

in figures 5-7. It is observed that increasing the temperature from 500 to

700°C significantly increases the deposition rates. A comparison of the

increase in deposition rate with temperature is made on figure 9. Data are shown for TCP deposition on stainless steel using 1.55% TCP in nitrogen.

These data indicate the expected increase in deposition rate with

temperature. At 500 and 600°C, the catalytic effect is evident only at

times of 5 minutes or less. The constant deposition rate above 5 minutes

is indicative of the lack of a catalytic effect. At 700°C, the deposition

rate is high and stainless steel shows significant catalytic activity for

the duration of the test.

Effect of Lubricant Vapor Concentration : The effect of increasing the

lubricant vapor concentration in the carrier gas on the deposition rate was examined. Figure 8 illustrates the effect of increasing TCP concentration

in nitrogen carrier gas from 0.049 to 1.23% on the deposition rate on stainless steel at 700°C. The data shown in this figure are for short test

times (less than 60 seconds).

During this short test period, stainless steel has a catalytic effect

on TCP deposition. Figure 8 shows that increasing the TCP vapor

concentration from 0.049 to 0.23% causes a significant increase in the TCP

deposition rate. It should be noted, however, that the yield in deposit

per mol of TCP vapor is highest for the lowest concentrations used. The

effect of increasing the TCP vapor concentration on deposition on iron is

illustrated in figures 10-13. Figures 10-12 show deposit data at 500, 600,

and 700°C, varying the TCP concentration in nitrogen from 0.126 to 1.55%.

These data show that at all three temperatures, the lowest TCP

concentration, 0.126%, causes the highest deposition rate after 30 minutes.

At 600 and 700°C, the 1.55% TCP vapor and the lower TCP vapor concentration

curves cross within the 30-minute test period. These changes in rate with

time and concentration appear to be associated with the coupling of the

catalytic effect of the iron surface with the diffusion process related to

the transport of iron through the coating. The catalytic effect of iron

diminishes with the decrease in availability of iron with increasing film

thickness. With higher TCP vapor concentrations, this diminishing

14
uoo

Time (min.)

Figure 10. Deposit thickness versus time for iron exposed to

various TCP concentrations at 500°C.

0 5 10 15 20 25 30

Time (min.)

Figure 11. Deposit thickness versus time for iron exposed

to various TCP concentrations at 600°C.

15
o 1.55^.

- a o.i26;S

1600

1200

800

koo

10 15

Time (mint)

20 25 30

Figure 12. Deposit thickness versus time for iron exposed to

two different TCP concentrations at 700°C.

2000

1 .8

Figure 13. 30 minute deposit versus TCP concentration for

iron at 700'C.

16
catalytic effect occurs In a shorter time period. This may be related to

the influence of lubricant concentration on the coating morphology and

subsequent diffusion rates. Figure 13 shows TCP deposit on iron as a function of TCP vapor concentration for tests of 30 minutes duration at

yOO'C. These data show that the maximum deposit occurs at a TCP vapor

concentration of 0.3%. In the range 0.1 to 0.3% TCP vapor concentration,

increasing the concentration increases the deposit. Using a TCP vapor

concentration higher than 0.3% decreases the deposit. Conventional

chemical kinetics would suggest that an increase in the TCP concentration

would always increase the rate of coating formation. It is possible that

changes in deposit morphology due to changes in concentrations of the vapor

could influence the diffusion of iron or iron corrosive compounds to the

surface.

The effect of increasing the TCP vapor concentration on deposition on copper is illustrated in figures 14-17. Figures 14-16 show deposit data at

500, 600, and 700°C, varying the TCP concentration in the nitrogen carrier

gas from 0.126 to 1.55%. The effect of vapor concentration on deposition

on copper differs substantially from that on iron. For copper all of the

conditions studied show a linear deposition rate from time zero. In all cases the higher the TCP vapor concentration the higher the deposition

rate. At 1.55% TCP vapor the deposit level after 30 minutes at 600 and

700 °C is much higher on copper than on iron. At 1.55% TCP vapor

concentration deposits at 500°C on iron and copper are about the same. At

0.126% TCP vapor concentration, the deposit levels on copper are higher

than on iron at all three temperatures. The high levels of deposit

formation on copper indicate the strong catalytic effect of the metal that

persists through the 30-minute duration of the tests. Figure 17

illustrates TCP deposit on copper as a function of TCP vapor concentration

for tests of 15 minutes duration at 700°C. The data show that increasing

the TCP vapor concentration from 0.126 to 1.55% causes a linear increase in

deposit level. In contrast to the results with an iron substrate, all the

data obtained with copper are consistent with conventional chemical

kinetics. It is the catalytic activity of copper has not been diminished

even when the coating is equivalent to over 30,000 monolayers thick.

Effect of Carrier Gas Concentration : The effect of carrier gas on TCP

deposition rates is illustrated in figures 18-21. Figures 18 and 19 show

TCP deposit data on iron, copper, stainless steel, and nickel using 0.0155

and 0.126% TCP vapor in nitrogen. Figures 20 and 21 show data from tests

conducted under conditions identical to those used in figures 18 and 19

except that air was used as the carrier gas instead of the inert nitrogen.

It should be noted that deposit formation in the presence of air may

involve a different mechanism than deposit formation in the presence of an

inert gas such as nitrogen. This behavior has been observed in liquid

phase TCP degradation where reaction in nitrogen gives rise to P-O-P

bonding whereas, reaction in air causes aryl-to-aryl linkages with the

formation of C-O-C bonds [15].

Comparison of figures 18 and 19 with figures 20 and 21 show that

deposition rates in the presence of air are comparatively higher than in

nitrogen. The influence of oxygen on the deposition rate is particularly

17
250

Time (min.)

Figure 14. Deposit thickness versus time for copper exposed

various TCP concentrations at 500 "C.

1000

J 5 10 15 20 25 30

Time (min.)

Figure 15. Deposit thickness versus time for copper exposed

to various TCP concentrations at 600°C.

18
uooo

0 5 10 15 20 25 30

Time (min.)

Figure 16. Deposit thickness versus time for copper exposed

to various TCP concentrations at 700**C.

4000

B

tsO 3.

4J •H

COO

a

3200 -

2A00 -

1600

800 -

1.5

% TCP

Figure 17. 15-ininute deposit versus TCP concentration for

copper at 700**C.

19
1200

900

A Iron

_ o Copper n Stainless Steel

V Nickel

1

6oo

o

PU

0) Q

300

15

Time (min.)

30

Figure 18. Deposition curves for 0.0155% TCP in nitrogen

stream on four metal substrates at TOO'C.

1200

Time (min.)

^

I Figure 19. Deposition curves for 0.126% TCP in nitrogen

stream on four metal substrates at 700°C. j

20
1200

e

u

H

m

o

P.eQ

900 -

600 -

300 -

15

Time (min.)

Figure 20. Deposition curves for 0.0155% TCP in air stream on four metal

substrates at 700°C.

1200

900

B

o

3^

*' 600 -

V)o.

0) Q

300

::o- ±

10 15

Time (mln.)

Iron A

Copper o

Stainless Steel

Mickel V

20 25 30

Figure 21. Deposition curves for 0.126% TCP in air stream on four metal

substrates at 700°C.

21
significant when the metal substrate has a strong catalytic effect.

Figures 18-21 show that the presence of air causes a significant increase

in the deposition rate on the catalytic substrates iron and copper. The

deposition rate on iron in the presence of air is initially high but

gradually decreases with increasing coating thickness indicating that the

catalytic effect of iron decreases with coating thickness. Tests on copper

were limited to short time tests since the thick coatings would not adhere

to the metal surface.

The catalytic effect of copper has not diminished even when relatively

thick coatings have formed. These data clearly show that both iron and

copper have significant catalytic effects in the presence of air. The

deposition data on stainless steel shows that the effect of stainless steel

is similar in both nitrogen and air. Stainless steel shows some catalytic

activity at short exposure times, but this effect soon diminishes with

coating thickness. The initial catalytic effect of stainless steel is more

significant with higher TCP vapor concentrations. The initial deposition

rate is higher in air than in nitrogen. Deposition data on nickel at low

TCP vapor concentrations (figs. 18,20) show that in both air and nitrogen

nickel is non-catalytic. At higher TCP vapor concentrations (figs. 19,21)

it is observed that in nitrogen nickel remains non-catalytic, but in air a high deposition rate at short exposure times indicates some catalytic

activity. This initial high rate is soon diminished to a non-catalytic

constant rate. It is possible that the initial weight increase of nickel

in air may be associated with oxidation of this substrate. These data

clearly show that the carrier gas composition has a significant effect

on the relative catalytic activity of metal substrates and the presence of

air accelerates the rate of deposition.

SEM and EDAX Analysis : The morphology of the TCP deposits formed on stainless steel, iron, and copper were examined using scanning electron

microscopy (SEM). Figure 22 shows the uncoated stainless steel surface.

Figures 23-29 are SEM micrographs of TCP deposits on stainless steel

ranging in thickness from 40 /xg/cm^ a relatively thin coating to 570

/ig/cm^ , a relatively thick coating. All deposits were formed using 1.55%

TCP in nitrogen at 700°C. These photographs show that the deposits have a granular structure. In relatively thin coatings, the granules appear

smaller. As the coating thickness increases the granules appear to be

larger. It is not clear whether the smaller granules grow to become larger

or whether they are covered by another layer of larger granules. This

increase in granular size with increase in coating thickness could be

responsible for the reduction in the catalytic effect of stainless steel

with coating thickness. These large granular structures could be acting as

a diffusion barrier. There is less iron available for reaction and

therefore, its catalytic effect on coating formation decreases. Figures 28

and 29 show the morphology of the deposit as a function of coating depth.

