reciprocating friction and wear behavior of a ceramic-matrix graphite

8/9/2019 Reciprocating Friction and Wear Behavior of a Ceramic-matrix Graphite

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Ž .Wear 225–229 1999 1338–1349

Reciprocating friction and wear behavior of a ceramic-matrix graphitecomposite for possible use in diesel engine valve guides

P.J. Blau a,), B. Dumont a, D.N. Braski a, T. Jenkins a, E.S. Zanoria b, M.C. Long c

aOak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6063, USA

bCaterpillar, Peoria ProÕing Ground, Peoria, IL 61656-1895, USA

cCaterpillar, Technical Center, Peoria, IL 61656-1875, USA

Abstract

Reciprocating ball-on-flat tests were conducted on prepared sections cut from cast iron, silicon nitride, and silicon nitrider

12.5 vol%

graphite composite valve guides over a range of temperatures, normal loads, speeds, and lubrication conditions. The purpose of this work

was to ascertain whether the latter ceramic composite would produce a beneficial lubricating film on the opposing surface and serve as a

self-lubricating material. Type 440C stainless steel was used as the counterface material. Machining practices were selected to provide theŽ .surface roughness and lay direction of grinding marks similar to that of actual valve guide bores. For comparison with the ceramic

composite material, both cast iron and silicon nitride matrix materials were also tested. Tests were also performed using graphite powder

on the silicon nitride matrix material to ascertain what frictional behavior might be observed in the most favorable case. Friction and wear

data, combined with surface chemical analysis confirmed that the current composite, while wear resistant, did not provide any lubrication

advantages over silicon nitride itself. No evidence for the sliding-induced formation of a beneficial graphite film was obtained by optical

examination, scanning electron microscopy, or surface chemical analysis. While the type of graphite used in the present composite

fractured into fine particles and did not form a lubricating film in our experiments, the results do not preclude the possibility of

developing other ceramic composites with self-lubricating properties. q 1999 Published by Elsevier Science S.A. All rights reserved.

Keywords: Friction; Wear; Graphite; Ceramic-matrix composites; Silicon nitride; Self-lubricating materials

1. Introduction

Self-lubricating composite materials, consisting of a

supporting matrix surrounding dispersed pockets of one or

more softer, lubricating species, have been used in a wide

range of tribological applications. Depending on the spe-

cific application, matrix materials can range from rela-

tively soft polymers to hard ceramics. Likewise, lubricat-

ing species can be soft metals, polymers, or other non-

metallics.

With efficiencies exceeding 42%, diesel engines are a

leading internal combustion option for propelling the next

generation of energy-efficient cars and trucks. Such en-

gines could benefit from the use of advanced tribomateri-

als like ceramics, light-weight metallic alloys, and compos-

ites to further improve their energy efficiency and perfor-

mance. In addition to parts like poppet valves, roller

followers, water pump seals, and turbocharger rotors, valve

)

Corresponding author. Tel.: q1-615-574-5377; fax: q1-615-574-

6918; e-mail: [email protected]

guides are a likely tribological application for these new

materials. In order to reduce diesel engine emissions from

oil emerging from the valve guides and entering the com-

bustion chamber, clearances between valve guides and

stems are being decreased. Thus, it becomes difficult for

liquid lubricants to be supplied to the valve stems, and so

it becomes desirable to fabricate the valve stems andror

guides from materials which have enough lubricating qual-

ities not to have to depend on a steady supply of oil in the

bore.

The current work compared the reciprocating slidingfriction and wear behavior of a continuous-fiber ceramic

Ž .composite CFCC of silicon nitride containing 12.5 vol%

of graphitic fibers to that of both a traditional cast iron

valve guide alloy and the ceramic matrix material without

a solid lubricant added to it. The CFCC material wasw xdeveloped under a U.S. Department of Energy project 1

as a candidate valve guide material. To better understand

the behavior which might be expected from the CFCC

material if a full graphitic film were developed on its

contact surface during sliding, we conducted additional

0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved.

