NASA Technical Memorandum 86943

The Role of Silver in Self-Lubricating

Coatings for Use at Extreme Temperatures

Harold E. Sliney

Lewis Research Center

Cleveland, Ohio

Prepared for the

Annual Meeting of the American Society

of Lubrication Engineers

Las Vegas, Nevada, May 6-9, 1985

NASA
THE ROLE OF SILVER IN SELF-LUBRICATING COATINGS

FOR USE AT EXTREME TEMPERATURES

Harold E. Sliney

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

SUMMARY

The advantages and disadvantages of elemental silver as a tribological

material are discussed. It is demonstrated that the relatively high melting

point of 961 °C, softness, marked plasticity, and thermochemical stability of

silver combine to make this metal useful in thin film solid lubricant coatings

over a wide temperature range. Disadvantages of silver during sliding, except

when used as a thin film, are shown to be gross ploughing due to plastic deformation under load with associated high friction and excessive transfer to counterface surfaces. This transfer generates an irregular surface topography with

consequent undesirable changes in bearing clearance distribution. This paper

describes research to overcome these disadvantages of elemental silver. A.

comparison is made of the tribological behavior of pure silver with that of

silver formulated with other metals and high-temperature solid lubricants.

The composite materials are prepared by co-depositing the powdered components

with an airbrush followed by furnace heat treatment or by plasma-spraying.

Composite coatings were formulated which are shown to be self-lubricating over

repeated, temperature cycles from low temperature to about 900 °C.

INTRODUCTION

There is a great need in current technology for solid lubricated systems

that will perform satisfactorily over a wide range of temperatures. For example, the achievement of major advances in high temperature lubrication are

essential for the development of more fuel efficient engines such as the adiabatic diesel and advanced turbomachinery (ref. 1). Other examples are the

advanced Stirling engine and numerous aerospace mechanisms. Maximum temperatures of 600 to 1100 °C are anticipated for critical sliding contacts in these

applications (refs. 2 to 5).

Solid lubricant candidate materials that are thermally and chemically

stable (nonreactive) at the temperatures quoted include certain soft oxides,

vitreous glazes, fluorides of alkaline earth metals, and soft noble metals

such as gold and silver. Gold and silver are of course costly, but the amount

used is often small enough to make the use of these metals cost-effective in

specialty lubricants. Silver in particular, is well-known to be ah important

tribological material for a wide range of applications. For example, it has

been used for many years as an electroplated coating on the retainer of rolling

element bearings; steel -backed electroplated silver bearings have been used as

hydrodynamically-lubricated crankshaft and main bearings in high performance

reciprocating engines for aircraft.

Silver, when present as a thin film between hard sliding surfaces., can

function as a solid lubricant film because of its very marked ductility. This
extreme ductility allows the silver film to plastically shear between the sliding surfaces and thereby provide a lubricating function. A problem with thin

electroplated silver is the difficulty in maintaining adequate bonding to the

substrate at elevated temperatures in an oxidizing environment (ref. 6). Also,

if the silver is too thick, it will deform excessively under the normal load

thus destroying dimensional accuracy and adding a significant ploughing force

to the total frictional (tangential) force. Nevertheless, it will be demonstrated in this paper that silver can be used as a lubricating component of

thick coatings from low temperatures to near its melting point of 961 °C if it

is either: (1) heterogeneously alloyed with harder metals; or (2) incorporated

as one component of composite, self-lubricating materials. It is possible to

use silver over this large temperature range not only because of its desirable

shear properties, but also because of its thermochemical stability (particularly oxidation resistance) at high temperatures.

In this paper, research is described in which silver was added to composite coatings containing high temperature solid lubricants in order to improve

low temperature tribological properties. Friction and wear were determined in

a pin on disk apparatus, the pin and disk materials were nickel base superalloys and titanium alloys. The disks were coated with the various experimental coatings that are the subject of this paper. Some of the data were

previously presented in NASA publications, but not in the open literature.