The structure close to the metal surface appears to be porous. There is an apparent decrease in porosity with larger granular size as the coating

depth increases. These photographs suggest that the overall coating may

not be a liquid at the deposition temperature but rather an amorphous

solid. Also, attempts to melt the resulting coating support the conclusion

that the coating exhibits solid- like behavior at the deposition

22
temperature. SEM micrographs of TCP deposits on iron are shown in figures

30-35. SEM micrograph of uncoated iron surface is shown in figure 30. All

deposits were formed using 1.55% TCP in nitrogen at 700°C. Figures 31-35

show deposits on iron ranging from 61 to 1200 /ig/cm^ . The deposits on iron

appear granular. The granules are smaller in thin coatings and larger in

relatively thicker coatings. This increase in size of granules and

corresponding decrease in availability of iron may be the cause of the

diminishing catalytic effect of iron with increasing coating thickness.

Figure 35 shows a side view of the coating as a function of depth.

This figure shows that the coating is relatively porous close to the

surface, and relatively non-porous on top, away from the surface. Figures

37-40 show TCP deposits on copper: figure 36 is an SEM micrograph of

uncoated copper surface. All deposits were formed using 1.55% TCP in

nitrogen at 700°C. The coating thicknesses on copper range from 194 to

4000 ^g/cm^ . TCP deposits on copper have a granular structure somewhat

different from the deposits on iron. The granules appear to have a coarser

texture. In figure 40, 4000 /lig/cm^ coating thickness, there appears to be

some secondary growth on the granules. The granular size appears to be

larger with increasing coating thickness. Figure 39 shows a side view of

the coating. The deposit appears porous closer to the surface. Away from

the surface the deposit appears to be more uniform.

The relative elemental composition of TCP deposits on iron and copper

were examined using Energy Dispersive Analytical X-Ray Spectroscopy (EDAX)

The deposits were formed using 1.55% TCP in nitrogen at 700°C for 30

minutes. The EDAX spectrum are shown in figures 41-42. The corresponding

SEM micrographs are shown in figures 43-45. These data indicate that for

deposits on iron, iron to phosphorous ratio is higher in coating material

next to the surface compared to coating material away from the surface.

The iron- to-phosphorous ratio next to the surface is approximately 1.5 to

1, while away from the surface the ratio is 0.75 to 1. The decrease in

iron catalytic effect with increasing coating thickness is due to this

decrease in availability of iron. For TCP deposits on copper, the ratio of

elemental composition of copper to phosphorous on the top side of the chip

away from the surface is 1.6 to 1. For both iron and copper a substantial

concentration of metal is in the deposit. The strong catalytic effect of

iron and copper in TCP vapor deposition is related to this increased

availability of the substrate.

23
Figures 22 through 29 are SEM photos taken on stainless steel wire surfaces

exposed to 1.55% TCP in nitrogen stream at 700°C. On figures 22 to 27, magnifications are 1,000X and 10,000X on the left frame and the right frame, respectively

The square window on the left frame is magnified 10 times and shown on the right

frame

.

Figure 22. Stainless steel wire surface [uncoated)

.

Figure 23. 2-second exposure forming a deposit of 40 pg/cm^

.

24
Figure 24. 5-second exposure forming a deposit of 55 pg/cm'
Figure 26. 10-minute exposure forming a deposit of 273 yg/cm^.

Figure 27. 30-minute exposure. The actual deposit could not be measured due

to the coating cracking off after cooling down to room temperature.

It is, however, estimated to be 570 yg/cm^.

26
Figure 28. Sideview on the cracked coating of figure 27. Magnification is

2,000X on the left frame. The square window on the left frame is

magnified 5 times and shown on the right frame.

Figure 29. Sideview on the coating of figure 25 after making a scratch using

a razor blade. Magnification is 50,000X.

27
Figure 30. SEM photograph of an iron wire surface. Magnification is lOOOX

on the left frame. The square window on the left frame is magnified

10 times and shown on the right frame. Each marker represents the

length on its frame.

Figure 31. SEM pliotograph of a deposit 61 pg/cm^ on an iron wire surface for

2 seconds with 1.55% TCP at 700°C. Magnification is lOOOX on the

left frame. The square window on the left frame is magnified 10

times and shown on the right frame. Each marker represents the

length on its frame.

28
Figure 32. SEM photograph of a deposit 100 yg/cm^ on an iron wire surface for

5 seconds with 1.55% TCP at 700°C. Magnification is lOOOX on the

left frame. The square window on the left frame is magnified 10

times and shown on the right frame. Each marker represents the

length on its frame.

Figure 33. SEM photograph of a deposit 678 \\g/cm^ on an iron wire surface for

5 minutes with 1.55% TCP at 700°C. Magnification is lOOOX on the

left frame. The square window on the left frame is magnified 10

times and shown on the right frame. Each marker represents the

length on its frame.

29
V

Figure 34. SEM photograph of a deposit 1200 yg/cm^ on an iron wire surface

for 30 minutes with 1.55% TCP at 700°C. Magnification is 2000X

on the left frame. The square window on the left frame is magnified

5X and shown on the right frame. Each marker represents the length

on its frame.

Figure 35. SEM photograph of a sideview of a deposit after fracturing the

coating on the iron wire surface. (Same condition as fig. 34.)

Magnification is 3000X. Marker represents the length.

30
Figure 36. SEM photograph of a copper wire surface. Magnification is lOOOX

on the left frame. The square window on the left frame is

magnified lOX and shown on the right frame. Each marker represents

the length on its frame.

Figure 37. SEM photograph of a deposit 194 yg/cm^ on a copper wire surface _ for 30 seconds with 1.55% TCP at 700°C. Magnification is 2000X

on the left frame. The square window on the left frame is

magnified 5X and shown on the right frame. Each marker represents

the length on its frame.

31
Figure 38. SEM photograph of a deposit 1260 yg/cm^ on a copper wire surface

for 5 minutes with 1.55% TCP at yOO^C. Magnification is lOOOX on the left frame. The square window on the left frame is magnified

lOX and shown on the right frame. Each marker represents the

length on its frame.

32
Figure 39. SEM photograph of a heavily deposited copper wire surface for 20

minutes with 1.55% TCP at 700°C. The actual deposit could not be

obtained due to the coating cracking off when cooling down to room

temperature. However, it is estimated to be 4000 yg/cm^. Note that

the upper right on the left frame is nearly the uncoated copper

surface after the deposit cracking off. Magnification is 573X on

the left frame. The square window on the left frame is magnified

5X and shown on the right frame. Each marker represents the length

on its frame

Figure 40. Top view on the coating of figure 39. Magnification is lOOOX on the left frame. The square window on the left frame is magnified

lOX and shown on the right frame. Each marker represents the

length on its frame,

33
Energy -

Ca)

Energy *•

(b)

Figure Al. EDAX spectra with Incident electrons Illuminating the top side (a)

and the back side (b) of cracked chips from TCP deposit on iron wire,

34
Figure 42. EDAX spectrum with incident electrons illuminating the top side

a cracked chip from TCP deposit on copper wire.

35
Figure 43. EDAX illuminated area of a cracked chip (top side) from TCP

deposit on iron wire.

Magnification is 2000X.

Figure 44. EDAX illuminated area of a cracked chip (backside) from TCP

deposit on iron wire.

Magnification is 5700X.

36
Figure 45. EDAX illuminated area of a cracked chip from TCP deposit

on copper wire.

Magnification is 3000X.

37
B. Diphenyl Ditertiarybutylphenyl Phosphate (GT)

Vapor deposition characteristics of GT on copper, iron, stainless

steel, and nickel substrates were examined. Deposition tests were conducted in nitrogen and air. Figures 46-48 show deposit data for GT in

nitrogen at 700°C. For this series of tests the GT vapor concentration in

nitrogen was varied from 0.013 to 1.75%. These data clearly indicate that

both the substrate material and lubricant vapor concentration have a significant effect on GT deposition rate. Deposition rates on iron and

copper indicate that iron and copper have a strong catalytic effect at both

low and high GT vapor concentrations. Copper maintains its strong

catalytic effect for the duration of the test whereas iron shows a diminishing catalytic effect with increasing coating thickness. This

decrease in iron catalytic effect with coating thickness is evident at all

three GT vapor concentrations. The diminishing catalytic effect of iron is

indicative of a decrease in the availability of iron with increasing

coating thickness. Stainless steel and nickel in contrast to iron and

copper have significantly less influence on GT vapor deposition. Stainless

steel shows an initial deposition rate indicative of a catalytic effect at

short exposure times; this rate soon diminishes to a non-catalytic constant

rate. Nickel shows little or no catalytic effect under these conditions.

Comparison of figures 46-48 clearly indicates that an increase in GT vapor

concentration increases the rate of deposition but does not change the

relative catalytic effects of the substrates. These trends on effect of GT

vapor concentration and effect of metal substrate on deposition

rate are consistent with the trends observed from the deposit data on TCP

vapor deposition.

