Ž .Wear 237 2000 33–38www.elsevier.comrlocaterwear

The tribological behaviors of copper-coated graphite filled PTFEcomposites

Fei Li, Feng-yuan Yan ), Lai-gui Yu, Wei-min LiuState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China

Received 7 January 1999; received in revised form 28 July 1999; accepted 28 July 1999

Abstract

Ž .Polytetrafluoroethylene PTFE composites filled with different fillers and various filler proportions were made by compressionŽ . Ž .molding. The powders of graphite, copper, copper-coated graphite CCG , and copper-mixed graphite CMG were selected as the fillers.

The tribological behaviors of composites in sliding against a stainless steel ring were evaluated on an MM-200 friction and wear tester.Ž .The morphologies and element chemical states of the worn composite surfaces were examined with scanning electron microscopy SEM

Ž .and X-ray photoelectron spectroscopy XPS , respectively. It was found that the CCG particles prepared by chemical replacement methodwere characterized by loose and uniform covering of graphite particles by many tiny copper particles. The preferential transfer of graphiteonto the surfaces of PTFE powders during the mixing process could be prevented by copper coating method, which was beneficial tofurther improving the wear resistance of composites. XPS analysis showed that no tribological reaction occurred during the frictionprocess of PTFE sliding against stainless steel ring, but there was radical -CF on the worn surfaces of PTFE composites, its generation3

mechanism and effect on the tribological behaviors of PTFE composites still need further study. q 2000 Elsevier Science S.A. All rightsreserved.

Keywords: PTFE; Composite; Copper-coated graphite; Friction and wear behavior

1. Introduction

With outstanding thermal stability, good resistance tosolvent, and low friction coefficient, polytetrafluoroethy-

Ž .lene PTFE has been used widely as an engineeringplastic. However, PTFE exhibits high wear rate at normalfriction conditions and cold-flow phenomenon under load.This is why a lot of efforts have been continuously madeto decrease the wear rate of PTFE by means of inorganic

w xor organic compound inclusion 1–5 . The mechanism offillers in reducing wear has also been largely focused on

w xRefs. 6–10 . However, little literature has been availabledealing with the interface effect between the filler and thepolymer matrix. Especially little attention has been paid tothe correlation between such an interface effect and thefriction and wear properties of the filled polymer compos-

w xites. Yan and Long 11 found that the wear behaviors ofPTFE composites filled with lamellar-structure fillers, to a

) Corresponding author. E-mail: fyyan@ns.lzb.ac.cn

certain extent, were related to the physical properties of thedefects in the composites, which could be well reflectedfrom the interfacial properties of crystalline and amor-phous regions in composites. They concluded that graphitecould preferentially transfer to the surfaces of PTFE parti-cles during mixing process by way of the so-called ‘‘solid

Fig. 1. Contact schematic diagram of the frictional pair.

0043-1648r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 99 00303-8

( )F. Li et al.rWear 237 2000 33–3834

Ž . Ž .Fig. 2. SEM micrographs of powders of a graphite and b CCG.

solvent effect’’, which was an important reason that thelamellar-structure inorganic filler, such as graphite andMoS , could reduce the mechanical strength of PTFE2

obviously. Accordingly, if one can control the preferentialtransfer phenomenon of graphite, it is expected to improvethe tribological behaviors of PTFE composites further.

2. Experimental details

The powder of PTFE with particle size of 20–100 mmwas supplied by Shanghai Electrochemical Plant andgraphite flake powder of particle size 10–50 mm byShanghai Colloid-Chemical Plant. The copper-coatedgraphite with 57.5 wt.% copper was produced by replace-

Fig. 3. Friction coefficients of unfilled PTFE and filled PTFE compositesŽin sliding against the stainless steel load: 196 N; sliding velocity: 0.445

.mrs; friction coefficient of unfilled PTFE: 0.26 .

w xment reaction method in our laboratory 12 . According tothe copper content in CCG, the CMG filler containing 57.5wt.% copper particles of diameter 50–70 mm and 42.5wt.% graphite powders was prepared by mechanical mix-ing. Specially noted here, the copper content in copper–PTFE composite is higher than in CCG–PTFE andCMG–PTFE at the condition of same filler volume con-tent. After fully mixing the PTFE powders with the filler,the slab specimens of a size of 6 mm=7 mm=30 mmwere prepared by compression molding at a pressure of 50MPa. The slab specimen of unfilled PTFE of the samedimension was prepared in the same way. After that, thespecimens were heated in hydrogen to 3758C at a rate of108Crmin, held there for 3 h, then cooled to ambienttemperature. The content of fillers in the prepared speci-mens were varied from 5 to 35 vol.%.

Fig. 4. Wear rate of PTFE composites in sliding against the stainless steelŽas a function of filler content load: 196 N; sliding velocity: 0.445 mrs;

y4 3 .wear rate of unfilled PTFE: 7.3=10 mm rN m .