New data are presented for a recently developed chromium carbide based composite coating.

MATERIALS AND COATING PROCEDURES

Descriptive data for coating materials and coating compositions are given

in tables I to IV.

Furnace-Fused Coatings

Reagent grade fluorides of greater than 99.9 percent purity and technical

grades of silver and Mo$2 of 99 percent purity were used in preparing the

furnace-fused coatings. The substrate material was a nickel base superalloy

Inconel 750 precipitation hardened to RQ 35. The disks were solvent cleaned

and sandblasted prior to applying the coatings. The fluoride and silver powders were suspended in a cellulose nitrate lacquer; then sprayed onto the disk

surface with a paint sprayer. The specimens were baked at 90 °C, then heated

in a hydrogen atmosphere at 1060 °C for 15 mih (this temperature is 100 °C

above the melting point of silver and 38 °C above the melting point of the

fluoride eutectic.) The specimens were then moved to a cooling chamber and

finally a nitrogen purge chamber before removal from the furnace. This procedure produced fusion-bonded coatings about 0.0038 cm thick in which microscopic

silver globules were distributed throughout a fluoride eutectic matrix.

Plasma-Sprayed Coatings

Whenever possible, commercially available powders, which are specifically

supplied for plasma spraying, were blended to give the compositions shown in

table III. The Ti-6Al-4V or Inconel 750 substrates were first sandblasted,

then plasma sprayed with a nichrome bond coat about 0.007 cm thick, and finally
with the lubricant coating to a total coating thickness of about 0.050 cm. The

coating was then surface ground to a total coating thickness of 0.025 cm. It

is necessary to grind in a manner that does not cause surface smearing. This

can be done by diamond grinding or with grinding wheels of cubic boron nitride!

Final grinding passes should be very light cuts of no more than about 0.0005 cm

depth.

Tribometer

Friction and wear experiments were run on a standard pin-on-disk apparatus

in which a pin with a hemispherical radius of 0.48 cm is placed in sliding contact with the flat, coated surface of a rotating disk. Normal load is applied

by dead weights and friction force is continuously measured with a temperaturecompensated strain gauge bridge circuit.

RESULTS AND DISCUSSION

It has been known for some time that thermally-fused (or fusion-bonded)

coatings of chemically stable fluoride such as calcium fluoride (CaF2) and

barium fluoride (BaF2) provide effective lubrication of nickel-base superalloys from about 500 to 900 °C (ref. 7). Unfortunately these fluorides are

not effective lubricants at lower temperatures. Metal-fluoride composites

prepared by powder metallurgy techniques (refs. 8 and 9) and by plasma spraying (ref. 10) have similar lubrication/temperature characteristics. Therefore,

formulation studies were conducted at NASA Lewis Research Center with the

objective of improving low temperature lubrication while retaining the good

high temperature lubricating properties of the fluorides (refs. 11 and 12).

Current research has the further objective of achieving harder, very wearresistant coatings with wide temperature spectrum capabilities. Results are

summarized below for the following three approaches: (1) The influence of

Mo$2 and of silver on lubrication with fused fluoride coatings; (2) the

effect of silver on lubrication with fluoride-superalloy, plasma-sprayed coatings^; (3) the lubricating characteristics of chromium carbide coatings containing BaF2~CaF2 eutectic and silver.

or Silver in Fused Fluoride Coatings

A research program was conducted with the objective of reducing the, lowtemperature friction of fused fluoride coatings by the addition of Mo$2 or

silver, both of which are known to form effective solid lubricant films at

moderate temperatures. Since MoS2 oxidizes rapidly above about 350 °C, its

use in this study was to determine whether it could be used in conjunction with

the fluorides for one-time applications where lubrication is required over only

one temperature cycle. Silver was studied as an additive to fluoride coatings

expected to be used repeatedly over a wide temperature range.

Effect of MoSg - Mo$2 was evaluated both as an overlay on, and as an

addition to, the fluoride coating composition. Figure 1 shows the effect of

MoS2 on the friction-temperature.characteristics of BaF2~CaF2 coatings.