The effect of changing the carrier gas from nitrogen to air on GT

vapor deposition was examined. The oxygen in air could have a significant

effect on GT deposition. Figure 49 shows deposition rates on copper, iron,

stainless steel, and nickel for tests conducted using 0.106% GT in air at

700°C. The influence of oxygen on the rate of GT deposition is

demonstrated by a comparison of figures 47 and 49. The experimental

conditions used to obtain the data presented in these two figures are identical except that nitrogen was used as the carrier gas for data

presented in figure 47 while air was used as the carrier gas for the data

presented in figure 49 . The data show the presence of oxygen in the

carrier gas does not cause any significant change in the deposition rate.

The effect of oxygen on GT deposition rate essentially follows the same trends that were observed with TCP deposition. In figure 49, the arrows on

the symbols for deposits on copper and iron signify that not all of the

deposits were measurable because part of the coating cracked off of the

substrate when the substrate was cooled from the test temperature of 700°C

to room temperature. The thickness of the coatings gave some integrity to

the deposit and upon cooling the differences in the coefficient of

expansion of the coating and the metal substrate caused part of the coating

to crack and flake off the metal substrate.

38
The morphology of GT vapor deposited films was examined using Scanning

Electron Microscopy. Figure 50 is an SEM micrograph of GT deposit on stainless steel of 72 /ig/cm^ thickness. This relatively thin coating has a granular structure somewhat similar to TCP coatings of the same film

thickness on stainless steel and iron. Since the deposition behavior for

GT is similar to that for TCP, it would be expected that thicker GT

coatings have a larger granular structure that is relatively non-porous

similar to that for TCP. This would be consistent with the diminishing

catalytic effect of iron with thick coatings. The larger granules could

impede diffusion and migration of iron or iron compounds through the

coating to contact the lubricant vapor. This decreased availability of

iron can cause a diminishing catalytic effect for GT deposition.

39
2000

Figure 46. Deposit thickness versus time for various substrates exposed to

vapor concentration of 0.013% GT in nitrogen stream at 700 C.

40
2000

Time (nln.)

Figure 48. Deposit thickness versus time for various substrates exposed to a vapor concentration of 1.75% GT in nitrogen stream at 700°C.

Time (mln.)

Figure 49. Deposit thickness versus time for various substrates exposed to

a vapor concentration of 0.106% GT in air stream at 700°C.

41
Figure 50. SEM: lOOOX. Stainless steel target at 700°C exposed 1 minute at 1.75% GT, 72 yg/cm^

.

42
C. Polyphenyl Ether (PPE)

The vapor deposition characteristics of PPE vapor were investigated.

The effect of metal substrate, operating temperature, and PPE vapor

concentration on deposition rate was examined. Tests were conducted using

copper, stainless steel, nickel, and iron substrates.

The effect of metal substrate on PPE deposition rate is shown in

figure 51. For this series of tests 1.0% PPE vapor concentration was used

in a nitrogen stream at 700°C. It is observed that for all four metals the

PPE deposition rate is linear from time zero for the duration of the test.

The deposition rates are different for each metal and can be considered

metal specific. This behavior can be attributed to a catalytic effect of

these metals. The linearity of the deposition rates suggests that the

coating thickness achieved in these PPE vapor deposition tests are not

sufficient to reduce the catalytic effect of the metal. It is expected,

as with the phosphate esters, that at longer test times or more severe conditions, after a sufficient coating depth is achieved the metal will

lose its catalytic effect and the deposition rate will reach a constant

non-catalytic rate. The highest PPE deposition rates were on iron,

followed by nickel, stainless steel, and copper. The relatively strong

catalytic effect of nickel and weak catalytic effect of copper are in

contrast to the deposition behavior in phosphate esters, where copper had a strong catalytic effect and nickel was essentially inert.

The effect of increasing temperature on PPE deposition rate is shown

in figure 52. Tests were conducted with 1.0% PPE in nitrogen using

stainless steel and iron substrates at 700, 800, and 850°C. The deposition

rate on both metals increases as the temperature increases. At all three

temperatures the deposition rates are linear from time zero for the

duration of the tests. The coating thicknesses achieved are not sufficient

to deter the catalytic effect of the metal. The deposition rates on iron

at all three temperatures are higher than the deposition rates on stainless

steel. The lower catalytic effect of stainless steel can be attributed to

lower iron corrosion rates with stainless steel. The effect of lubricant

vapor concentration on PPE deposition rate is shown in figure 53. Tests

were conducted with 0.6 and 1.0% PPE in nitrogen using stainless steel

substrates at 700, 800, and 850°C. At all three temperatures, the

deposition rates increase with increasing PPE vapor concentration. The

deposition rates are linear from time zero for the duration of the test.

The time and severity of the tests are not sufficient to deter the

catalytic effect of the metal substrate. Again as in the case

of the phosphate esters, the lowest concentration studied is the most

efficient in producing a unit of film thickness per mole of PPE vapor.

Scanning Electron Microscopy (SEM) was used to examine the morphology of

PPE deposits. Figures 54, 55, and 56 are SEM micrographs of PPE deposits

from tests conducted using stainless steel exposed to 1.0% PPE in nitrogen

at 700 and 800°C. Figure 54 is an SEM micrograph of a relatively thin PPE

coating (60 /zg/cm^ ) . Figures 55 and 56 are SEM micrographs of relatively

thicker PPE coatings (278 and 315 /ig/cm^ ) . The PPE deposits appear to have

granular structure somewhat different from phosphate ester deposits. As

43
the coating thickness increases from 60 to 278 /ig/cra^ , the size of the

granules appears larger.

SEM micrographs of PPE deposits on iron, nickel, copper, and stainless

steel are contrasted in figures 56-59. The deposits were formed using 1.0%

PPE in nitrogen at 700°C. The structure of the deposits on iron and nickel

appear to be similar. It should be noted that both iron and nickel had

strong catalytic effects on PPE deposition. The structure of deposits on copper and stainless steel appear to be similar to each other but different

from the structure of deposits on iron and nickel. The deposits on copper

and stainless steel appear more granular compared to deposits on iron and

nickel. The lower catalytic effect of stainless steel and copper may be

related to differences in deposit structure morphology.

44
400 ^ I ron

Time (ill In.)

Figure 51. Deposit thickness versus time for different substrates exposed

to 1.0% PPE concentration in nitrogen at 700°C.

I 1 nic (hi in.)

Figure 52. Deposit thickness versus time for stainless steel and iron

substrates exposed to 1.0% PPE concentration in nitrogen at

700, 800, and 850°C.

45
Figure 53. Deposit thickness versus time for stainless steel substrates

exposed to two different PPE concentrations in nitrogen at

700, 800, and 850°C.

46
Figure 54. SEM photo of a deposit 60 /zg/cm^ on a stainless steel wire

surface for 3.5 minutes with 1.0% PPE at 800°C. Magnification

is 2000X on the left frame. The square window on the left

frame is magnified 5X and shown on the right frame.

Figure 55. SEM photo of a deposit 315 /ig/cm^ on a stainless steel wire

surface for 20 minutes with 1.0% PPE at 800°C. Magnification

is 2000X on the left frame. The square window on the left

frame is magnified 5X and shown on the right frame.

47
Figure 56. SEM photo of a deposit 278 ng/cm^ on a stainless steel wire

surface for 30 minutes with 1.0% PPE at 700°C. Magnification

is 2000X on the left frame. The square window on the left

frame is magnified 5X and shown on the right frame.

7.5 minutes with 1.0% PPE at 700°C. Magnification is 2000X.

48
Figure 58. SEM photo of a deposit 243 /xg/cm^ on a nickel wire surface for

15 minutes with 1.0% PPE at 700°C. Magnification is 2000X.

Figure 59. SEM photo of a deposit 166 /ig/cm^ on a copper wire surface for

20 minutes with 1.0% PPE at 700°C. Magnification is 2000X.

49
D. Trimethylolpropane Triheptanoate (TMPTH)

The vapor deposition characteristics of TMPTH on copper, iron,

stainless steel, quartz, and nickel were investigated. The effects of

metal substrate, operating temperature, lubricant vapor concentration, and

carrier gas composition on TMPTH deposition rate were examined.

The effect of metal substrate on deposition rate is shown in figure

60. For this series of tests 1.0% TMPTH in nitrogen was used at 700°C.

The data show that the deposition rates are metal specific. The deposition

rates are unique for each substrate. The deposition rates on all

substrates were linear from time zero for the duration of the tests. The

highest rate was on iron followed by nickel, stainless steel, and copper

and quartz which essentially have the same rate. The similarity of

deposition rates on copper and on inert material, quartz, indicates that

copper can be considered inert for TMPTH deposition. The relatively higher

deposition rates on stainless steel, nickel, and iron indicate that these

substrates have a catalytic effect on TMPTH deposition. The rate on iron

is substantially higher than on the other substrates. The linearity of

these rates indicates that for the duration and severity of these

deposition tests catalytic metal substrates maintain their catalytic

effect. It is expected that with thicker coatings under more severe conditions the catalytic effect of the metal substrates will diminish and

the deposition rates will reach a constant non-catalytic rate similar to

that on quartz. It should be noted that the effect of metal substrates on TMPTH deposition is similar to the effect of metal substrates on PPE

deposition. In both cases iron had a strong catalytic effect followed by

nickel and stainless steel. In both cases copper was essentially inert.

