Tribology of Fiber-Reinforced Polyimides Sliding Against Steel and

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Tribology

of

Fiber-Reinforced

Polyimides

Sliding

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Against Steel and Silicon Nitride

Paul Sutor Midwest Research Institute, Kansas City, MO 64110

Graphite-polyimide composites, containing s o l i d l u b r i cant additives, were found to give low f r i c t i o n and wear when s l i d i n g against steel at 316°C. The same composites were found to give higher f r i c t i o n , with more wear and surface damage, when s l i d i n g against s i l i c o n n i t r i d e under i d e n t i c a l conditions. High i n t e r f a c i a l temperatures, graphite f i b e r abrasiveness, and adverse additive reactions contributed to the poor lubricant effectiveness of the composites s l i d i n g against s i l i c o n n i t r i d e . Composites which contained a MoS -based s o l i d lubricant additive were found to give substantially lower f r i c t i o n against s i l i c o n n i t r i d e than composites which contained a WSe /GaIn-based additive. Composites which contained a coarse f i b e r weave, which produced a greater concentration of graphite fibers at the composite s l i d i n g surface, were found to give generally higher wear and f r i c t i o n against s i l i c o n n i t r i d e than composites which contained a fine f i b e r weave. 2

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Graphite fiber-reinforced polyimides possess a t t r a c t i v e properties as l u b r i c a t i n g materials of construction, p a r t i c u l a r l y f o r elevated temperature operation. Polyimides as a class are thermally stable and exhibit reasonably low f r i c t i o n and wear up to 300°-350°C. Graphite f i b e r reinforcement provides the strength and s t i f f n e s s required i n structural components, and can also improve the t r i b o l o g i c a l properties of polyimides. Graphite-polyimide composites may be used as bearings, bearing separators, bushings, gears, and s l i d ing seals. Much of the recent work on the f r i c t i o n and wear behavior of polyimides, with and without graphite f i b e r reinforcement and/or s o l i d lubricant additives, has been conducted by Fusaro (_1) , who has recently reviewed the f i e l d (2). Reduction of f r i c t i o n and wear of polyimides by inclusion of graphite f i b e r s , p a r t i c u l a r l y at elevated temperature, i s i l l u s t r a t e d i n a study by Fusaro and Sliney (3). In

0097-6156/85/0287-0269$06.00/0 © 1985 American Chemical Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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experiments i n which stainless steel pins were s l i d against s o l i d polyimide disks at 25°C, these authors found that addition of 50% graphite fibers to one polyimide markedly improved the f r i c t i o n c o e f f i c i e n t (from 0.40 to 0.19) and wear rate (from 35 x 1 0 " to 0.6 x 10"" m /m), compared to the base polyimide. At 300°C, the f r i c t i o n c o e f f i c i e n t of the graphite fiber-reinforced polyimide was even lower (0.05), but the wear rate was nearly the same (0.7 x 10" m /m) as at 25°C. Sliney and Johnson investigated graphite fiber-polyimide composites f o r spherical bearings to 340°C (4); Sliney and Jacobson investigated graphite fiber-polyimide composites i n s e l f - a l i g n i n g p l a i n bearings to 315°C (5). Gardos and McConnell subsequently i n i t i a t e d a long term program to develop graphite-polyimide composites suitable f o r use as s e l f - l u b r i c a t i n g separators f o r high speed b a l l bearings, operating at 316°C, i n small, limited l i f e gas turbine engines (6). The composite separators were to provide s o l i d l u b r i cation f o r bearings consisting of hot-pressed s i l i c o n n i t r i d e b a l l s and M50 steel races. Inorganic s o l i d lubricant additives were i n corporated i n these composites, with the aim of lowering f r i c t i o n and f a c i l i t a t i n g lubricant transfer to the bearing surfaces, while maintaining low wear. Development and t r i b o l o g i c a l evaluation of these s e l f - l u b r i cating composites over the past several years has recently been reviewed by Sutor and Gardos (7, 8). This work demonstrated that many of the composites which were excellent lubricants for s t e e l were poor lubricants f o r s i l i c o n n i t r i d e . Thus, achieving e f f e c t i v e s o l i d l u b r i c a t i o n of s i l i c o n n i t r i d e was of p a r t i c u l a r concern. The work reported here was the culmination of our study of s e l f - l u b r i c a t i n g composites intended f o r use as l u b r i c a t i n g separators i n high speed ceramic and metal b a l l bearings. In t h i s work, the f r i c t i o n , wear, and transfer behavior of four graphite-polyimide composites, containing, s o l i d lubricant additives, was evaluated i n s l i d i n g versus M50 steel and hot-pressed s i l i c o n n i t r i d e , at 316°C in a i r . The effects on composite t r i b o l o g i c a l properties of s o l i d lubricant additive type, and of the manner i n which the graphite fibers were woven, was examined. 14