In one case, MoS2 is employed as an overlay (top coat) on the fluoride coating; in the other it is an additive within the fluoride coating. Control

experiments were performed with pre-oxidized but otherwise unlubricated
Inconel 750 pins and disks and with BaF2~CaF2 coated disks without Mo$2- The

unlubricated specimens gave high wear rates and friction coefficients of 0.6

at room temperature gradually decreasing to 0.45 at 800 °C. Wear was much

less at all temperatures with the fluoride coatings, but friction was characteristically high up to about 300 °C, then gradually decreased to about 0.2 at

500 °C and remained at that value to 800 °C. With the Mo$2 overlay, there was

a predictable, dramatic reduction in friction coefficients below 300 °C. Upon

oxidation of Mo$2 above 300 °C, friction was controlled by the fluorides and

was therefore the same as observed for BaF2~CaF2 coatings without Mo$2. Similar behavior was observed when Mo$2 was incorporated into the coating composition, friction coefficients never exceeded 0.2 over the entire temperature

range, but again, the Mo$2 oxidized and the beneficial effect was only obtainable over one temperature excursion from room temperature to 800 °C.

Effect of silver - Metallic silver powder was added to the fluoride coating composition in amounts up to 50 wt %. The data in figure 2 show that the

friction coefficients at room temperature were at a minimum for 35 wt % of

silver. At this silver concentration, friction coefficients were 0.2 or less

over the entire temperature range studied. Figure 3 compares the frictiontemperature characteristics of fluoride coatings with and without a 35 percent

silver addition.

Silver in Plasma-Sprayed Fluoride-Metal Self-Lubricating Coatings

The thermally-fused coatings just discussed require heat treatment (firing) at high temperatures above the melting point of at least some of the

coating components in order to achieve fusion bonding. (The procedure is very

similar to that employed in applying porcelain enamels to metals or glazes to

ceramics.) This heat treatment is not acceptable for some alloys including

those of titanium. An alternative method is plasma spraying which needs only

minimally to heat the substrate surface. Plasma spraying is also more convenient and can be used to deposit combinations of materials which are not amenable to fusion bonding.

It was reported in (ref. 12) that plasma-sprayed composite coatings of

nichrome, CaF2, silver, and glass lubricated nickel-base superalloys from

cryogenic temperatures to 900 °C. Figure 4 is a photomicrograph of the composite coating. Figure 5 shows the friction-temperature characteristics of

such a coating compared to a similar coating without silver. It is clear that

the silver addition reduces friction at low temperatures without a significant

adverse effect on lubrication at high temperature.

In a program to solve a specific lubrication problem with titanium alloys

at 430 °C, plasma-spray coatings of increasing complexity from a one component

(pure silver) to the four component composition reported in (ref. 12) were

investigated (ref. 13). All multicomponent coatings contained a one-to-one

weight ratio of silver and nichrome (1:1 Ag-NiCr). CaF2 was the added material in the three component coatings, and a sodium-free glass was the fourth

component. The purpose of the nichrome was to provide a harder, less deformable coating material than pure silver. CaF2 was used because of its wellknown high temperature lubricating ability. Glass was expected to provide a

glazing tendency on the sliding surface as well as providing a degree of

oxidation-protection to the nichrome. The identification numbers and compositions of the coating are given in table III. The coatings were applied to a
titanium alloy (Ti-6 Al-4 V). A bond coat of nichrome about 0.07 mm thick was

plasma-sprayed onto the disks prior to application of the lubricant coatings.

The nichrome bonded we'll to the titanium alloy and the top coats all bonded

well to the intermediate nichrome coating. An excess of top coat was applied,

then ground back to give a total coating thickness of about 0.25 mm (0.010 in)

except where otherwise specified.

The friction characteristics of the coatings in sliding contact with

titanium alloy pins over test durations of up to 20 hr are shown in figure 6.