In contrast to these trends, copper had a strong catalytic effect and

nickel essentially no effect for deposition of phosphate esters. The order

of catalytic activity observed for PPE and TMPTH deposition closely follows

the order observed in hydrogenation-dehydrogenation and hydrogen© lysis

reactions of hydrocarbons [17].

The effect of increasing operating temperature on TMPTH deposition is

shown in figure 61. Tests were conducted using 1.0% TMPTH in nitrogen

using stainless steel substrates at 600, 700, and 800°C. The data show

that increasing the operating temperature increases the deposition rates.

At 600°C the rate appears linear from time zero. At 700 and 800°C the

deposition rate decreases after 15 to 20 minutes test time. This decrease

at the more severe conditions indicates a decrease in catalytic effect of

the metal due to an increase in coating thickness and subsequent decrease

in availability of metal. It should also be noted that at these higher

temperatures the TMPTH deposit may not be stable and may be degrading [18].

Deposit degradation at these higher temperatures may be an additional

factor causing an overall decrease in deposition rate.

The effect of carrier gas composition on TMPTH deposition rate is

shown in figure 62. For this series of tests 1.0% TMPTH was used with

stainless steel substrates at 700°C. The oxygen concentration in nitrogen

carrier gas was varied from 0 to 21% by mixing nitrogen and air streams at

different ratios. The data show that the highest deposition rates are in

50
air (21% oxygen) and the lowest in nitrogen. Increasing the oxygen

concentration increases the deposition rate. 1.0% oxygen in nitrogen causes

a deposition rate similar to that with just nitrogen. Increasing the

oxygen concentration to 5% increases the deposition rate. Increasing the

oxygen concentration to 21% increases the deposition rate further. For the

conditions used in these tests the deposition rates are linear from time zero indicating that the metal catalytic effect is retained. For more

severe conditions or longer test times it would be expected that the

deposition rates would decrease due to diminishing catalytic effect of the

metal with increasing coating thickness.

The effect of lubricant vapor concentration on TMPTH deposition rate

is shown in figure 63. Tests were conducted using 0.4 and 1.0% TMPTH in

nitrogen on stainless steel substrates at 700°C. The data show that

increasing the TMPTH vapor concentration from 0.4 to 1.0% increases the

deposition rate. The rates are linear from time zero indicating that for

this severity of tests the stainless steel substrate retains its catalytic

effect. Also, the rate is approximately linearly related to the lubricant

concentration

The surface morphology of TMPTH deposits was examined by Scanning

Electron Microscopy (SEM) . Figure 64 shows an uncoated stainless steel

surface. Figures 65-67 show TMPTH deposits on stainless steel at a coating thickness of 78 and 113 /xg/cm^ . These deposits were formed using

1.0% TMPTH in nitrogen. Figures 68-71 show thicker coating of TMPTH

deposit on stainless steel vising combinations of nitrogen and air as the

carrier gas. Coating thicknesses range up to 413 ixg/cm^ (fig. 71). It is

observed that TMPTH deposits are quite different from phosphate esters and

PPE deposits. TMPTH deposits have a filamentous structure in contrast to

the globular structure that was observed with phosphate esters and PPE

deposits. The morphology of TMPTH deposits look very similar to that of

coke deposits [19]. Several investigators have observed

filamentous- tubular deposits in the formation of coke from hydrocarbons

[20-22]. It has been reported that these deposits often contained highly

dispersed metal particles [23] . Some researchers have observed metal

particles concentrated at the tip of these filamentous deposit structures

[22] . It was suggested that these metal particles are carried away from

the substrate surface by the growth process. Figures 72-80 show SEM

micrographs of TMPTH deposits on iron, nickel and copper. It is observed

that for iron and nickel, the deposits appear similar to that on stainless

steel. With thicker coatings the filamentous structure of the deposits

become clearer. TMPTH deposits on copper appear to be different than that on iron, stainless steel, and nickel. Deposits on copper appear to have a more integral structure, while deposits on iron, stainless steel, and

nickel appeared to be more dispersed. It should be noted that copper

provided an essentially non-catalytic surface for TMPTH deposition. This

lack of catalytic activity may be related to the different surface

morphology observed on copper. It is interesting to note that SEM pictures

of coke deposits on copper surfaces also show a different morphology than

the filamentous deposit observed on nickel and iron surfaces [23]. The

similarity in surface morphology between TMPTH deposits and coke deposits

suggests similarities in mechanisms of deposit formation.

51
H

CM

rHO

0) •M W

U)

V)

4) rHc J

p. •H u

o Pi O

O +-> 3 • 1-1 u z

O < > •

oo

( uiD/3rl) aisodaa

o

o>

O

+-> t3U

o

X

<u

0)o

1/1

0)

o • m uo

<U O

S o

+->

+-> 10 rj 3

U 03 >

a>

o o

+-> •HO -H

(U

o

O

U

a

•H

52
5 10 15 20 25

Time (min .

Figure 61. Effect of temperature on TMPTH deposition on stainless steel,

substrates

TMPTH concentration; 1.0% in nitrogen

53
c

•HE

4-> HHHmo

?-i • o uo > o

aJ •M O

+->

'T3 (D

<D t>0 10 O O U X -H

(1) c

--1 c

0) -rH

+-> V) o\°

t/) CM

(/) (D

O ^ +J c „ •H o\°

rt o

to ( uio/3rt) jisodoQ o O H s c

•HO

+J 'H

•P

!/) Oj

tn !-> > O

in o

tn o c p: ^ (U O M

•HA

X P O

+J (/) •H ;3o O -H a >

VI3

Oh

5A
200

150

100

50

Time (min.)

Figure 63. Effect of TMPTH vapor concentration on Deposition rate of

TMPTH on stainless steel at 700°C.

55
Figure 64. SEM picture of a stainless steel surface,

preheated to 700°C.

Magnification: 2000X
Figure 66. SEM picture of a deposit 113 ug/cm on a stainless steel surface, 1.0% TMPTH in

nitrogen, 700°C.

57
Figure 68. SEM picture of a deposit 122 ug/cm on a stainless steel surface 1.0% TMPTH in

+ 5% 0^, 700°C.

Magnification: 2000X

Figure 69. SEM picture of the same deposit shown in

figure 68.

Magnification: 10,000X

58
Figure 70. SEM picture of a deposit (413 ug/cm ) on

a stainless steel surface 1.0% TMPTH in air.
Figure 72. SEM picture of an Iron surface preheated

to 700°C,

Magnification: 2000X

60
Figure 74. SEM picture of a deposit (2220 ^g/cm) on

an iron surface.

1.0% TMPTH In nitrogen, 700°C.

Figure 75. Same deposit as in figure 74.

Magnification 10,000X.

60
Figure 76. SEM picture of a copper surface preheated

to yoo^c.

Magnification: 2000X

Figure 77. SEM picture of a deposit (173 ug/cm ) on a copper surface.

1.0% TMPTH In air, 700''C.

Magnification: 2000X

62
Figure 78. SEM picture of a nickel surface, preheated

to 700°.

Magnification: 2000X
Figure 80. SEM picture of a deposit (173 ug/cm ) on

a copper surface.

1.0% TMPTH In air, TOO^C.

Magnification: lO.OOOX

64
E. Comparisons of Deposition Rates of Phosphate Esters and Non-Phosphate

Hydrocarbons

A comparison of the initial rates of deposit formation within the

first 1 minute of deposition of phosphate esters TCP and GT, and

non-phosphate hydrocarbons PPE and TMPTH is shown in table 1. Deposition

rates were compared on iron, copper, stainless steel, nickel, and quartz

with lubricant vapor in nitrogen stream at 700°C. These data clearly

indicate that deposition rates are metal specific. For the phosphate

esters TCP and GT the ranking of catalytic activity of metal substrates is

iron > copper > stainless steel > nickel. Nickel and quartz can be

considered to be inert substrates, while iron and copper are strong

catalysts for deposit formation even when relatively thick films are present. Stainless steel falls between the two extremes.

Although the relative catalytic activity of metals is the same for the

phosphate ester, the ranking of catalytic activity is different when

deposits are being formed from hydrocarbon lubricants which do not contain

phosphate. The main difference between deposit formation from phosphate

ester lubricants and non-phosphate ester lubricants and non-phosphate

hydrocarbons is the change in catalytic behavior of nickel. Nickel has the

characteristics of a strong catalyst for deposition from non-phosphate

hydrocarbons, while it behaves essentially as an inert substrate for

deposition of phosphate esters. It is also observed that under equivalent

conditions, deposition rates are higher for deposits formed from phosphate

esters than from non-phosphate hydrocarbons. The ranking of catalytic

activity of metals for deposition of non-phosphate hydrocarbons PPE and

TMPTH closely follows the order observed in hydrogenation-dehydrogenation

and hydrogeno lysis reactions of hydrocarbons.

Table 1. Initial rates of deposit formation

Substrate Lubricant Vapor in Nitrogen Stream at 700°C

1.55% TCP 1.75% GT 1% PPE 1% TMPTH

Iron 290 430 19 127

Copper 210 280 6 1 Stainless Steel 110 45 9 8

Nickel 15 10 16 24

Quartz 15 10 2 1

*All rates were based on initial 1 minute of deposition.

*All rates are in units of /ig/cm^ min. To convert this value to molecular

layers the multiplication factor is approximately 10.