Ž .P II: S 0 0 4 3 -1 6 4 8 9 9 0 0 0 5 9 -9


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Table 1

Kinetic friction coefficients for various materials on graphitic materials

Ref. Materials Test conditions Minimum Av

w x2 copper on natural Madagascar reciprocating Bowden-Leben – 0.0

graphite flakes apparatusw x2 copper on fibrous Ceylon graphite reciprocating Bowden-Leben – 0.1

apparatusw x2 copper on compacted natural flake reciprocating Bowden-Leben – 0.1

graphite apparatusw x3 self-mated highly-graphitic carbon annular ring, flat-on-flat, 50% – 0.0

relative humidityw x4 M-50 tool steel on carbon graphite reciprocating ball-on-flat, 45"10% 0.11 0.1

Ž .materials 11 grades relative humidity

w x4 M-50 tool steel on carbon graphite reciprocating ball-on-flat, 45"10% 0.07 0.1Ž .materials 11 grades , in 1508C relative humidity

Ž .synthetic oil Mobil 1 15W-50w x4 Silicon nitride type NBD 100 on reciprocating ball-on-flat, 45"10% 0.07 0.1

Ž .carbon graphite materials 11 grades relative humidityw x4 Silicon nitride type NBD 100 on reciprocating ball-on-flat, 45"10% 0.06 0.1

Ž .carbon graphite materials 11 grades , relative humidityŽ .in 1508C synthetic oil Mobil 1 15W-50

w x5 Stainless steel type 440C on graphite micro-friction apparatus, stroke- 0.15 –

foil by-stroke, ball-on-flat device,

68"3% relative humidityw x5 Stainless steel type 440C on graphite micro-friction apparatus, stroke- 0.08 –

powder-covered aluminum by-stroke, ball-on-flat device,

66"1% relative humidity


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( )P.J. Blau et al.r Wear 225–229 1999 1338–13491340

experiments on the matrix material alone, but lubricated

with a deposit of commercial graphite powder as well as

with some of the loose fibers of the kind used in the

composite.

In order to explore the possible benefits of the CFCC

and not to overlook important trends in behavior, a range

of loads, speeds, lubrication states, and temperatures were

used. Tests, supplemented with surface chemical analysis,

were conducted both dry and with diesel engine oil sup-plied by Caterpillar A range of loads, speeds, and states of

lubrication was used to compare the friction and wear

behavior of the three valve materials under controlled

laboratory conditions, and to determine the extent to which

the CFCC formed a lubricious film during sliding contact.

Further studies, under more engine-like conditions, were

planned if the CFCC had demonstrated favorable behavior

in these bench-scale tests.

Table 1 lists typical kinetic friction coefficients forw xgraphite-covered surfaces of various types 2– 5 . From the

data, friction coefficients between 0.05 to 0.15 would be

expected for the CFCC if an effective graphite lubricating

film had formed. One of the key technical issues for

commercial application of the CFCC material is the stabil-

ity and persistence of such lubricating films. Williams etw xal. 6 showed that the friction and wear of carbon–gra-

phite materials against stainless steel exhibited running-in

behavior which, in favorable friction and wear cases,

resulted in the formation and replenishment of cushioning

transfer layers on the steel. Highly graphitic materials

initially wore faster than non-graphitic materials during

running-in until a stable transfer layer could be established.

Since the presence of a stable transfer layer is critical to

maintaining favorable tribological behavior, it was of inter-

est in the current studies to see whether such a layer wouldbe formed on the current carbon fiber–silicon nitride com-

posite.

Previous research has indicated the feasibility of using

graphite lubricants to reduce the friction and wear of

ceramics and ceramic–metal couples. For example, Liuw xand Xue 7 conducted reciprocating ball-on-flat tests in

room temperature air with Cr-steel sliding against tetrago-Ž .nal zirconia polycrystalline TZP ceramic matrix compos-

ites containing 0 to 25 vol% graphite. The friction coeffi-

cient decreased from about 0.56 to 0.30 as graphite content

was increased up to 24.4 vol%. However, the wear rate of

the composites seemed to drop slightly at low graphitecontents, then increase significantly when it exceeded 15

vol%. In an earlier set of experiments by Gangopadhyayw xand Jahanmir 8 , graphite–silicon nitride couples, among

other combinations, were subjected to pin-on-ring tests in

which the ceramic pin specimen was drilled and filled with

the graphite to model the composite behavior. The ring

was type AISI 52100 steel. After a higher-friction running-

in period, a transfer film containing graphite was formed

on the silicon nitride, leading to a persistent kinetic friction

coefficient of about 0.17. Maximum gains in benefits from

graphite with silicon nitride required more than 20 area %

of graphite in the contact region. The wear rate of the

silicon nitride against the steel rings either remained aboutŽ .the same or increased with high )40% area percents of

graphite. The wear rate of the steel rings, on the other

hand, decreased slightly as graphite composition was in-

creased from 0 to nearly 50 area %. This work was laterw xrevisited in a review article by Gangopadhyay et al. 9 .