Surface profiles of the wear tracks at the completion of the tests are shown

in figure 7. Pin and coating wear rates are given in figure 8. Pin wear rates

from (ref. 14) against the uncoated Ti-6 Al-4 V and against a titanium diboride

hard coat on this alloy are also given for comparison.

The lowest sliding friction early in the tests was observed with pure

silver. Friction coefficients were less than 0.1 for 5 hr then increased

abruptly. No wear occurred on the hemispherical pin, but silver immediately

transferred to it and the sliding combination was essentially silver on silver.

9Surface profilometry (fig. 7(a)) showed that the coating was very severely

plastically deformed, but the pin had not penetrated through the silver to the

substrate. This excessive plastic deformation and the heavy silver transfer

make these relatively thick silver coatings unacceptable for the type of sliding contact employed in these tests. The effect of coating thickness will be

described later.

The addition of nichrome produced coatings with a low initial friction

coefficient of 0.1; however, the friction coefficient gradually and continuously increased during the test to 0.33 after 9 hr. The coating wear track

profile (fig. 7(b)) shows considerable coating wear but much less indication

of gross plastic deformation than pure silver. Also, the heavy transfer,

which is characteristic of pure silver, did not occur. Pin and coating wear

rates are given in figure 8.

Calcium fluoride addition (coating PS106) resulted in significant reductions in pin and coating wear rates. Initial friction coefficients were higher

than with silver and one-to-one silver-nichrome, but were very steady and constant at a value of 0.2 for the entire 9-hr test duration.

The four-component coating (PS101) had about the same friction coefficient

as PS106 (0.2). A small increase in pin wear rate occurred compared to that

observed with PS106, but the coating wear rate was so low that the test was

continued for a duration of 20 hr with very little penetration of the coating

(fig. 7(d)).

. These results indicate that plasma-sprayed silver is very effective as a

dry lubricant coating but must be combined with other components to reduce

plastic deformation and excessive silver transfer to the counterface material.

Calcium fluoride did not reduce the friction coefficient for short test durations, but resulted in a coating with a very stable, moderate friction coefficient of about 0.2 over a long duration of sliding. It also had a beneficial

effect in reducing pin and coating wear. The glass addition caused a small

increase in pin wear but a substantial reduction in coating wear.

Effect of thickness of silver coatings - The main problems with the relatively thick (0.17 mm) silver coatings were severe plastic deformation of the
silver and heavy adhesive transfer of silver to the titanium alloy pin, therefore, thinner silver coatings were also evaluated. It was expected that plastic deformation would necessarily be reduced for the thinner coatings for which

the harder substrate would provide a more effective support.

The effects of silver coating thickness on friction and wear are shown in

figures 9 to 11. The friction coefficients for 0.07 mm coating was about 0.1

or nearly the same as that of the 0.17 mm.coating during the first 5 hr but

remained at this low value for the total duration of 7 hr while the friction

of the thicker coating took a sharp upturn after 5 hr. Surface profiles of

the wear tracks on the coating (fig. 10(b)) showed much reduced plastic deformation of the silver. The depth of the track was about 0.03 mm or about onehalf the coating thickness; therefore, no contact occurred between the titanium

alloy pin and the coating substrate. Silver transfer to the pin was reduced

and some pin wear occurred (fig. 11).

Finally a 0.02-mm coating was evaluated. Friction coefficients were about

50 percent higher (0.15) than in the case of the thicker coatings, but remained

constant for 8 hr except for a time about halfway through the test when sliding

was a little rough and the friction coefficient briefly peaked at 0.2. This

was probably caused by a temporary penetration of the coating over a small area

of the wear track which subsequently "healed" by plastic flow of silver over

the failed area and the pick up of additional silver from the sides of the wear

track as wear of the hemispherically tipped pin progressed. The wear data of

figure 11 and the surface profiles of figure 10(c) show that both pin wear and

coating wear rates were very low. The silver flowed readily under the shear

stress of the sliding contact. Figure 10(d) shows that, in some of the tests,

the silver was displaced and redeposited in other areas as particles or patches

which were up to 0.03 mm thick. This flow characteristic can be advantageous

as previously described for "healing" small areas where the coating is worn

away. However, redistribution of the silver can also be a problem if it

results in undesirable changes in bearing clearances.