14

14

3

3

Experimental Materials. The composite lubricants contained four components: polyimide matrix resin, woven graphite f i b e r reinforcement, i n organic s o l i d lubricant additives, and (NH ) HP0 . A l l composite formulations consisted of 40% graphite f i b e r , 45% polyimide, 13% inorganic lubricant pigment, and 2% (NH ) HP0 , based on the weight of each component before molding. Composition based on volume of each component after molding d i f f e r e d f o r each composite, as shown in Table I, due to the d i f f e r e n t densities of some components and to process variables which were not e a s i l y controlled. Four composites were studied. Each contained the same polyimide matrix resin, graphite f i b e r , and (NH ) HP0 . The composites d i f f e r e d i n the type of s o l i d lubricant additive employed and i n the manner i n which the fibers were woven. Two types of inorganic s o l i d lubricant additives, denoted LI and L2, and two types of f i b e r weave, denoted Wl and W2, were employed. The materials to which 4

4

2

2

4

4

4

2

4


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Tribology of Fiber-Reinforced Polyimides

these designations refer are described i n d e t a i l below. For brevity, the composites w i l l be i d e n t i f i e d i n this paper as L1W1, L1W2, L2W1, and L2W2, indicating the lubricant additive and f i b e r weave employed. Table I describes i n d e t a i l the composite formulat i o n to which each designation refers. The polyimide matrix resin employed i n a l l composites was THERMID 600, a thermosetting resin polymerized from acetyleneterminated oligomers. Its chemical structure i s shown below (9).

VAC

c

,

C = CH

THORNEL 50 graphite fibers were employed for reinforcement. These f i b e r s , obtained by pyrolysis of p o l y a c r y l o n i t r i l e , have a t e n s i l e strength of 2.41 GPa and t e n s i l e modulus of 391 GPa. Tows containing several thousand THORNEL 50 filaments were woven into three dimensional, orthogonal preforms i n one of two manners, i l l u s trated i n Figure 1. Weave 2 (W2) was r e l a t i v e l y coarser than weave 1 (Wl), although f i b e r volume i n the preforms was similar (30.0% for Wl and 31.2% for W2). One of two inorganic lubricant mixtures was employed. Lubricant 1 (LI), f i r s t reported by Boes (10), i s formed by heating a compact of 80% WSe and 20% Ga/In eutectic to 540°C. The resultant mixture contains WSe , gallium and indium selenides, W, Ga, and In. The compact was ground to form a powder for inclusion i n the composi t e s . LI r e s i s t s oxidation at remarkably high temperatures due to formation of a passivating layer of gallium and indium oxides. Lubricant 2 (L2) was TURBOLUBE (ASU Composants), a mixture of layer l a t t i c e t r a n s i t i o n metal dichalcogenides. Elemental analysis i s consistent with the approximate composition 75% MoS , 20% MoSe , and 5% WS or WSe , by weight. L2 was furnished as a powder. Dibasic ammonium phosphate, (NH ) HP0 , was included i n a l l composites, acting both to improve fiber-matrix bonding and to reduce the f r i c t i o n and wear of the graphite fibers at elevated temperature, as reported by Lancaster (11). The composites were fabricated by mixing the inorganic additives with the resin, i n f i l t r a t i n g f i b e r preforms with this mixture, pressing i n a hydraulic press, and curing. Test specimens were then machined from the composites. Although mechanical properties of the four new composites were not measured, similar previously fabricated materials had t e n s i l e strengths and moduli of approximately 200 MPa and 8 GPa, respectively. One s l i d i n g counterface was M50 steel (alloying elements: 4.0% Cr, 4.2% Mo, 1.0% V, 0.8% C, 0.2% S i , 0.2% Mn, by weight), hardened to Rockwell C 58-60, with an average surface roughness of 0.150.20 pm r.m.s. The other counterface was NC-132 hot-pressed s i l i c o n n i t r i d e (Norton Co.), which contains approximately 1% MgO as a sintering aid, of the same surface f i n i s h . 2