This earlier work suggested the potential for self-lubricat-ing ceramics to play a part in advancing diesel engine

technology. The question addressed here was whether one

particular CFCC would provide low friction and wear; and

if so, do carbon films produced by rubbing play a part in

that performance?

2. Materials

A CFCC consisting of approximately 12.5 vol%Ž .AMOCO P-75 graphite fibers 75% graphitic was incor-

porated into a matrix of polycrystalline, fine-grained hotŽiso-statically-pressed silicon nitride Allied Signal, grade

.GS-44 in an attempt to produce a self-lubricating material

A typical microstructure of this material is shown in Fig.

1. The fiber type and composition of the CFCC was

selected based on the results of previous studies in which

there was an emphasis on not only friction and wear, but

also on thermal expansion coefficient and material process-

Fig. 1. Optical photomicrograph of the CFCC material. Translucency of

the matrix permits one to see some of the near-surface fibers. Bright areas

are the edges of pits in this obliquely-lit image.


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( )P.J. Blau et al.r Wear 225–229 1999 1338–1349 1341

Fig. 2. Schematic representation of the ball-on-flat test geometry used in

the current work.

w xing 1 . Some of the tests described here involved applyingŽloose lubricating graphite powder Southwestern Graphite,

.Grade 1651, 0.7 mm or loose P-75 graphite fibers to

GS-44 specimens in order to study the effects of these

lubricants on the matrix material alone.

Wear specimens were fabricated from diesel enginevalve guides by slicing the guides down the center and

preparing a flat test surface parallel to the plane of the

central slice. The test orientation, with the sliding direction

perpendicular to the lay, is shown in Fig. 2. Measurements

of the surface finish of the actual valve guide bores were

used to obtain surface finish requirements for the test

specimens. Surface grinding parameters were selected and

specimens were ground in the Oak Ridge National Labora-

tory, Machining and Inspection Research User Center to

achieve finishes on flat test surfaces approximating thoseŽ .of the bores Table 2 . Grinding marks on the surface of a

CFCC specimen are shown on Fig. 3.

3. Test matrix and procedures

This investigation used two reciprocating, ball-on-flat

testing machines, run under a variety of conditions. No

reliable information was available on the actual side loads

experienced in valve guide bores. Therefore, loads were

Fig. 3. Area of the unworn CFCC showing grinding marks and granularŽ .areas where the fibers emerge at the surface SEM .

selected to produce measurable amounts of wear on the

test specimens and to produce wear features similar to

those observed on specimens of engine-tested ceramic

valve guides. The early work was conducted at lower loadsŽ .5 N on a reciprocating test machine developed at Oak

Ridge National Laboratory. That machine moves the flat

specimen stage back and forth under a fixed ball. The 5 N

experiments were designed to detect the effects of graphite

and graphite in water on the friction of GS-44 and the

CFCC at room temperature. In addition, loose P-75 graphite

fibers were dispersed on the surface of GS-44 to compare

the response with that of commercial graphite powder.

Later work was conduced on a commercial Plint andŽ .Partners, Wokingham, UK machine that holds the flat

specimen fixed and moves the ball back and forth. That

work investigated higher normal forces, higher reciprocat-

ing frequencies, and the effects of temperature. In both the

early and later work, 9.525 mm diameter AISI Type 440C

stainless steel balls were used as sliders. The nominal

composition of AISI 440C, in wt.%, is 0.95–1.2 C, 1 Mn,

Table 2

Materials

Ž . Ž . Ž .440C Stainless Cast iron gray GS-44 b CFCC bŽ . Ž . Ž .steel a matrix material GS-44r 12.5% Cgr

3Ž .Density grcm 7.68 7.3 3.2 –y6Ž . Ž .Coefficient of thermal expansion 10 r8C 10.1 10.5 d 3.4 –

Ž . Ž .Elastic modulus GPa 200 117 a 310Ž .Poisson’s ratio 0.28 0.17 c 0.26 0.26

Surface finish, 5 N load studies AFBMA Grade 10 – R s 0.38 mm R s 0.45 mma aSurface finish, higher-load tests AFBMA Grade 10 R s 0.35"0.05 mm R s 0.35"0.05 mm R s 0.35"0.05 mma a a

Ž . Ž .a Properties from the ASME Handbook—Metal Properties, McGraw-Hill 1954 .Ž . Ž .b Properties data obtained from AlliedSignal Ceramic Components, Torrance, CA 1997 .Ž . Ž .c Properties data reported in ORNL Report TMr8959, Oak Ridge National Laboratory 1983 .Ž .d Data from M.C. Long, Caterpillar Technical Center, Peoria, IL.