In general, the results indicate the following: (1) Low friction coefficients (0.05 to 0.15) can be achieved with plasma-sprayed silver coatings of

0.02 to 0.17 mm thickness. However, excessive plastic deformation of the

coatings occur in all but the 0.02 mm thick coatings; (2) among thick plasmasprayed coatings (0.17 mm), the best results were obtained with a coating

weight percent composition of 30Ag-30NiCr-25CaF2-15 glass (PS101).

Chromium Carbide Coatings Containing Silver and a BaF2/CaF2 Eutectic

In a further attempt to achieve coatings with lower wear rates over a

wide temperature range, a bonded chromium carbide wear control coating composition was modified by the addition of silver and other materials with the

objective of achieving lower friction while retaining the wear resistance of

the unmodified chromium carbide coating composition. Chromium carbide was

chosen for this study because of its excellent combination of wear resistance

and superior oxidation resistance; chromium carbide is more oxidatively

stable; than for example, the well-known wear resistant hard materials such as

tungsten carbide and titanium carbide or nitride (ref. 15).

Table V gives some.preliminary friction and wear data for a new coating

composition consistency of 80 wt % bonded chromium carbide, 10 wt % silver,
and 10 wt % of a CaF2/BaF2 eutectic. This composition is designated as

PS200; composition details are given in table III.

The data show that friction coefficients (typically 0.35) are higher than

those shown for PS101 on figure 5. However friction coefficients were lower

than those of the unformulated carbide coating, and coating wear was, in most

cases, too low to be measured by profilometry tracer across the wear tracks.

Pin wear rates were about the same as previously reported for sliding against

PS101. It should be noted; however, that in these preliminary experiments,

test duration was only 20 min. Therefore the wear rates shown are probably

run-in wear rates which are generally higher than steady-state wear rates.

These data, although preliminary, indicate that the new chromium carbide based

coating formulations are promising candidates for self-lubricating bearing

application where a long wear life over a wide temperature spectrum is

required.

SUMMATION

This paper summarizes research at NASA Lewis Research Center on the influence of silver on various types of self-lubricating materials. Some conclusions that may be drawn from the data presented are:

1. Silver can be an effective lubricant additive in composite materials

for use from low temperatures to 900 °C in an oxidizing atmosphere (air).

2. The primary benefit achieved with silver addition was to improve low

and intermediate temperature lubricating properties without adversely affecting high temperature lubrication by other components of the composite coatings.

Silver is chemically stable at the higher temperatures, thus allowing the

coatings to be used over repeated temperature cycles.

3. A new plasma-sprayed composite coating composition consisting of bonded

chromium carbide formulated with silver and CaF2/BaF2 eutectic was exceptionally wear resistant up to 900 °C and had better frictional characteristics

than the unmodified carbide coatings. This composition, designated as PS200,

appears to be a promising candidate for long-duration, high-temperature applications for which a moderately high friction coefficient of about 0.3 is

acceptable.

REFERENCES

1. Pinkus, 0. and Wilcock, O.F., "Strategy for Energy Conservation Through

Tribology." ASME, (1977).

2. Kamo, R., Bryzik, W., "Cummins/TACOM Advanced Adiabatic Engine,"

Proceedings of the Twenty-First Automotive Technology Development

Contractors' Coordination Meeting." (SAE P-138), SAE, (1984), pp.

121-134. (Also published as SAE SP-571, 1984).