( )F. Li et al.rWear 237 2000 33–38 35

Ž . Ž . Ž .Fig. 5. SEM micrographs of the worn surfaces of a CCG–PTFE and b CMG–PTFE the sliding direction is shown by arrows .

The friction and wear tests were conducted on anMM-200 friction and wear tester. The contact schematicdiagram of frictional pairs is shown in Fig. 1. A stainless

Ž .steel ring 1Cr18Ni9Ti of diameter 40 mm was used asthe counterpart. Sliding was performed under ambient

Ž .conditions temperature: 20 C8, humidity: 50%"5% at asliding velocity of 0.445 mrs, a normal load of 196 N, andtest duration of 60 min, which is corresponding to the totalsliding distance of 1602 m for the steel ring. The frictionforce was measured with a torque shaft, provided withstrain gauges, and the average of the friction coefficientover the final 10 min of each test was reported as thesteady-state value. Before each test, the surfaces of speci-mens and counterpart ring were finished with No. 800abrasive paper in water, then cleaned with acetone in an

Ž .ultrasonic bath. The resultant surface roughness R ofa

specimen and stainless steel ring ranged from 0.2 to 0.4mm. At the end of each test, the sizes of wear scar weremeasured with a digital-reading microscope, then the wearvolume loss of the composite specimen was calculated.The wear rate v of each specimen was calculated from the

Ž .relationship: vsV SP , where V is the wear volume incubic millimeters, S the sliding distance in meter, and Pthe applied load in Newton. in this work. The averages ofthe three replicate friction coefficients and wear rates werecited as the last results.

The worn surface morphologies of the composites wereexamined with a JEM-1200EX scanning electron micro-

Ž .scope SEM . A PHI-5702 X-ray photoelectron spectrome-Ž .ter XPS with an excitation source of Mg-Ka radiation at

a pass energy of 50 eV was utilized to analyze the elementchemical states on the worn composite surfaces. The bind-

Ž . Ž . Ž .Fig. 6. X-ray dot mapping of the elemental Cu distribution on the worn surfaces of a Copper–, b CMG– and c CCG–PTFE composites.

( )F. Li et al.rWear 237 2000 33–3836

Ž . Ž . Ž . Ž .Fig. 7. XPS spectra of a C1s and b F1s for 1 unworn and 2 worn PTFE.

ing energy of the amorphous carbon was used as theŽ .reference 284.6 eV and the binding energies of the tested

elements were determined with a deviation of 0.3 eV.

3. Results and discussion

3.1. The morphological features of graphite and CCGpowders

Fig. 2 shows the SEM micrographs of graphite andCCG particles. It can be seen that the appearance of the

Ž .graphite particle is characteristic of flake Fig. 2a . Incontrast, it is difficult to observe the flake structure of thegraphite particles after coated with many fine reductive

Ž .copper particles Fig. 2b . It indicates that the graphiteparticle has been coated completely by the loose and tinyparticles of copper. Though the coated reductive copperwith loose and tiny structure could not offer obviouslyextra strength for the graphite particles, it could effectively

prevent the direct contact of graphite with PTFE powdersduring initial mixing process.

3.2. The friction and wear properties of PTFE composites

Fig. 3 shows the friction coefficients of various filledPTFE composites in sliding against stainless steel ring. Itshould be mentioned that the unfilled PTFE registers afriction coefficient of about 0.26 in this work, which ishigher than that of composites under the same test condi-tions. This indicates that fillers are beneficial to increasingthe friction-reducing ability of PTFE. However, it shouldbe pointed out that since PTFE itself has good solid-lubric-ity, the unfilled PTFE and PTFE composites in slidingagainst a stainless steel ring register marginal differencesin the friction-reducing ability.

Fig. 4 shows the average wear rates of PTFE filled withCCG, copper, graphite and CMG. It is seen that theinclusion of fillers in PTFE has increased wear resistance

Žof PTFE considerably Unfilled PTFE had a wear rate of

Ž . Ž .Fig. 8. XPS spectra of a C1s and b F1s on the worn surface of graphite–PTFE composite.

( )F. Li et al.rWear 237 2000 33–38 37

7.3=10y4 mm3rN m which was measured repeatedly in.this work . It is also interesting to note that though CCG-

PTFE did not exhibit the best friction-reducing behaviorŽ .Fig. 3 , it has better wear resistance than graphite–PTFEand CMG–PTFE. It is supposed that CCG as filler inPTFE was beneficial to strengthening the bonding betweenfillers and the polymer matrix, which might contribute toimproving the wear resistance of PTFE composites.

3.3. Microscopic studies of the worn surfaces

The SEM micrographs of worn surfaces of CCG–PTFEand CMG–PTFE composites are shown in Fig. 5. It is seenthat the wear scar of PTFE filled with CCG is relativelysmooth and shows no apparently plucked and ploughedmarks. This indicates that adhesion is the dominant wearmechanism for CCG–PTFE composite. Contrary to theabove, the micrograph of the wear scar of CMG–PTFEcomposite shows marks of hard copper particle erosionand plastic deformation. This observation conforms with

w xwhat was reported by Yu and Bahadur 10 . Moreover, thedifferent PTFE composites would undergo different wearmechanisms in sliding against steel counterface, which wasgoverned by the different actions of various fillers.