65
F. Deposition Studies With Evaporating Liquid Delivery System

As mentioned in the experimental section, some of the early deposition

data was obtained with a system in which liquid lubricants held in a crucible or boat at elevated temperatures evaporated into a flowing gas

stream. The gas stream then passed over the metal specimen. This was a convenient technique to obtain data at low contact times since the

direction of the gas flow could be changed quickly. However, the exact

concentration of lubricant in the gas phase is not known, and it is

possible that liquid-phase reactions in the crucible could modify the

lubricant. Consequently, the syringe delivery system was developed and the

preponderance of deposition data were obtained with that improved

technique. However, some unique studies involving tributyl phosphate (TBP)

and mineral oil were conducted with the evaporating liquid delivery system,

and the results are reported in this section. These are the only

deposition data available for these materials. The exact concentration of

the lubricant in the vapor is not known for these studies, but the maximum

concentrations which would be realized if the liquid in the crucibles

saturated the gas phase are reported in the following tables. The actual

lubricant concentration is approximately 5% of the saturation value.

Results presented in the wear test section of this report show that

TBP is the most effective material studied for providing a lubricating film

at low temperatures. A comparison of the deposition characteristics of TBP

with other lubricants is presented in table 2. All of these results were obtained with the liquid evaporation delivery system so that they are comparable even though the exact lubricant concentrations in the vapor are not known. The TBP-produced deposits are nearly an order of magnitude

thicker than the coatings produced by TCP under identical conditions.

Similarly, the deposition effectiveness of TMPTH approaches TBP only when

the deposition temperature is 250°C higher.

Further information concerning the deposition kinetics of coatings

formed from TBP are presented in table 3. Measurable quantities of coating

from TBP could be detected at low temperatures and very short contact

times. These results are consistent with the wear results which indicated

that TBP is the most effective lubricant studied for low- temperature

applications. Deposition data for mineral oil are presented in table 2.

Coatings can be formed from mineral oil vapors but the thickness of these

coatings are low compared to the other materials considered in this study.

As one might expect, the presence of oxygen might have a tendency to

improve the coating characteristics of mineral oil. Liquid-phase oxidation

studies indicate that low molecular weight oxidation products of mineral

oil form high molecular weight materials and deposits by a polymer

condensation process. It is possible that in the presence of oxygen, low

molecular weight oxidation products that would form in the gas phase with

these materials would then polymerize to form a coating on the metal

surfaces. The influence of oxygen concentration on the deposition of three

different lubricants is presented in table 4. The formation of coatings

from mineral oil vapors shows a trend which is consistent with this

66
proposed mechanism. An increase in the concentration of oxygen in the

carrier gas causes a corresponding increase in the amount of coating formed

by mineral oil.

In contrast, the presence of oxygen does not appear to influence the

formation of deposits formed from DEHS , while an increase in oxygen

concentration reduces the formation of a deposit from vaporized TMPTH. It

is possible that the reaction of oxygen with vaporized lubricants creates an oxidized product which is more susceptible to decomposition and the

formation of deposits. In contrast, an alternative mechanism would involve

the oxidative decomposition of a coating which would reduce the net amount

of deposit present at any time. These data seem to indicate that this

oxidative degradation may be occurring with TMPTH. However, studies

concerned with the influence of oxygen on coatings formed from TMPTH using

the syringe pump delivery systems show contradictory results. For example,

figure 62 in the earlier section which discussed the deposition studies

with TMPTH showed that oxygen increased the rate of deposit formation from

this ester. The conflicting results obtained with these two deposition

techniques may be due to one of the problems inherent with the evaporating

liquid-delivery system. It is possible that oxidation of the liquid TMPTH

occurs in the crucible at the elevated temperatures in the evaporation

section of the apparatus. This oxidation process could lead to

pol3rmerization of the TMPTH in the liquid which would reduce the vapor

pressure of the liquid and, consequently, reduce the amount of TMPTH in the

gas stream when it impinges on the stainless steel substrate. Because of

this and other possible complications associated with the evaporating

liquid-delivery system, the syringe pump delivery system was the preferred

technique in these studies. It should be noted, however, that the delivery

system in an actual application may, in some cases, more closely simulate

the evaporating liquid experimental technique. In conclusion, the

concentration of oxygen in the gas stream can influence the kinetics of

film formation. Oxygen can enhance or inhibit deposition rates depending

upon the specific characteristics of the lubricant and the delivery system

configuration

67
Table 2. Comparison of deposit formation of various lubricants

Temperature Carrier Lubricant

(°C) Gas Deposit

10 sec. 5 min.

6% 1.2%

Tributyl Phosphate (TBP) 650 N2 120 1350

Tricresyl Phosphate (TCP) 650 N2 17 146

Mineral Oil 750 5% O2 in N2 22 132

Polyol Ester (TMPTH) 900 1% O2 in N2 170 1007

* Deposits were formed on stainless steel substrates

* 1 ng/crn^ coating thickness corresponds to 10 monolayers

Table 3. Effect of temperature on deposit formation of

tributyl phosphate (TBP) on stainless steel

Time (sec.) Deposit (uz/cm—)

320°C 430°C 650°C

3 5 10 38

5 6 25 70

10 8 73 ,120

15 10 85 175

20 12 113 190

* 6% TBP was used in nitrogen carrier gas

* 1 fig/cm^ coating thickness corresponds to 10 monolayers

68
Table 4. Effect of oxygen concentration on deposit

formation of various lubricants

Temperature Deposit (^g/cm^)

Lubricant °C Oxygen Concentration (%)

1 2 3 4 5

Trime thy 1 o Iprop ane Triheptanoate (TMPTH)

800 390 353 315 231 189

Di-2-Ethyl hexyl Sebacate

(DEHS)

800 105 103 104 107 106

Mineral Oil 750 93 107 107 118 132

* 1.2% of lubricant vapor was used in carrier gas

* Deposits were formed on stainless steel substrates

* Deposits shown are after 5 minutes test time

* 1 ^g/cm^ coating thickness corresponds to 10 monolayers

I

69
IV. WEAR STUDIES OF VAPOR-DEPOSITED LUBRICANT FILMS

The discussion in the preceding section on the kinetics of vapor

deposition clearly shows that at elevated temperatures, films of

substantial thickness are formed from lubricant vapors at relatively low

vapor concentrations. A modified four-ball wear tester was used to

investigate the wear characteristics of vapor-deposited lubricant films.

The wear tester and its operating procedure have been described in section

II of this report. All tests were conducted for 30 minutes using M-50

tool-steel balls and a rotational speed of 600 rpm. For most of the tests

the operating temperature was maintained at 371°C (yoO^F) . With TBP , some tests were conducted at 270°C.

The wear scar diameter, measured using an optical microscope, was taken to be a good indication of the wear resistance of the lubricant film

The effect of load, lubricant vapor concentration, and carrier gas

composition on wear were examined.

A. Phosphate Esters (TCP, TBP, and GT)

The anti-wear effectiveness of vapor-deposited films from tricresyl

phosphate (TCP) , tributyl phosphate (TBP) , and diphenyl ditertiary butyl

phenyl phosphate (GT) were examined for films formed on M-50 tool-steel

balls. The selection phosphate esters exhibited substantially different

properties with respect to liquid phase thermal, oxidative, and

hydrolytic stabilities. The temperatures at which the wear tests were conducted were always above the temperature at which water can exist in th

contact zone; therefore, hydrolytic stability was not considered an important property.

TBP has relatively poor oxidation stability; TCP has good oxidation

stability; and GT has excellent oxidation stability. The thermal stabilit;

of the alkyl phosphate (TBP) is generally not as good as that of triaryl

phosphates, TCP and GT [15]. The four-ball wear tests on vapor-deposited

lubricant films show no general correlation with either liquid-phase

thermal or oxidative stabilities.

The effect of load on wear scar is shown in figures 81 and 82 for

TBP, TCP, and GT vapor-deposited films. These tests were conducted at

371°C. For data in figure 81, 0.5% lubricant vapor concentration in

nitrogen was used. For data in figure 82, 0.75% lubricant vapor

concentration in nitrogen was used. In figures 81 and 82, the Hertz elasti(

deformation line is shov/n to indicate zero wear.

The wear caused by the lubricated system can be considered to be the

difference between the wear scar and the Hertz elastic deformation value.

A good lubricant will show wear parallel to the Hertz deformation line

until seizure occurs. Seizure or scuffing produces wear that can be

several times that noted during pre -seizure conditions.

70
The data in figures 81 and 82 show that at both 0.5 and 0.75%

lubricant vapor, TCP provided excellent wear protection over the range of

loads studied, with no seizure even at 50 kg load. A comparison of figures

81 and 82 shows that increasing the TCP vapor concentration from 0.5 to

0.75% caused no significant change in wear values for TCP. The data for

TBP indicate that while using 0.5% TBP in nitrogen (fig. 81), seizure

occurred at 30 kg load. During pre-seizure at lower loads, wear values

using TBP were similar to that using TCP. Increasing TBP vapor

concentration from 0.5 to 0.75% (fig. 82) increased the wear scar at

equivalent loads, but the seizure load was raised to 40 kg. The increase

in wear values maybe attributable to an increase in coating rate and a subsequent increase in three -body wear caused by trapping of wear particles

in the coating.