3. Groseclose, I.E. Helms, H.E., and Johnson, R.A., "AGT 100 Advanced Gas

Turbine Technology Project," Proceedings of the Twenty-First Automotive

Technology Development Contractors' Coordination Meejting, (SAE P-138),

SAE, (1984), pp. 263-272.
4. Kreiner, D. and Kidwell, J.R., "ACT 101 Advanced Gas Turbine Technology

Development Program," Proceedings of the Twenty-First Automotive

Technology Development Contractors' Coordination Meeting. (SAE P-138),

SAE, (1984), pp. 273-285.

5. Titran, R.H. and Stephens, J.R., "Advanced High Temperature Materials for

the Energy Efficient Automotive Stirling Engine," DOE/NASA/51040-54, NASA

TM-83659, (1984).

6. Peterson, M.B., Murray, S.F., and Florek, J.J., "Consideration of

Lubricants for Temperatures Above 1000 °F," ALSE Trans.. 2, 225-234

(1960).

7. Sliney, H.E., Strom, T.N., and Allen, G.P., "Fused Fluoride Lubricants

for Extreme Temperatures and Corrosive Environments," ASLE Trans.. 8,

307-322 (1965).

8. Sliney, H.E., "Self-Lubricating Composites of Porous Nickel and

Nickel-Chromium Alloy Impregnated with Barium Fluoride-Calcium Fluoride

Eutectic," ASLE Trans.. 9, 331-347 (1966).

9. Longson, B., "Lubrication of High Temperature Ceramic Materials," Tribol.

Int., 1_6, 221-225, (1983).

10. Sliney, H.E., "Plasma-Sprayed Metal-Glass and Metal-Glass-Fluoride

Coatings for Lubrication to 900 °C," ASLE Trans. V7. 182-189, (1974).

11. Olson, K.M. and Sliney, H.E., "Addition to Fused-Fluoride Lubricant

Coatings for Reduction of Low-Temperature Friction," NASA TN-D-3793,

(1967).

12. Sliney, H.E., "Wide Temperature Spectrum Self-Lubricating Coatings

Prepared by Plasma Spraying," Thin Solid Films. 64, 211-217, (1979).

13. Sliney, H.E., and Wisander, D.W., "Plasma-Sprayed Coatings for

Lubrication of a Titanium Alloy in Air at 430 °C," NASA TP-1509, (1979).

14. Wisander, D.W., "Friction and Wear of Titanium Alloys and Copper Alloys

Sliding Against Ti-6 A1-4V Alloy in Air at 430 °C," NASA TN-D-8207,

(1976).

15. Sliney, H.E., "A Commentary on Solid Lubricants and Wear-Resistant Solids

for Use in Extreme Environments," Solid and Liquid Lubricants for Extreme

Environments. (ASLE SP-15), ASLE, (1984), pp. 1-6.
TABLE I. - MATERIALS USED IN PREPARATION OF FURNACE-FIRED

COATINGS

Powder material

Silver (Ag)

CaF2

BaF2

Eutectic

(62BaF2-38CaF2)

Molybdenum

Disulfide, MoS2

Range of

particle sizes,

yd)

Submicron

5 to 20

5 to 20

5 to 20

50 to 75

Grade

Technical

Reagent

Reagent

Reagent

Technical

99 percent

M.P. °C

961

1410

1280

1022

Decomp.

Below M.P.

TABLE II. - MATERIALS USED IN PREPARATION OF PLASMA-SPRAYED COATINGS

Powder material

Silver (Ag)

Nichrome (NiCr)

80 Ni-20 Cr

Calcium fluoride

(CaF2)