2

2

2

2

2

4

2

4

Methods. S l i d i n g f r i c t i o n and wear experiments were conducted using a Hohman A-6 dual rub shoe machine. The s l i d i n g configuration



WSe /GaIn

MoS /MoSe

L1W2

L2W1

2

WSe /GaIn

L1W1

Fine

4

Coarse

Fine

Fiber Weave

42.4

54.0

49.4

Fiber

46.1

40.0

44.6

3.4

2.2

2.4

1.6

6.5

1 .58

1 .65

2.4

3

Density @ 25°C) (g-cm

1.4

Porosity

1.65

4

2.0

2

1.6

4

Composition* (volume % a f t e r molding) Lubricant (NH ) HP0 Polyimide

4

2

2.0 1 .68 3.2 1.6 L2W2 49.4 43.8 MoS^/MoSe? Coarse * For a l lcomposites, formulation was based on weight % before molding = 40.0% f i b e r , 45.0% polyimide, 13.0% lubricant, and 2.0% (NH ) HP0 .

2

2

2

Lubricant Additive

Composite

Table I. Composition of Self-Lubricating Composites

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COARSE WEAVE W2 Figure 1. Unit c e l l comparison of fine weave (Wl) and coarse weave (W2).


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consists of two conforming rub shoes loaded against the outside d i ameter of a rotating disk. The rub shoes were made of composite laminates, adhesive bonded to stainless steel base blocks, and the disks were made of steel or s i l i c o n n i t r i d e . Relative humidity of the a i r i n the laboratory was 30 ± 8%. A 111 N (25 lb) load was app l i e d to the rub shoes and the specimens were heated to 316°C by an oven which enclosed the test zone. Temperature was monitored and controlled by a thermocouple inserted i n one of the rub shoe steel base blocks, 5.5 mm from the s l i d i n g interface. After temperature e q u i l i b r a t i o n , the steel or ceramic counterface disk was rotated at a v e l o c i t y of 1.10 m/s. Specimens were s l i d for 3.85 km, removed for wear measurement and o p t i c a l microscopy, s l i d an additional 3.85 km against s i l i c o n n i t r i d e and 13.8 km against s t e e l , and removed for wear and SEM/EDX analysis. F r i c t i o n a l torque was recorded continuously. Wear was measured as weight loss, to the nearest mg, and converted to volume loss using the measured composite densities. The composites were hygroscopic. In order to obtain accurate and reproducible measurements of wear, i n the absence of dehydration, by the method of weight loss, the following procedure was employed. Before each test, the composite rub shoes were heated to 316°C for 30 min and weighed immediately upon cooling to 30°C. After each test, they were also weighed immediately upon cooling to 30°C. Between tests, they were stored at 25°C i n a desiccator.

Results 3

Table II presents the s p e c i f i c wear rates ( i n mm/Nm, volume loss per unit load per unit s l i d i n g distance) and average kinetic f r i c t i o n c o e f f i c i e n t s of the composites s l i d i n g versus steel and s i l i c o n n i t r i d e , upon i n i t i a l and continued s l i d i n g . Plots of f r i c t i o n coe f f i c i e n t versus s l i d i n g duration i n kilocycles (1 kc = 107 m) are shown i n Figures 2-5, for a l l experiments. A l l composites formed transfer films upon s l i d i n g against both steel and s i l i c o n n i t r i d e . The films were similar i n appearance under o p t i c a l microscopy, and covered approximately 30% of the contact region. Steel Counterface. Wear and f r i c t i o n against s t e e l , for a l l composi t e s except L1W1, indicate excellent l u b r i c a t i o n . F r i c t i o n c o e f f i cients are near the minimum, and wear rates near the maximum, of the "low f r i c t i o n , low wear" regime delineated for a wide variety of polyimides by Fusaro (2). The experiments i n which composite L1W1 was s l i d i n g against steel were conducted at a reduced s l i d i n g v e l o c i t y , 0.73 m/s, compared to the s l i d i n g v e l o c i t y of 1.10 m/s employed i n a l l other reported experiments. The s l i d i n g distance was the same, so that wear rates may be j u s t i f i a b l y compared. The lower v e l o c i t y was required for L1W1 because the composite soon began to glow red at the higher speed. Temperature measured by an o p t i c a l pyrometer focused on the s l i d i n g interface quickly rose to 510°C at the higher speed, but the red glow indicates that i n t e r f a c i a l temperatures may have been at or above 650°C. No gross thermal decomposition of the L1W1 composite