The elemental distributions of Cu on the worn surfacesof various PTFE composites are shown in Fig. 6. It is seenthat there is a considerable difference in the planar distri-bution of Cu in the worn surfaces of the composites.Specifically speaking, Cu distributes at higher content inthe worn surface of copper– and CMG–PTFE, while it hasa non-uniform distribution and low content in the wornsurfaces of CCG–PTFE. This indicates that the Cu coatingon graphite particles caused changes in the filler character-istics, which subsequently led to variations in the frictionand wear behaviors of PTFE composites. The enrichmentof Cu on the worn surface of CCG–PTFE was slightbecause the reductive Cu in small grit size could be easilyextruded out of the contact region with soft polymerduring the friction process. The enrichment of Cu on the

Ž . Ž .Fig. 9. XPS spectra of C1s on the worn surfaces of 1 copper–, 2Ž .CCG– and 3 CMG–PTFE.

Ž . Ž .Fig. 10. XPS spectra of F1s on the worn surfaces of 1 copper–, 2Ž .CCG– and 3 CMG–PTFE.

worn surfaces of copper–PTFE and CMG–PTFE helped inimproving the load-carrying capacity of the composites.However, such a positive effect could be offset by theabrasion of the counterface by hard copper particles. At thesame time, the differences in the friction and wear behav-iors of various PTFE composites could also be correlatedwith the different surface structures of the fillers. That is,

Žfor CCG–PTFE composite, the adhesion phase PTFE. Ž .matrix could wet the dispersion phase CCG completely

due to the coated copper preventing the ‘‘solid solventeffect’’ of graphite to PTFE particles, while the case wasnot so for graphite– and CMG–PTFE composites.

3.4. XPS analysis of the worn composite surfaces

Fig. 7 shows the XPS spectra in unworn and wornPTFE blocks. Both worn and unworn PTFE have the samepeaks of C1s and F1s. This indicates that no tribochemicalreactions occurred for the pure PTFE in sliding against the

Fig. 11. XPS spectra of C1s on the unworn surface of copper–PTFEcomposite.

( )F. Li et al.rWear 237 2000 33–3838

Fig. 12. XPS spectra of F1s on the unworn surface of copper–PTFEcomposite.

stainless steel ring. Contrary to the above, as shown in Fig.Ž .8 a,b , the C1s peak at 294.20 eV and F1s peak at 691.6

eV indicated that the surface chemical states of thegraphite–PTFE composite had changed during the fric-tional process. As for Cu–, CCG– and CMG–PTFE com-posites, there are also new C1s and F1s peaks respectively

Ž .at close 294.20 eV and 691.6 eV Figs. 9 and 10 . Here wesuppose the two peaks are assigned to the radical -CF . It3

Ž .is interesting to notice that these C1s 294.20 eV and F1sŽ .691.6 eV peaks did not appear in the XPS spectra of the

Ž .unworn Cu–PTFE composite surface Figs. 11 and 12 . Sowe might infer that the molecular chain of the PTFE wasbroken into chain fragments by breaking C–C andror C–Fbonds under the severe mechanical stress and associatedfriction heat, which might caused the generation of radical

w x-CF . Richardson and Pascoe 13 , in studying the catalytic3

effect of a clean surface of iron film on the decompositionof n-C F , found the compound to rupture into radical5 12

chain fragments: CFq, C Fq and C Fq which were ac-3 2 5 3 7

tive to react and chemically bond with the metallic ele-ments on the counterface. Here we should point that thereare radical -CF on the surfaces of PTFE composites, but3

there are none on that of unfilled PTFE, from which itcould be deduced that filler could affect the chemicalstates of polymer composites. However, the affirmedmechanism of the generation of -CF and its effect on the3

tribological behaviors of PTFE composites still need fur-ther study.

4. Conclusions

Ž .1 For the particles of copper-coated graphite, eachgraphite particle was loosely and uniformly coated by thetiny copper particles. The copper layer possessed enoughadhesion strength with graphite particle surface to prevent

the direct contact of graphite and PTFE during mixingprocess.

Ž .2 Graphite–PTFE composite in sliding against thestainless steel presented the lowest friction coefficient, buthigher wear rate compared with CMG–PTFE and CCG–PTFE.

Ž .3 The wear mechanism of graphite–PTFE and CCG–PTFE composites was mainly due to the adhesion wear,while CMG–PTFE to both the abrasion wear and theadhesion wear.

Ž .4 XPS analysis showed that there was radical -CF3

generated on the surface of PTFE composites, which mightbe supposed that fillers could change the surface chemicalstates of this polymer.

Acknowledgements

We thank Mr. Shangkui Qi for his help with running ofthe XPS, and Miss Yuanping Wang for the SEM pictures.We also gratefully acknowledge helpful discussions withProfessor Xushou Zhang and Dr. Jinjun Lu.

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