Glass

(58 Si02-21.2 BaO

7.8 CaO-13.0 K20)

Chromium Carbide

Nickel Aluminum

Blend

Range of

particle sizes,

50 to 150

50 to 150

5 to 20

50 to 150

10 to 70

Source

Commercial, plasma spray powder

Commercial, plasma spray powder

Reagent grade chemical

Laboratory preparation from

reagent grade chemicals

Commercial plasma

Spray powder

TABLE III. - COMPOSITIONS OF PLASMA-SPRAYED COATINGS

Identification

number

PS109

PS108

PS106

PS101

PS100

PS200

Composition,

wt %

100 Ag

50 Ag-50 NiCr

35 Ag-35 NiCr-30 CaF2

30 Ag-30 NiCr-25 CaF2-15 glass

67 NiCr-16.5 CaF2 -16.5 glass

80 NiAl bonded chromium

carbide-10 Ag-10 BaF2/CaF2

eutectic
TABLE IV. - BULK PROPERTIES OF SOME HARD COAT

MATERIALS3

Material

B4C

TiC

SiC

Cr3C4

we

Si3N4

TiN

Cr203

Microhardness,

kg/mm2

4200

3200

2900

2650

2050

2000

1950

C1800

Oxidation

temperature,**

°C

1090

540

1650

1370

540

1400

540

aOata from: Engineering Properties of

Ceramic Materials, Battelle Memorial

Institute. Published by American

Ceramic Society, Columbus, Ohio, 1966.

''Temperature for appreciable detrimental oxidation (passivating oxide

films form at lower temperatures).

cEstimated conversion from published

Moh hardness of 9.

TABLE V. - SUMMARY OF PRELIMINARY FRICTION AND WEAR DATA FOR PS200 (80 PERCENT

BONDED CHROMIUM CARBIDE-10 PERCENT Ag-10 PERCENT BaF2/CaF2 EUTECTIC)

[0.8 m/s (300 rpm, 5 cm diam track), 0.5 kg, 20 min test duration.]

Materials

Pin

Inc 750

oSiC

HS6B

Disk

PS200

PS200

Unformulated

Bonded Chrome

Carbide

Temperature,

°C

75

760

900

25

760

900

25

530

Wear factors, K

(run-in wear),

cnvj/cm • kg

Pin

2x1 0~9

3.5xlO-9

1.5x10-9

1.2x10-11

1.0x10-1°

6-OxlO-11

2.9x10-9

7.5xlO-10

Coating

Nondetectable

Nondetectable

5.1x10-9

Nondetectable

Typical

friction

coefficients

0.40*0.05

0.35*0.05

0.35+0.05

0.40+0.05

0.26+0.05

0.35+0.05

0.55+0.05

0.45+0.05
o

u_

O

O

O

Of

O UNLUBRICATED

D FURNACE-FUSED BaF2/CaF2 EUTECTIC COATING

A EUTECTIC COATING WITH BURNISHED MoS2 TOP COAT

Q EUTECTIC COATING WITH 25 wt % MoSo ADDITIVE

200 400

TEMPERATURE. °C

600 800

Figure 1. - Effect of MoS2 as a burnished top coat and as an

additive to BaF2/CaF2 eutectic furnace-fused coatings.

Dry air atmosphere, 2.3 m/s, 500-gram load cast Inconel

rider, Inconel 750 disks with 0.004 cm coatings.

o

o

o

TEMPERATURE,

°C

.6 —

10 20 30

SILVER, wt%

40 50

Figure 2. - Optimization of silver content in furnace-fused

eutectic coatings.
.6

i—

LJJ

O

E .4

uj

o

o

I-'

O

UNLUBRICATED

BaF2/CaF2 EUTECTIC COATING

EUTECTIC COATING WITH 35 wt % Ag ADDITIVE

200 400 600 800

TEMPERATURE, °C

Figure 3. - Effect of silver addition to BaF2/CaF2 furnacefused coating.

;-80-20£l

ORIGINAL MAGNIFICATION X100

Figure 4. - Microstructure of plasma sprayed composite.
o

u_

u_

LU

O

o

O

S3

O PRE-OXIDIZED RENE 41 CONTROL

D PS 100(NiCr-CaF2-GLASS)

A PS 101 (NiCr-Ag-CaF2-GLASS)

1

-t_> ¥J. — —

1 1

"TD— — U

1 1

6 ro;

S

00

-200

GALLING

WEAR

200 400 600

TEMPERATURE. °C

1000

Figure 5. - Effect of silver addition on the friction and wear of

plasma-sprayed, nichrome-matrix coatings (from Ref. 12).