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Table 3

Summary of test conditions used in this and the previous study

Preliminary study Current study

Machine reciprocating, ball-on-flat, built at ORNL reciprocating, ball-on-flat, Plint Model TE-77 machine

Surrounding environment air, %RH s 70–74 air, %RH s 40–60

Ball material and diameter 440C stainless steel, 9.525 mm 440C stainless steel, 9.525 mmŽ .Normal force N 5.0 25 to 200Ž .Stroke length mm 10 mm 8 mm

Ž .Oscillation frequency Hz 0.5 5, 10, 20

Ž .Sliding distance km 0.0003 0.026 to 18.0Lubrication none, loose graphite powder, loose P-75 none, loose graphite powder,

carbon fibers, graphite powder in water, SAE 30 diesel oil

SAE 30 diesel oilŽ . Ž . Ž .Temperatures 8C room temperature ;23 room temperature ;23 , 150, 175

Measurements average kinetic friction coefficient; average kinetic friction coefficient;

wear rate of the ball wear rates of both the ball and flat

1 Si, 0.04 P, 0.003 S, 16–18 Cr, 0.75 Mo, bal. Fe. While

440C stainless steel is not normally used as a valve

material, it was similar to Cr-plated valve stems in the

sense that it had similar hardness and was covered by aprotective film of chromium oxide.

Table 3 summarizes the test conditions. Friction data

were obtained using force transducers mounted on the ball

holder on the low-load test machine and on the flat speci-

men stage on the higher-load machine. Wear rates for ball

specimens were calculated using the diameter of the wear

scar on the ball to compute volumetric loss rate per unit

load and distance slid. Wear rates for the flat specimens

other than cast iron, expressed in similar units, were

computed using wear groove cross-sectional stylus profilesŽTalysurf 10, Rank Taylor-Hobson, Leicester, UK; 2 mm

.tip radius . The cast iron specimens had deeper wear scarsand were measured by a laser surface mapping instrument

Žwith a greater depth range Rodenstock, RM 600, Munich.Germany .

4. Results

4.1. Friction results

Ž .Average kinetic friction coefficients m for 5 N,

short-duration tests of 440C sliding on GS-44 matrix in the

presence of various media, are summarized in Fig. 4. Each

point represents the average of three tests. After 30 s, the

dry graphite powder and dry carbon fibers gave similar

results. Using carbon fibers added to distilled water, the

friction coefficient was approximately doubled, and when

tests were run on GS-44 with no lubricant, the frictioncoefficient doubled again. The flat specimens of CFCC

Ž .Fig. 4. Summary of previous friction data ms friction coefficient when using 5 N loads.


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Table 4Ž .Summary of test conditions used in the current study all tests used 440C as the slider material

ID Flat specimen Number Temperature Lubrication Normal force Oscillation Sliding distanceŽ . Ž . Ž . Ž .Number material of tests 8C N frequency Hz km

1 cast iron 3 room none 25 5 0.11

2 GS-44 3 room none 25 5 0.11

3 CFCC 3 room none 25 5 0.11a4 cast iron 3 room none 100 10 0.25

5 GS-44 3 room none 100 10 1

6 CFCC 3 room none 100 10 1a7 cast iron 3 150 none 100 10 0.25

8 GS-44 3 150 none 100 10 1

9 CFCC 3 150 none 100 10 1a10 cast iron 3 175 oil 200 20 0.25

11 GS-44 3 175 oil 200 20 2

12 CFCC 1 175 oil 200 20 2b

13 CFCC 1 175 oil 200 20 6b

14 CFCC 1 175 oil 200 20 18

aFor cast-iron specimens, the amount of wear was so large that the duration of the test then the sliding distance were reduced.

bFor the CFCC, longer tests were running to try to quantify the amount of wear of the flat specimen.

run dry and in oil under otherwise similar conditions,reached final friction coefficients of 0.60"0.02 and 0.11

"0.01, respectively. Thus, the CFCC running dry had

slightly higher friction than the unlubricated GS-44, show-

ing no obvious benefit of the incorporated carbon fibers.