3

0.09

0.05

2.83

3.40

L2W1

5

L2W2

2.51

5.02 11.8

0.09 0.16

12.1

3

0.14

f

12.0 x 10""

1.85 x 10"

3

3

f

5

0.08

0.29

9.44

0.53

0.24

0.22

5

2.44

17.9

0.34 0.10

13.8 x 10"

0.19

NC-132 S i l i c o n N i t r i d e Continued S l i d i n g I n i t i a l Sliding Wear Rate Wear Rate k (mm/Nm) (mm/Nm)

0.15

2.69

21.8 x 10*"5

f

M50 Steel Continued S l i d i n g I n i t i a l Sliding Wear Rate Wear Rate k k (mm/Nm) (mm/Nm)

L1W2

L1W1

Composite

Counterface

Table I I . Wear Rates and Average F r i c t i o n Coefficients (f, ) at 3l6°C


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Composite • L1W1 • L1W2

• •

^

10

\

o—•.

20

30

40

Sliding Duration, kc

Figure 2. F r i c t i o n c o e f f i c i e n t as a function of s l i d i n g duration for LI composites s l i d i n g against s t e e l .

Figure 3. F r i c t i o n c o e f f i c i e n t as a function of s l i d i n g duration for L2 composites s l i d i n g against s t e e l .


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Figure 4. F r i c t i o n c o e f f i c i e n t as a function of s l i d i n g duration for LI composites s l i d i n g against s i l i c o n n i t r i d e .


Figure 5. F r i c t i o n c o e f f i c i e n t as a function of s l i d i n g duration for L2 composites s l i d i n g against s i l i c o n n i t r i d e .



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was observed during s l i d i n g at the reduced v e l o c i t y , for which the wear and f r i c t i o n results are reported. A l l composites, except L1W1, gave similar wear and f r i c t i o n when s l i d i n g against s t e e l . The anomalously high wear of L1W1, even at reduced v e l o c i t y , was due to complete s p a l l i n g of the polyimide matrix from the region between the f i b e r s , as shown i n Figure 6. This extensive matrix s p a l l i n g was not observed for the other composites, implying r e l a t i v e l y weak fiber-matrix bonding i n the L1W1 specimens. Weak bonding was possibly due to poor impregnation of the fibers by the matrix during f a b r i c a t i o n , or a deficiency of the phosphate fiber-matrix binder i n the near surface region of the composite . S i l i c o n N i t r i d e Counterface. Increased wear, f r i c t i o n , and surface damage was apparent for every composite when s l i d i n g against s i l i c o n n i t r i d e , as compared to s l i d i n g against s t e e l . Figure 6 compares the n e g l i g i b l y damaged, smooth surface of composite L1W1 after s l i d ing against steel with the same composite after s l i d i n g against s i l i c o n n i t r i d e , i n which gross breakage of f i b e r tows and chaotic mixing of fibers and matrix i s apparent. Except for L1W1, wear of the composites was consistently greater against s i l i c o n n i t r i d e than against s t e e l . More s i g n i f i c a n t l y , when s l i d i n g against s i l i con n i t r i d e , f r i c t i o n of the composites containing LI was p a r t i c u l a r l y high. In the case of L1W2, the f r i c t i o n c o e f f i c i e n t increased monotonically to values approaching 1.0 with continued s l i d i n g . On continued s l i d i n g , f r i c t i o n of composites containing L2 also i n creased, although more modestly. It i s clear that only L2W1 can be considered an e f f e c t i v e lubricant for s i l i c o n n i t r i d e under the conditions investigated.