The data for GT indicate that while using 0.5% lubricant vapor

concentration (fig. 81), wear values were somewhat higher than for the

other phosphate esters under equivalent conditions. Seizure occurred at a load of 30 kg. Increasing the GT vapor concentration from 0.5 to 0.75%

(fig. 82), the wear values improved slightly but there was no improvement

in seizure load. Seizure still occurred at 30 kg load.

It should be noted that in wear tests conducted with a typical liquid

mineral oil lubricant at 75 °C, wear values are about the same or somewhat

higher than that observed with the vapor-deposited films from phosphate

esters. With liquid mineral oil lubricants, seizure occurs at 30 to 50 kg

load depending on the type and grade of mineral oil. Also, the best

correlation between four-ball wear tests and hydraulic pump lubrication is

in the range of 5 to 10 kg load. On this basis, it may be concluded that

the vapor-deposited films from the phosphate esters studied, TCP, TBP, and

GT, provide good lubrication.

The effect of lubricant vapor concentration on wear is shown in figure

83. A series of tests were conducted using TBP, TCP, and GT in nitrogen at

371 °C. The wear data were compared at a constant 10 kg load. The actual

wear caused by the lubricated system is the difference between the measured

wear scar and wear scar caused by elastic deformation (Hertz line) . These

data show that TCP has a minimum wear value at about 0.5% TCP vapor. This

behavior has been attributed to deposit formation. At low vapor

concentrations, the deposition rate may be adequate in providing an easily

sheared film that is being continuously replenished. At higher lubricant

vapor concentrations and, hence, higher deposition rates, excess deposit

may trap or collect wear debris which could cause abrasive three -body wear. The data in figure 83 show that with GT vapor, there is a decrease in wear

over the concentration range of 0.2 to 0.8%. If there is an optimum GT

concentration for minimum wear, it would be expected to be in the

concentration range above 0.8%. For TBP, the data in figure 83 show that over the concentration range 0.2 to 1.0%, the wear scar observed is about

the same. At concentrations less than 0.1%, there is a significant

increase in wear scar due to seizure.

71
Load (kg)

Figure 81. Four-ball wear scar diameter versus load for 0.50 lubricant

vapor concentration in nitrogen stream. Tests were conducted

using M-50 tool steel balls at 371°C, 600 rpm.

0.1 1 I I I I I I I I

t I I I I I I I I 3 5 7 10 20 30 50 70 loo

Load (kg)

Figure 82. Four-ball wear scar diameter versus load for 0.75 lubricant

vapor concentration in nitrogen stream. Tests were conducted

using M-50 tool steel balls at 371°C, rpm.

72
1.0

0.2 o7k oTb oT8

f» Lubricant

1.0 1 .2

Figure 83. Four-ball wear scar diameter versus lubricant vapor

concentration in nitrogen stream. Tests were cdnducted using

M-50 tool steel balls at 371''C with 10 kg load, 600 rpm.

73
l.C o 371 °C

- 0

" / /U

0.9

0.8

0.7

0.6

0.5

£k

0.4 O

o O O O

0.3 o

0.2 Hert: Line

1 1 1 1 f 0 0.2 0.4 Q.b 0.8 1.0

% TBP in NFigure 84. Four-ball wear scar diameter versus concentration of tributyl

phosphate at 270°C and 371°C, M-50 steel balls, 600 rpm, 10 kg

load.

74
Figure 84 compares the effect of TBP vapor concentration on wear at

two different temperatures (371 and 270°C) . The data show that at 270°C

minimum wear occurs at an optimum TBP vapor concentration of 0.5%. Lower

vapor concentrations cause higher wear. Lower deposition rates at lower

concentrations may not be adequate in replenishing the lubricant film at a sufficient rate. Increase in wear at higher vapor concentrations is caused

by abrasive wear due to wear particles trapped in excess deposit.

Comparison of wear data at 371 and 270°C indicate that at all vapor

concentrations wear values at 270 °C are lower than wear values at 371 °C.

With a decrease in temperature, there is an apparent decrease in wear.

The effect of carrier gas composition on wear was examined using GT

vapor in nitrogen and air at three different concentrations. Wear data

from tests conducted using 0.03, 0.20, and 0.46% GT vapor in nitrogen and

air are shown in figures 85 and 86. These figures also include the Hertz

elastic deformation line to indicate zero wear. These data clearly

demonstrate that even at low vapor concentrations in both nitrogen and

oxygen, effective lubricant films are formed that have good anti-wear

characteristics at elevated temperatures. Comparison of figures 85 and 86

indicates that the influence of oxygen in air on the lubricating

characteristics of the vapor-deposited films is not nearly as significant

as the influence of oxygen in air on the rates of film deposition. Also,

wear appears to be a significant function of lubricant vapor concentration

when nitrogen rather than air is the carrier gas. Figure 85 shows that the

influence of the lubricant concentration on wear reverses as one traverses

across a range of loads. At low loads, the minimum wear is realized at low

concentration while the optimum concentration is higher at high loads.

The basis of good wear characteristics is a balance of the rates of

competing processes. The lubricant coating is formed from the gas phase at

a rate that is usually enhanced by increases in temperature and lubricant

concentration in the vapor. The coating is dissipated or removed by wear

at a rate that is enhanced by variables such as load and velocity. If the

rate of coating formation is too small compared to the rate of removal, the

resulting coating will be too thin to give sufficient lubrication, and

extensive wear of the metal surface will occur. If the rate of formation

overwhelms the rate of removal, excess coating will gather on the surface

and trap debris particles thereby causing undesirable three-body wear.

Figure 85 indicates that at the low loads, the lowest lubricant

concentration is adequate to maintain an optimum lubricating coating. In

contrast, at the high loads, higher vapor concentrations are required to , m.aintain adequate coating thickness. *

The data in figure 86 show that in air with a lubricant vapor

concentration of 0.03%, seizure occurs at a 20 kg load. With higher

lubricant vapor concentrations, seizure has not yet occurred at a 20 kg

load, but conditions may be close to seizure. The relative insensitivity

of lubricant vapor concentration to wear in the presence of oxygen may be

due to higher deposition rates in the presence of air even with low

lubricant vapor concentrations.

75
These studies on wear characteristics of phosphate ester

vapor -deposited lubricant films have shown that films formed from TCP, TBP

and GT are effective and have excellent anti-wear characteristics at

elevated temperatures.

76
5.0

A.O

u

3u

«

3.0-

2. CI -

l.( _

0.8

o.e

0.

Figure 85

0. 2 -

o.r

0.033Z GT

O 0.20AX CT ^ 0.463Z GT

J 1—I I I t I I J I 1 I 1 I .1

4 5 6 7 89 10 20 30 40 60 80 100 Load (kg).

Four-ball wear scar diameter versus load for various GT

vapor concentrations in nitrogen stream at 371 "C. Tests

were conducted using M-50 tool steel balls at 600 rpm for

30 minutes

Load (kg)

Figure 86. Four-ball wear scar diameter versus load for various GT

vapor concentrations in air stream at 371 °C. Tests were

conducted using M-50 tool steel balls at 600 rpm for 30

minutes

77
B. Non-Phosphate Hydrocarbons (PPE and TMPTH)

The modified four-bali wear tester was used to examine the anti-wear

characteristics of vapor -deposited films formed from PPE and TMPTH. The

effect of load, carrier gas composition and lubricant vapor concentration

was investigated. Figures 87-89 show wear data for PPE vapor-deposited

films. The effect of load on wear scar diameter is shown in figure 87.

For these tests, 0.11% PPE vapor was used in an air stream at 371°C. The

Hertz elastic deformation line is shown to indicate zero wear. These data

show that the PPE vapor-deposited film acts as a good lubricant at 5 and 10

kg loads. Seizure with recovery occurs at a 20 kg load. Based on the

criteria that the best correlation between four-ball wear tests and

hydraulic pump lubrication is in the range of 1 to 10 kg load, the PPE

vapor-deposited film can be considered a good lubricant at these load

conditions. Figure 87 also shows a comparison of the anti-wear behavior of

PPE vapor-deposited lubricant and PPE liquid lubricant. The liquid

lubricant tests were conducted at standard four-ball wear test conditions

(10 ml liquid lubricant at 75°C) . At a 10 kg load, the wear scar diameter

using vapor-deposited lubricant at 371 °C is less than the wear scar diameter using the liquid lubricant at 75°C. These data indicate that at

the relevant load conditions, the vapor-deposited lubricant provides better

wear protection than the liquid lubricant.

The effect of oxygen concentration in the carrier gas on wear is shown

in figure 88. 0.11% PPE was used in a carrier gas that was a mixture of

nitrogen and air where the oxygen concentration was varied from 0 to 21%.

These data show that in this concentration range there is no significant

change in wear scar diameter with oxygen concentration. The effect of PPE

vapor concentration on wear is shown in figure 89. Tests were conducted in

air at 371 °C at a 10 kg load. The PPE vapor concentration was varied from

0.045 to 0.5%. The data show that an increase in PPE vapor concentration

causes an increase in wear. This increase is more significant at low PPE

vapor concentrations. Increase in wear with increasing lubricant vapor

concentration can be due to abrasive three -body wear caused by wear

particles trapped in excess deposit.

Figures 90-91 show wear data for TMPTH. Figure 90 shows the

effect of load on wear scar for tests conducted using 0.5% TMPTH in air.