Lubricated with oil, the CFCC had friction coefficients

similar to GS-44 lubricated with dry 0.7 mm graphite

powder.

Steady-state friction data were obtained using the more

severe testing conditions given in Table 4. Results are

shown in Table 5. Except for the CFCC tested under oil

lubrication, where there was only one test per sliding

distance, each value in the table is the average of three testresults. In most cases, the friction coefficient was steady

Ž .after the running-in period. However for GS-44 IDs8 ,

there was a slight rise in friction during the test.

As shown in Fig. 5, the friction coefficient of the CFCC

couple was slightly higher than that for GS-44. For both

materials, friction increased with increasing load and speedŽ .at room temperature . For cast-iron we saw no influence

of load or speed. Under dry conditions and a 100 N load,

the coefficients of friction of the CFCC and GS-44 areŽ .relatively high ms 0.88 and 0.83 . In both cases, the

friction coefficient decreased to 0.5 at higher temperature.

Table 5

Friction and wear data from plint TE-77 tests

ID Flat specimen Average friction Flat specimen wear rate Ball specimen wear ratey6 3 y6 3Ž Ž .. Ž Ž ..number material coefficient =10 mm r N m =10 mm r N m

a1 cast iron 0.62"0.02 180 not detectableb2 GS-44 0.55"0.01 not detectable 12b3 CFCC 0.63"0.01 not detectable 8.2

a4 cast iron 0.55"0.03 450 not detectable

5 GS-44 0.83"0.03 0.6 5.1

6 CFCC 0.88"0.01 1.3 7.2a7 cast iron 0.63"0.02 480 not detectable

c b

8 GS-44 0.47 to 0.52 not detectable 5.7d b9 CFCC 0.5"0.03 not detectable 6a10 cast iron 0.23"0.01 157 not detectable

b11 GS-44 0.125"0.005 not detectable 0.6b12 CFCC 0.120"0.05 not detectable 1.1b13 CFCC 0.120"0.005 not detectable 1.2b14 CFCC 0.100"0.005 not detectable 1.1

aOval shape of the wear scar invalidated the wear volume calculation method.

bWear amount was not measurable with standard profilometric method.

cThe friction coefficient decreased from 0.47 to 0.52 with the sliding distance.

d y 6 3 Ž .For the CFCC, another test was done with a polished specimen. The wear rate of the flat specimen was measurable: 0.44=10 mm r N m , and wasy6 3 Ž .less than that obtained at room temperature. The ball wear rate was 4.7=10 mm r N m , a little less than at room temperature.


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Fig. 5. Comparison of average steady state friction coefficients for cast

iron, GS-44 and CFCC materials under various testing conditions.

However for normal forces of both 25 and 100 N at room

temperature, the CFCC exhibited higher friction than GS-

44, the same behavior as was seen in the 5 N tests.Ž .When tested in oil, the friction coefficient m de-

Ž .creased to values typical of boundary lubrication m;0.1 .

For the CFCC, friction did not vary greatly with increasing

sliding distance, even up to 18 km.

4.2. Wear results

The wear rates of the 440C ball against the CFCC under

low-load conditions were 19.2=10y6 and 9.3=10y6

3 Ž .mm r N m for unlubricated and oil-lubricated sliding,

Fig. 6. Comparison of the wear rates of the ball and flat specimens under

various test conditions.

respectively. The wear rates obtained under higher-load

conditions are reported in Table 5. Each value represents

an average of at least three tests except for tests numbered

12, 13 and 14.

As shown in Fig. 6, the cast-iron wear rate always

exceeded that of the ceramics. In fact, the sliding distance

per test had to be reduced for cast-iron because of the deep

grooves which were produced. Under 100 N load, oil

reduced the wear rate of cast iron by 50%, making itsimilar to that produced by a load of 25 N under dry

conditions. As Fig. 7 shows, the original machining texture

was obliterated owing to extensive plastic deformation

which is characteristic of ‘severe metallic wear.’