Discussion Comparison with Previous Work. Our previous work had shown that i n t e r n a l l y lubricated graphite-polyimide composites experienced more wear and gave much higher f r i c t i o n when s l i d i n g against s i l i c o n n i t r i d e , as compared to s t e e l , at 3l6°C i n a i r (8). We had also observed that composites containing inorganic lubricant additive L2 gave lower f r i c t i o n against s i l i c o n n i t r i d e than those containing additive LI. The present work confirms these effects for four new composites. Previous composites contained a r e l a t i v e l y t i g h t weave of graphite f i b e r s , corresponding to 46 volume-% f i b e r i n the preforms. We hypothesized that the t r i b o l o g i c a l properties of the composites might be improved i f a r e l a t i v e l y looser weave of graphite fibers was employed. This i s because a looser weave would presumbaly allow better impregnation of the graphite f i b e r tows by the polyimide matrix and inorganic lubricant additives. Therefore, composites containing the looser Wl and W2 weaves (approximately 30 volume-% f i b e r i n the preforms) were fabricated for this study. Composites similar i n a l l respects to those reported here, but reinforced with the tight graphite f i b e r weave, were previously evaluated i n our laboratory, s l i d i n g against s i l i c o n n i t r i d e under conditions corresponding to those of the " i n i t i a l s l i d i n g " results


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Figure 6. Surface of composite L1W1 after s l i d i n g against s t e e l (a & b) and s i l i c o n n i t r i d e (c & d).


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reported here. For these composites, the material containing l u b r i cant additive LI gave a wear rate of 8.53 x 10 mm/Nm and average f r i c t i o n c o e f f i c i e n t of 0.99;_the material containing additive L2 gave a wear rate of 6.79 x 10 mm/Nm and average f r i c t i o n c o e f f i cient of 0.08. Comparison with the results i n Table II shows that no s i g n i f i c a n t change i n t r i b o l o g i c a l properties of composites containing L2 occurs upon changing reinforcement from the former t i g h t weave to the present loose weaves. For composites containing LI, on the other hand, considerable improvement i n f r i c t i o n i s associated with loose weave reinforcement, compared to t i g h t weave reinforcement. 5

5

3

3

Effects of Counterface and Composite Variables. Some of the greater surface damage observed i n composites s l i d i n g against s i l i c o n n i t r i d e , as compared to s t e e l , i s undoubtedly due to the lower thermal conductivity of s i l i c o n n i t r i d e . I t i s well known that i n t e r f a c i a l temperatures between s l i d i n g surfaces are often much higher than the temperature of the test environment, and higher even than the temperatures which can be measured close to the interface (12). This i s p a r t i c u l a r l y true at higher s l i d i n g v e l o c i t i e s . We have measured temperatures of 100°C as f a r as 5.5 mm from the interface for composites s l i d i n g against s i l i c o n n i t r i d e i n 26°C a i r , at the load and v e l o c i t y conditions reported here. The present results for L1W1 s l i d i n g against s t e e l indicate that true i n t e r f a c i a l temperatures during s l i d i n g may be double the 316°C temperature of the specimen enclosure. Since s i l i c o n n i t r i d e has a much lower thermal cond u c t i v i t y than s t e e l , true surface temperatures during s l i d i n g are expected to be much higher. As observed by Fusaro (2) and substantiated i n our previous work (7), the properties of the graphite fibers tend to dominate the t r i b o l o g i c a l performance of graphite-polyimide composites. Photomicrographs of composites reinforced with the W2 weave show that the composite surfaces are considerably enriched i n f i b e r s , compared to composites reinforced with the Wl weave. The generally higher wear and f r i c t i o n of composites reinforced with the W2 weave may be attributed to the higher concentration of graphite fibers at the surface. At elevated temperature, graphite can be abrasive rather than l u b r i c a t i n g , due to desorption of moisture, unless additives (such as dibasic ammonium phosphate) are incorporated. This abrasiveness appears to be more severe i n the case of a s i l i c o n n i t r i d e counterface. Surface profilometry of the s i l i c o n n i t r i d e disks i n the region of contact showed that the ceramics had been polished and, i n some cases, grooves had been worn i n the ceramic surfaces. Wear of the s i l i c o n n i t r i d e counterface was substantiated by EDX detection of S i on the composite surfaces a f t e r s l i d i n g . This i s shown most c l e a r l y i n the EDX of Figure 7, for L2W2. A d i f f i culty i n detecting S i for the LI composites i s the presence of W i n the additive; the major EDX peak for W overlaps that of S i . S l i d i n g experiments with similar Ll-containing composites have previously shown that S i - r i c h surface layers formed on the composites during s l i d i n g against s i l i c o n n i t r i d e may be from 0.6-3.0 um thick, and may completely prevent the inorganic lubricant additives from reaching the contact interface.