Lubricating plain spherical bearings at a radial load of 34 MPa.

Bearing material, precipitation hardened nickel-chromium

alloy.

.4 r—

D PS 109

O PS108

A PS106

Q PS 101

6 8

DURATION, hr

Figure 6. - Friction characteristics in air of 0.17-millimeter-thick plasma-sprayed

composite coatings contain ing silver. Ti-5AI-2.5Sn pin material; 430°C;

250-gram load; 13-centimeter-per-second sliding velocity at 50 rpm.
0.025

mm

ORIGINAL SURFACE

LEVEL

9hM

^ORIGINAL RIDER

// CONTOUR

DEBRIS SCRATCH

(a) PS109 (silver). fl>) PS108 (Ag-NiCr).

9hr

(c) PS106 (Ag-NiCr-CaF2).

20 hn ,

DEBRIS /1I

SCRATCH^ "

(d) PS101 (Ag-NiCr-CaF2 glass).

Figure 7. - Wear track cross sections showing wear and deformation of 0.17-millimeter-thick plasmasprayed composite coatings containing silver.
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(b) Coating wear.

Figure 8. - Effect of coating composition on pin and coating wear in air.

Plasma-sprayed coatings, 0.17 mm thick.
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Figure 9. - Effect of coating thickness on friction of

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(a) 0.17-Millimeter coating 6-hour duration. (b) 0.07-Millimeter coating: 6.5-hour

duration.

WEAR TRACK-

(c) 0.02-Millimeter coating: 6.9-hour

duration (deformed area).

WEAR TRACK

. L-REDEPOSITED

0.03mm /T SILVER PARTICLE

0.32mm

(d) 0.02-Millimeter coating: 6.9-hour duration

(built up area).

Figure 10, - Wear track cross sections showing effect of coating thickness on wear and deformation of

plasma-sprayed silver coatings.
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Figure 11. - Effect of silver coating thickness on pin and coating wear in air.
1. Report No.

NASA TM-86943

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle 5. Report Date

The Role of Silver in Self-Lubricating Coatings for

Use at Extreme Temperatures

6. Performing Organization Code

533-05-12

7. Authors)

Harold E. Sliney

8. Performing Organization Report No.

E-2428

10. Work Unit No.

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared for the Annual Meeting of the American Society of Lubrication

Engineers, Las Vegas, Nevada, May 6-9, 1985.

16. Abstract

The advantages and disadvantages of elemental silver as a tribological material

are discussed. It is demonstrated that the relatively high melting point of

961 °C, softness, marked plasticity, and thermochemical stability of silver combine to make this metal useful in thin film solid lubricant coatings over a wide

temperature range. Disadvantages of silver during sliding, except when used as

a thin film, are shown to be gross ploughing due to plastic deformation under

load with associated high friction and excessive transfer to counterface surfaces. This transfer generates an irregular surface topography with consequent

undesirable changes in bearing clearance distribution. This paper describes

research to overcome these disadvantages of elemental silver. A comparison is

made of the tribological behavior of pure silver with that of silver formulated

with other metals and high-temperature solid lubricants. The composite materials are prepared by co-depositing the powdered components with an airbrush followed by furnace heat treatment or by plasma-spraying. Composite coatings were

formulated which are shown to be self-lubricating over repeated, temperature

cycles from low temperature to about 900 °C.

17. Key Words (Suggested by Author(s))

Solid lubrication

Tribology of silver

19. Security Classif. (of this report)

Unclassified

18. Distribution Statement

Unclassified

STAR Category

20. Security Classif. (of this page)

Unclassified

- unlimited

27

21. No. of pages 22. Price*

*For sale by the National Technical Information Service, Springfield, Virginia 22161
National Aeronautics and

.'Administration

Lewis Research Center

and Ohio 44135

Official Business

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