Both GS-44 and the CFCC wore so little that in most

cases their wear loss was not measurable. At a normal

force of 100 N at room temperature, however, the CFCCŽ y6had about twice the wear rate of GS-44 1.3=10 vs.

y6 3 Ž . .0.6=10 mm r N m , respectively . Oil effectively pre-

vented significant wear of the CFCC even at a total sliding

distance of 18 km. Energy-dispersive X-ray analysisŽ .EDXA of the worn CFCC contact surfaces indicated the

presence of S and Ca presumably from the diesel oil

additives. The Mg was probably from the sintering aids in

the silicon nitride, as confirmed by analysis of unlubricated

wear surfaces.

Roughness measurements on both ceramics were ob-

tained before and after testing. For 100 N dry tests, theŽ .GS-44 surface was smoother after testing R s 0.05 mm ,a

because asperities were abrasively truncated and low spots

were filled in by third bodies. For the CFCC the opposite

was found. The CFCC surface was rougher after testing

Fig. 7. Detail view of the edge of the wear scar on cast iron. Ductile

features in the worn area suggest severe metallic wear. The origina

surface finish is visible at the lower right outside the wear scar areaŽ .SEM .


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Ž . R s0.88 mm . This was probably caused by the pull-outaof fractured carbon fibers so as to leave deep pits, and thus

produce higher roughness values.Ž . ŽScanning electron microscope SEM observations e.g.,

. Ž .Fig. 8a–c and Auger electron spectroscopy AES of the

Ž .Fig. 8. Worn area of the CFCC material run under three conditions: a

unlubricated at room temperature showing localized material removalw x Ž . w x Ž .Test ID: 3 ; b run under hot, unlubricated conditions Test ID: 6 ; c

w xrun in hot oil Test ID: 14 .

Ž . ŽFig. 9. Worn areas of the ball a, optical and the flat CFCC specimen b.SEM produced under hot, unlubricated conditions.

CFCC specimens under all test conditions failed to reveal

any trace of a sliding-induced carbon film. No pre-sputter-

ing results were obtained for the CFCC tested in hot oil

because of an insulating residue film from the diesel oil.

The thickness of the carbon film due to the oil residue was

about 140 nm. Similar results were previously reported forŽ . w xlubricated sliding of Ti CN against 1045 steel 10 . In that

case, a carbon film of 90 nm was detected and friction

coefficients similar to what we measured were reportedŽ .ms 0.12 to 0.16, depending on speed and normal force .Ž .Metallic transfer from the ball Fe and Cr was ob-

served on the CFCC and GS-44 tracks generated under dry

conditions. Based on EDXA, transfer was much reduced

under lubricated conditions. In hot dry conditions, a non-Ž .uniform layer was observed Fig. 9b and Fig. 10b . Based

on the high amount of Fe and O present, we propose that

this layer consists of a mechanical mixture of iron oxidesŽ .Fe O or Fe O and silicates. This patchy layer did not2 3 3 4seem to cover the graphite fibers exposed at the surface. In


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Ž . ŽFig. 10. Worn areas of the ball a, optical and flat GS-44 specimen b,.SEM produced under hot, unlubricated conditions.

the center of Fig. 9b, we can observe debris trapped in a

hole, this debris contained mainly O, Fe and Cr from

metallic oxides but no silicates.

The wear rate of the 440C ball against cast-iron was

negligible, its having been protected by an oval-shapedŽdeposit of cast-iron with the shorter dimension in the

.sliding direction . In contrast, the ball wear scars produced

by sliding against both ceramic materials were always

round and flat.

The 440C balls wore more rapidly than the ceramic flat

specimens under all conditions, and the ball wear rate

against the CFCC was slightly higher than against GS-44,

except for the lowest load. The reason for this behavior is

not completely clear; however, it is possible that the edges

of the pits left when the graphite fibers were removed hadan abrasive cutting effect on the ball and thereby slightly

raised its wear rate over the smoother GS-44 surface.

Under hot dry conditions, abrasion grooves were formedŽ .on the ball in the sliding direction Fig. 9a and Fig. 10a ,

but some of them seem to be coated by a non-uniform film

transferred from the flat specimen. EDXA indicated that

this film consisted of O, Fe, Cr, and Si, probably some Fe-

and Cr-oxides and SiO . Back-transferred deposits were2observed under dry testing conditions at room temperature,

not in the form of a patchy film, but rather as particles

embedded inside the grooves. No transfer to the ball was

observed for hot oil tests, but their wear rates were lower

than those for dry conditions at a 25 N load by factors of 7

and 20, for the CFCC and GS-44, respectively.