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Since no N was found at the composite surface, S i was presumed to be i n the form of s i l i c a , S i 0 . Two hypotheses have been advanced to explain the presence of s i l i c a i n terms of ceramic oxidat i o n . F i r s t , at i n t e r f a c i a l temperatures above 700°C, oxidation of s i l i c o n n i t r i d e to s i l i c a can occur (13). The s i l i c a can then be abraded by the graphite fibers and wear p a r t i c l e s may be imbedded i n the composite surface. Second, the presence of l o c a l l y reducing conditions (due to graphite) at the s l i d i n g interface can allow the so-called "active oxidation" of s i l i c o n n i t r i d e to take place, whereby v o l a t i l e SiO i s formed (14). Both hypotheses predict adverse affects due to a higher concentration of fibers at the surface, and the W2 composites do indeed give higher wear and f r i c t i o n than the comparable Wl composites. Since the major component of lubricant 1 (WSe ) and a l l components of lubricant 2 are layer l a t t i c e t r a n s i t i o n metal dichalcogenides, i t i s at f i r s t d i f f i c u l t to explain the large differences in f r i c t i o n a l behavior observed between LI and L2 composites s l i d i n g against s i l i c o n n i t r i d e . We advance here a hypothesis for involvement of the phosphate additive i n adverse reactions both with the ceramic counterface and with lubricant 1. Dibasic ammonium phosphate melts with decomposition at 155°C. In fact, a large proportion of the porosity i n the composites may be attributed to generation of v o l a t i l e ammonia at the processing temperature. Globules of molten phosphate can be seen on the surface of composite L1W1 i n Figure 8. EDX of a l l composite surfaces, except for the best performing L2W1, showed p a r t i c u l a r l y high concentrations of P. Phosphates as a class are well known to be glass formers; phosphate glasses are expected to have high adhesion to hot-pressed s i l i c o n n i t r i d e , i n which the n i t r i d e grains are bound together by magnesium s i l i c a t e glasses. This high adhesion can cause high f r i c t i o n and promote counterface wear. We postulate that the free metals (W, Ga, and In) or metal oxides present i n LI may react p r e f e r e n t i a l l y with phosphate decomposition products to produce a glassy, b r i t t l e surface layer on the composites and hence decrease lubricant effectiveness. A more complete chemical and surface analysis than was possible i n this work would be needed to confirm or deny this postulate. 2

2

Conclusions Internally lubricated, graphite-polyimide composites are excellent lubricants for steel at 316°C. They give very low f r i c t i o n and reasonably low wear, while maintaining exceptional strength and s t i f f n e s s . When s l i d i n g against hot-pressed s i l i c o n n i t r i d e , however, both composite and counterface wear are higher, and f r i c t i o n can be very high. The adverse i n t e r f a c i a l chemistry which may contribute to high f r i c t i o n against s i l i c o n n i t r i d e can be substant i a l l y mitigated when the concentration of graphite fibers at the s l i d i n g surface i s decreased (as by f i b e r weave Wl discussed i n the paper) and an inorganic lubricant pigment containing only molybdenum dichalcogenides (additive L2 discussed i n the paper) i s incorporated in the composites.


282


Figure 7. Silicon-containing layer on composite L2W2 after s l i d i n g against s i l i c o n n i t r i d e .

Figure 8. Phosphate globules on surface of composite L1W1 s l i d i n g against s i l i c o n n i t r i d e .


after

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283

Acknowledgments The author wishes to acknowledge the contributions of Messrs. Carl Windisch, Ed Harper, and Rick Bacon. This work was supported by the DARPA S o l i d Lubricated R o l l i n g Element Bearing Program under A i r Force Contract F33615-80C-5190; Mr. Karl R. Mecklenburg was project monitor.

Literature Cited 1. 2. 3. 4.

5. 6.

7. 8. 9. 10. 11. 12. 13. 14.

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R E C E I V E D January 23, 1985


Tribology of Fiber-Reinforced Polyimides Sliding Against Steel and

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