A range of loads from 5 to 60 kg was used. The Hertz elastic deformation

line is shown to indicate zero wear. These data show that even up to a load of 50 kg, the TMPTH vapor-deposited film is acting as a good lubricant

and seizure has not occurred. It is apparent from the somewhat greater

increase in wear scar at a 60 kg load that seizure conditions are being

approached. These data indicate that at the relevant load conditions of 1 to 10 kg where there is good correlation between four-ball test data and

hydraulic pump lubrication, the TMPTH vapor-deposited film acts as an excellent lubricant. Even with more severe conditions up to a load of

50 kg, the TMPTH vapor-deposited film is a good lubricant.

78
The effect of oxygen concentration in the carrier gas on wear for

TMPTH vapor-deposited films is shown in figure 92. For this series of

tests, 0.5% TMPTH was used in a carrier gas that was a mixture of nitrogen

and oxygen. The oxygen concentration was varied from 0 to 20% The data

are shown for a range of loads from 5 to 50 kg. The Hertz deformation line

is shown to indicate zero wear. The data show that at low oxygen

concentrations, increasing the oxygen concentration decreases wear. At

higher oxygen concentrations, increasing the oxygen concentration has

little effect on wear. In more severe conditions at high loads and high

oxygen concentrations, there may be a slight increase in wear with

increasing oxygen concentration. This behavior indicates that at low

oxygen concentrations, increasing oxygen concentration leads to a corresponding increase in deposition rate that is beneficial in decreasing

wear, but, at high oxygen concentrations under more severe conditions,

increasing oxygen concentration leads to a corresponding increase in

deposition rate to levels that can cause additional wear due to wear

particles being trapped in excess deposit and causing abrasive three-body

wear.

The effect of TMPTH vapor concentration on deposition rate is shown in

figure 92. For this series of test, a lubricant vapor concentration of

0.2 to 1.45 % was used in air at 371 °C. The tests were conducted at a constant load of 10 kg. The Hertz deformation line is shown to indicate

zero wear. These data show that there is an optimum TMPTH vapor

concentration for minimum wear. This optimum occurs at about 0.8% TMPTH

vapor concentration. This optimum compares with an optimum of 0.5%

obtained for TCP and TBP . An optimum was not observable in the

concentration ranges studied for GT and PPE . At low lubricant vapor

concentrations, increase in TMPTH concentration causes an increase in

deposition rate. An increase in coating thickness under these

conditions decreases the wear scar. At higher lubricant vapor

concentrations , increase in TMPTH vapor concentration and corresponding

increase in deposition rate causes additional wear due to wear particles

being trapped in excess deposit and causing three-body wear. At the

optimum TMPTH vapor concentration, the deposition rate is such that a sufficient coating thickness is maintained without an excess deposit and

the corresponding abrasive wear.

These studies on wear characteristics of PPE and TMPTH vapor-deposited

films have shown that at elevated temperatures lubricant films formed from

PPE and TMPTH vapor are effective and act as good lubricants in reducing

wear

It should be noted that the limitations of the modified four-ball wear

test limited the temperature levels at which measurements could be

conducted. The temperatures considered are relatively low when the

kinetics of deposition formation are presented in the previous section are considered. The deposition studies would indicate that only very thin

films would be formed under the temperature and lubricant concentration

conditions of these wear studies. The anticipated film thickness would be

reduced even further by the short exposure time of the wear test.

Consequently, the most significant result of these wear studies is that

79
effective lubrication can be realized with a very thin vapor-deposited

coating. The vapor-deposition kinetic measurements indicate that higher

temperatures would be more conducive for the formation of coatings, and

effective lubrication can probably be realized with a very low lubricant

concentrations at elevated temperatures.

80
2.0

S 0- 8

03

vapur I'cposiLCU lAIDnCallL

Polyplicny 1 r.thcr A Luquid Lubricant

A

- — ^ " »\ctti

1 r 10 20 30 40 50 Load (kg)

Figure 87. Four-ball wear scar diameter versus load for vapor

deposited lubricant (0.11% lubricant vapor concentrtion

in air stream at 371°C) and liquid lubricant (10 ml liquid

lubricant at 75°C). Tests were conducted using M-50 tool

steel balls at 600 rpm for 30 minutes.

0.8 -

0.7 -

0.6 -

0.5 -

0.4

2 0.3 -

0.2 -

0.1 -

Figure 88.

15 20 Percent O.xyccn

Four-ball wear scar diameter versus oxygen concentration

in vapor stream. Tests were conducted using M-50 tool steel

balls at 371°C with 10 kg load, 600 rpm, for 30 minutes.

81
Figure 89. Four-ball wear scar diameter versus lubricant vapor

concentration in air stream. Tests were conducted

using M-50 tool steel balls at 371°C with 10 kg load,

600 rpm, for 30 minutes.

62
1

1 I I I I I—

o

o

83
84
in

•H

cno

COo

•H

td

!>

•u

cCC

0)bOO

J-i ^

•H0C

•HCM O

+Jaa

1-1 QJp. o

cn o

Cfl

>-i "

<U CO

V4

0)

CO

1-1 o

cd in o I CO a

CO

O

0) o

15

O

.-H CO

.-Io S

CNJ3

•H

85
V. CONCLUSIONS

The vapor-phase lubrication studies discussed in this report have

shown that lubricating films of substantial thickness can be formed by

vaporizing a liquid lubricant in a carrier gas and depositing thermal

and/or thermal-oxidation products of the vaporized lubricant on the

substrate surfaces at elevated temperatures. These vapor-deposited

lubricating films exhibited excellent anti-wear characteristics. The

formation of these lubricating films was found to be dependent on lubricant

type, substrate material, temperature, lubricant vapor concentration, and

carrier gas composition. Tests conducted using phosphate esters, organic

acid esters, polyphenyl ether, and mineral oils demonstrated that these

conventional lubricants, when delivered from the vapor phase at

temperatures from 370 to 850°C, formed deposits with good lubrication

characteristics. An increase in temperature generally caused an increase

in deposition rate. Deposition rates were metal specific; i.e., some metals had a significant catalytic effect on the rate of deposition. Iron

had a strong catalytic effect for both phosphate esters and non-phosphate

hydrocarbons. Copper was catalytic with phosphate esters but essentially

inert with respect to non-phosphate hydrocarbons. Nickel was catalytic

with non-phosphate hydrocarbons but was essentially inert with phosphate

esters. Stainless steel showed catalytic activity that diminished rapidly

with increasing coating thickness. All of the metals lost their catalytic

effect after a sufficient coating thickness was achieved. The coating

thickness needed for this decrease in metal catalytic effect was dependent

on type of metal, type of lubricant, temperature, lubricant vapor

concentration, and carrier gas composition. The ranking of catalytic

activity of metal substrates for deposition of non-phosphate hydrocarbons

closely followed the order observed in hydrogenation-dehydrogenation and

hydrogenolysis reactions of hydrocarbons. The decrease in catalytic effect

with increasing coating thickness was attributed to a decrease in the

availability of metal or metal organic product in the coating. With

thicker coatings, there may have been a diffusion barrier for metallubricant interaction.

Increasing lubricant vapor concentration or oxygen concentration in

carrier gas increases the rate of deposition. The level of increase is

dependent on lubricant type, metal substrate, and temperature. Increases

in rate of deposition are dependent on coating thickness. With thicker

coatings, the effect of increasing lubricant vapor concentration or oxygen

concentration on rate of deposition decreases.

SEM studies of the surface morphology of the vapor-deposited films

show that the structure of the deposits can vary with lubricant type, metal

substrate, and coating thickness. With phosphate esters and PPE, the

deposits appear to have a granular structure. These granules appear

smaller for thin coatings and larger for thick coatings. These larger

granules with thicker coatings could be acting as a diffusion barrier for

metal- lubricant interaction. The coating appears to be more porous near

the metal surface and less porous away from the surface. TMPTH deposits,

unlike phosphate esters and PPE, have a filamentous structure. These

86
filaments appear more dense with thicker coatings. EDAX analysis of the

coating shows that there is a substantial concentration of metal in the

coating. The metal concentration is higher in the coating next to the

metal surface compared to coating away from the surface. This further

indicates that the coating is acting as a diffusion barrier for

lubricant-metal interaction. Four-ball wear tests show that the

vapor-deposited lubricant films exhibit excellent anti-wear

characteristics. The wear characteristics were dependent on parameters

that had an effect on coating rate such as lubricant type, temperature,

lubricant vapor concentration, and carrier gas composition. At the

temperatures and lubricant vapor concentrations used in this study,

very thin coatings would be expected. Even at these thin coating levels,

good lubrication was achieved. When the coating thickness becomes too

small, there is a lack of lubricant at the contact and wear can increase.

When the coating thickness becomes too large, wear can increase also. With

thicker coatings, excess deposit can trap or collect wear particles and

cause abrasive three-body wear. There exists an optimum coating thickness

for optimum wear. Both phosphate esters and non-phosphate hydrocarbons,

when delivered to the contact from the vapor phase, form lubricant films

that exhibit excellent anti-wear characteristics.

87
VI . REFERENCES

[1]. Pinkus, 0. and D. E. Wilcock, "Strategy for Energy Conservation

Through Tribology," 174 pages, The American Society of Mechanical

Engineers, N.Y. , 1977

.