4.3. Friction results for tests with graphite films on GS-44

As indicated earlier, additional tests were performed to

determine how low a friction coefficient would be ob-

tained if a graphite lubricating layer was formed on GS-44.

Therefore, powdered graphite was intentionally introduced

in a series of experiments summarized in Table 6. In

particular, the friction coefficients arising from thick films,Ž .;1–2 mm thick Tests 17 and 20 compared well to those

reported in Table 1.

The higher-load friction results obtained with the Plint

TE-77 machine are in good agreement with the previousŽ .work at low 5 N loads on the ORNL machine. Friction

decreased as graphite layer thickness increased under both

low and high load. Low friction coefficients, m;0.2,Žoccurred only during the first few minutes of testing e.g.

.5 min for Runs a 17 and 20 . Low m values were

Table 6

Results of room temperature tests of 440C against GS-44 with graphite powder lubricant

ID Lubrication Normal Oscillation Sliding Kinetic frictionaŽ . Ž . Ž .number force N frequency Hz distance m coefficient behavior

15 none 25 5 110 steady-state ms 0.55"0.01b16 thin layer 25 5 48 0.28"0.04 during the first 90s then 0.52"0.02

b17 thick layer 25 5 26 steady state ms 0.22"0.02

18 none 100 10 1000 steady state ms 0.83" 0.03

19 thin layer 100 10 192 0.62"0.08 during the first 6 min then 0.8"0.02

20 thick layer 100 10 45 steady state ms 0.19"0.02

aThe friction behavior during these different tests induced to change the sliding distance from one test to the other.

bThe ‘thin’ layer is estimated to be several microns thick, and ‘thick’ layer is estimated to be a few mm thick.


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associated with the formation of transfer layers on both

specimens. Indeed, SEM and EDXA results from Fig. 11a

and b, substantiate the formation of a protective film on

the ball. This film contained a large amount of C and very

low amounts of O and Si. The patchy and brittle film on

the opposing specimens consisted of a mixture of iron

oxide, silicates and graphite. Similar film structures andw xcompositions were reported by Gangopadhyay et al. 9 .

Other authors found even lower friction coefficients withŽ . w xself-lubricating ceramics ms 0.05 7 , but those values

Ž . Ž .Fig. 11. Energy dispersive X-ray spectra: a the ball specimen rubbed on GS-44 covered with graphite power at room temperature; b a graphitic film on

GS-44 tested at room temperature.


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Fig. 12. Similar trends in average friction coefficient were observed over

a range of test conditions.

were obtained only for short duration tests and for tests

with water lubrication. Fig. 12 shows trends in average

friction coefficient over a range of test conditions.

5. Discussion

Being much harder than the cast iron, both the GS-44

and the CFCC had considerably better wear resistance

against 440C for all test conditions. The 440C wore into

the cast-iron but showed little or no wear itself. A small

amount of oxidation of the cast-iron surface was suggested

from EDXA, but mechanical effects of plastic deforma-

tion, coupled with surface fatigue and micro-fracture domi-

nated the wear process. Stainless steel wore more rapidly

than GS-44 or the CFCC whatever the conditions were.

The use of diesel oil reduced the friction and wear of all

four tested materials and kept functioning for tests as long

as 18 km in sliding distance.In the case of GS-44 lubricated with graphite powder,

friction decreased to a low value due to the formation of a

patchy, load-bearing film on the flat specimen. Image

analysis of SEM photomicrographs showed that approxi-

mately 60% of the ceramic surface was covered by the

film. Similarly, the wear of the 440C counterface was

reduced due to the formation of a film containing a large

amount of C. The problem in trying to capitalize on this

effect is to be able to maintain a constant supply of solid

lubricant. To observe the formation of a film, we had to

stop the test as soon as the friction coefficient began to

increase. If tests were allowed to continue beyond thistransition, evidence of film formation was not observed.

The CFCC material failed to produce tribological im-

provements over the matrix material alone. The graphite

particles were intended to supply an interfacial film to help

to support the load and lubricate the surface, thereby

reducing wear. But, under the conditions of the present

study, there seemed to be an inadequate supply of shear-

able graphite to perform this function. Instead, we ob-

served an abrasive effect by the edges of the graphite-con-

taining pockets in the microstructure. Instead of simply

smearing over the surfaces to lubricate them, we suggest

that the graphite reinforcements fractured instead, adding

third bodies to cement the mechanically mixed interface

layer, and raising the friction coefficient as the normal

force increased.