[2]. Kamo, R. , "The Adiabatic Diesel," Tribology Workshop, February 1977,

Washington, DC.

[3]. Archard, J.F., "The Temperature of Rubbing Surfaces," Wear 2, p. 455

(1959)

.

[4]. Fein, R.S., "Transition Temperatures With Four-Ball Machine," ASLE

Trans. 3, p. 34 (1960).

[5]. Francis, H.A., "Interfacial Temperature Distribution Within a Sliding Hertzian Contact," ASLE Trans. 14, p. 41 (1970).

[6]. Bos, A., "The Temperature at the Wear Scars of the Four-Ball

Apparatus," Wear 31, p. 17 (1975).

[7]. Hsu, S.M. and E.E. Klaus, "Estimation of Molecular Junction

Temperatures in Four-Ball Contacts by Chemical Reaction Rate

Studies," ASLE Trans. 21 (3), p. 201 (1978).

[8]. Turchina, V., D.M, Sanborn and W.O. Winer, "Temperature Measurements

in Sliding Elastohydrodynamic Point Contacts," J. Lub. Tech. 96, p.

464 (1974) D, p. 909.

[9]. Ausherman, V.K. , H.S. Nagaraj , D. M. Sanborn, and W.O. Winer,

"Infrared Temperature Mapping in Elastohydrodynamic Lubrication," J.

of Lub. Tech. 98 F(2) , p. 236 (1976).

[10]. Klaus, E.E. and E.J. Tewksbury, "Microcorrosion Studies With

Functional Fluids," ASLE Trans. 29 (5), p. 205 (1973).

[11]. Diamond, H. , H.C. Kennedy and R.G. Larsen, "Oxidation

Characteristics of Lubricating Oils at High Temperatures," Ind. and

Eng. Chem. 44, P- 1834 (1952).

[12]. Brook, J.H.T., "A Circulatory Oxidation Test," J. Inst. Petrol. 48

(457), p. 7 (1962).

[13]. Oberright, E.A. , S.J. Leonard and H.L. Hepplewhite "DepositForming Tendencies of High-Temperature Lubricants," ASLE Trans. 7 (1) , p. 64 (1964)

.

[14]. Cvitkovic, E. , E.E. Klaus and F.E. Lockwood, "A Thin Film Test for

Measurement of the Oxidation and Evaporation of Ester Type

Lubricants," ASLE Trans. 22, p. 395 (1979).

88
[15]. Cho, L. and E.E. Klaus, "Oxidative Degradation of Phosphate Esters,"

ASLE Trans. 24, p. 119 (1981).

[16]. Lahijani, J., F.E. Lockwood, and E. E. Klaus, "The Influence of

Metals on Sludge Formation," ASLE Preprint No. 80-LC-1C-4 (1980).

[17]. Sinfelt, J.H., Progress in Solid State Chemistry 10(2), p. 55

(1975).

[18]. Gunsel, S., "Development and Evaluation of a High-Temperature

Lubrication System," Ph.D. Thesis, The Pennsylvania State University

(1986) .

[19]. Marek, J.C., L.F. Albright, "Surface Phenomena During Pyrolysis,"

Coke Formation on Metal Surfaces, ACS Symposium Series 202 (1982).

[20]. Marek, J.C., L.F. Albright, "Formation and Removal of Coke Deposited

on Stainless Steel and Vycor Surfaces from Acetylene and Ethylene,"

ACS Symposium Series 202 (1982).

[21]. Lacava, A. I., et al . , "Effect of H2 on the Iron and Nickel

Catalyzed Formation of Carbon from Benzene," ACS Symposium Series

202 (1982).

[22]. Baker, R.T.K., D.J.C. Yates, J. A. Dumesic, "Filamentous Carbon

Formation Over Iron Surfaces," ACS Sjrmposium Series 202 (1982).

[23]. Lacava, A. I., et al .

, "Mechanism of Surface C-Formation During the

Pyrolysis of Benzene in the Presence of Hydrogen," ACS Symposium

Series 202 (1982).

89
Appendix A. COOPERATIVE EFFORTS

Cooperation between Pennsylvania State University (PSU) and industry

has been expanded as a result of the present studies on vapor phase

lubrication. A second year of a project supported by ALCOA has begun. The

graduate students involved in that project do part of their research at PSU

and part at the ALCOA Research Center. The two graduate students involved

in this program thus far have conducted vapor -delivered wear tests on the

ALCOA high- temperature tests in the range of 300 to 600°C. This tester has

been described in the literature by Dr. Robert Bruce. Surface analytical

facilities are also available for evaluation of coatings of the wear

specimens before and after the wear tests. Results from that work can be

compared with four-ball wear tests at 371 °C and static deposition tests up

to 850°C as used in the present report. Tests using steel-on-steel

specimens in the ALCOA (Bruce) wear tester have shown that vapor-phase

pre- lubrication on only one of the specimens provides good high- temperature

wear and friction values for the four classes of lubricants evaluated thus

far. The lubricants include alkyl and aryl phosphate esters, polyphenyl

ethers, and organic acid esters.

A proposal was prepared jointly by PSU and Martin-Marietta to

develop a vapor-phase lubricated forging system. The feasibility phase of

this proposal was completed successfully with EPA and Forging Council

sponsorship. Support funding for the next phase of this program is not

available at this time. The program has demonstrated the lubricating

ability of vapor-delivered phosphate esters as well as the reduced amount

of lubricant used and the significant reduction of pollution.

The Chemical Engineering Department of PSU and the Ceramics Division

of the National Bureau of Standards have begun the third year of a cooperative program. Graduate students from PSU are conducting thesis

research at NBS under the guidance and supervision of Dr. S. M. Hsu. This

work includes a high- temperature wear tester developed at NBS for the study

of metal and ceramic specimens at high temperatures and for vapor-delivered

or more conventional liquid lubrication.

The Gas Research Institute has recently established a Center for

Advanced Materials (CAM) at PSU. The CAM is an engineering center for

research in ceramics including high- temperature tribology. Research

sponsored by CAM is focused on the specific effects that result from the use of natural gas as a fuel. Current plans call for research to be

conducted at PSU, NBS, and the University of Illinois.

90
Appendix B. PUBLICATIONS, THESES, AND PRESENTATIONS

Publications :

1. Klaus, E. E. and C. W. Lai, "VAPOR-DEPOSITED LUBRICANTS FOR EXTREME

CONDITIONS," ASLE Special Publication 15, "Solid and Liquid Lubricants

for Extreme Environments," pp. 75-79 (1983).

2. Pinto, N. D. G., J. L. Duda, E. E. Graham and E. E. Klaus, "IN SITU

FORMATION OF SOLID LUBRICATING FILMS FROM CONVENTIONAL MINERAL OIL AND

ESTER BASE LUBRICANTS," ASLE Proceedings, 3rd International Conference

on Solid Lubrication, ASLE Pub. 14, pp. 98-104 (1984).

3. Graham, E. E. and E. E. Klaus, "Lubrication from the Vapor Phase at

High Temperatures," ASLE Preprint No. 85AM1B1 , May (1985).

Theses

1. Gunsel, S., "Development and Evaluation of a High-Temperature

Lubrication System," Ph.D. Thesis, The Pennsylvania State University,

University Park, PA 1986.

2. Jeng, G. S., "Interaction at High Temperatures of Phosphate Esters in

the Vapor Phase With Metal Surfaces," Ph.D. Thesis, The Pennsylvania

State University, University Park, PA (1986).

Presentations :

1. Klaus, E. E. , "Vapor-Phase Lubrication in Engines," Gas Research

Institute Workshop, June 5 (1986).

91
NBS-114A (REV. 2-80) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NIST/SP-744

2. Performing Organ. Report No 3. Publication date

September 1988

4. TITLE AND SUBTITLE

Formation of Lubricating Films at Elevated Temperatures

From the Gas Phase

5. AUTHOR(S)

E. E. Klaus, J. L. Duda, and S. K. Naidu

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

(formerly NATIONAL BUREAU OF STANDARDS)

U.S. DEPARTMENT OF COMiMERCE

QAITHERSBURG. MO 20899

7. Contract/Grant No. 60NANB5D0548

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

8. Type of Report & Period Covered

Topical

SAME AS ITEM #6 ABOVE.

and Department of Energy

Washington, DC 20585

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 88-600554

I I Document describes a computer program; SF-185, FlPS Software Summary, Is attached.

11. ABSTRACT fA 200-word or less factual summary of most si gnificant information. If document includes a significant

bi bliography or literature survey, mention it here)

Conventional liquid lubricants, when subjected to temperatures of 250'*C and

above for extended time in an air atmosphere, degrade rapidly to make large amounts

of solid sludge and deposits. Based on boundary lubrication of bearings, these same lubricants, when subjected to 250*'C to the melting point of the bearing metal, produce

in the micro- to milli-second residence time in the bearing contact enough "friction

poljnner" to result in good lubrication. This report describes the use of these

conventional liquid lubricants delivered in a homogeneous vapor phase where the

carrier gas is nitrogen, air or mixtures of these two gases. The lubricants studied

include alkyl and aryl phosphate esters, organic acid esters, polyphenyl ethers,

and mineral oil.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Boundary lubrication; gas phase; high temperature; tribology.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

1 yl Unlimited

1 1 For Official Distribution, Do Not Release to NTIS

rvl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

97

15. Price

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

USCOMM-DC 6043-P80