Under hot dry testing conditions, the friction coefficient

of the CFCC and the GS-44 were reduced by the presence

of a smooth, lower shear strength transfer layer which also

protected the slider’s surface. Similar observations werew xreported by Skopp et al. 11 . In that work, monolithic

silicon nitride exhibited less wear at 4008C than at room

temperature because of the formation of a protective film

which allowed some plastic deformation.

We suggest that the composition of the layer formed on

the CFCC during hot dry sliding consisted more of ironŽ .and chromium oxides than did the mechanically mixed

film of oxides and C observed with lubrication by graphite

at room temperature. The formation of low shear strength

debris layers was enhanced by increased tribochemical

reactivity at 1508C compared to room temperature. The

area percent of the in situ formed film was, however, less

than the 60% observed for GS-44 lubricated with graphite

powder, and as a result its friction coefficient was not as

low.

As indicated in the introduction, graphite in various

natural and synthetic forms exhibits a range of frictionalw xcharacteristics. This was reported in publications 2,3 and

w xobserved in our earlier work 4 . The AMOCO P-75

graphite fibers used in the subject composite were selected

based on considerations of processing, and on a series of

earlier proprietary studies. In the composite studied here, it

seemed to fracture into powder rather than smear along the

surface to lubricate. Furthermore, even if the graphite

fibers were able to perform well as a solid lubricant, thesilicon nitride matrix in all likelihood would not wear

away fast enough to resupply the graphite to the sliding

surface. But if the matrix material did wear fast enough to

resupply the graphite to the surface, then the valve stem to

guide clearance in the engine would increase too much to

be acceptable based on oil leakage considerations.

Considering the preceding factors, current results indi-

cate no convincing tribological benefits of substituting the

current CFCC material for polycrystalline silicon nitride in

diesel engine valve guides.

6. Conclusions

Reciprocating ball-on-flat tests were conducted using

440C steel balls on three materials prepared by grinding

flat areas on the sides of diesel engine valve guides. The

materials were cast iron, silicon nitride, and a silicon

nitrider12.5 vol% graphite composite. Tests were con-

ducted over a range of temperatures, normal loads, speeds,

and lubrication conditions. Friction tests were also per-


12/12


formed using a commercial graphite powder lubricant on

the silicon nitride matrix material to determine what fric-

tional behavior might be observed in the most favorable

case where the graphite phase from a self-lubricating

composite would coat the contact surface and lubricate it

effectively.

1. Friction and wear data, combined with surface chem-

ical analysis, confirmed that the current composite, while

more wear resistant than cast iron, did not provide anylubrication advantages over the silicon nitride matrix mate-

rial alone.

2. No evidence for the formation of a sliding-induced,

beneficial graphite film was obtained either by optical

examination, scanning electron microscopy, or surfaceŽ .chemical analysis AES and EDXA . This finding was true

for tests done under both lubricated and unlubricated con-

ditions.

3. Tests with powdered graphite initially placed on the

surfaces of silicon nitride and subjected to sliding against

440C steel exhibited the low frictional behavior typical of

graphite lubricants, but the effect did not last long, and

transitions to much higher friction coefficients occurred as

soon as the transient film wore away.

4. Despite differences in apparatus design, results ob-

tained on two different reciprocating testing machines

indicated the same trends in friction for the silicon nitride

and ceramic matrix composite materials.

5. While the type of graphite fiber used in the present

composite did not provide the needed lubricating effect

under our experimental conditions, these results do not

preclude the possibility of developing other ceramic com-

posites with self-lubricating behavior. In the development

of such materials, it is important to select the proper

lubricant phase content, morphology, and properties toallow it to be supplied as the matrix slowly wears away.

Commercial successes with polymeric composites have

shown than effective self-lubricating materials can be made,

but the properties of ceramics, being much harder that the

incorporated lubricating material, make such materials

more difficult to design.

Acknowledgements

This research was supported in part by the U.S. Depart-

ment of Energy, Office of Industrial Technologies, Contin-

uous Ceramic Fiber Composites Program, under contract

DE-AC05-96OR 22464 with Lockheed Martin Energy Re-

search. The authors would like to express their apprecia-

tion to Rick Lowden, Oak Ridge National Laboratory, for

his guidance and comments during the course of perform-

ing this work.

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