Materials Science and Engineering A 392 (2005) 359–365

Study on the friction and wear properties of glass fabric composites filledwith nano- and micro-particles under different conditions

Feng-Hua Sua,b, Zhao-Zhu Zhanga,∗, Wei-Min Liua

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR Chinab Graduate School of the Chinese Academy of Sciences, China

Received 1 July 2004; accepted 26 September 2004

Abstract

The glass fabric composites filled with the particulates of polytetrafluoroethylene (PTFE), micro-sized MoS2, nano-TiO2, and nano-CaCO3,respectively, were prepared by dip-coating of the glass fabric in a phenolic resin containing the particulates to be incorporated and the successivecuring. The friction and wear behaviors of the resulting glass fabric composites sliding against AISI-1045 steel in a pin-on-disk configurationat various temperatures were evaluated on a Xuanwu-III high temperature friction and wear tester. The morphologies of the worn surfaces oft he elementald A). It wasf pertieso lassf endent ont ow1 he bondings e particulatefi t fillers.M riction andw could bes©

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he filled glass fabric composites and the counterpart steel pins were analyzed by means of scanning electron microscopy, and tistribution ofF on the worn surface of the counterpart steel was determined by means of energy dispersive X-ray analysis (EDX

ound that PTFE and nano-TiO2 particulates as the fillers contributed to significantly improve the friction-reducing and anti-wear prof the glass fabric composites, but nano-CaCO3 and micro-MoS2 as the fillers were harmful to the friction and wear behavior of the g

abric composites. The friction and wear properties of the glass fabric composites filled with the particulate fillers were closely dephe environmental temperature and the wear rates of the composites at elevated temperature above 200◦C were much larger than that bel50◦C, which was attributed to the degradation and decomposition of the adhesive resin at excessively elevated temperature. Ttrengths between the interfaces of the glass fabric, the adhesive resin, and the incorporated particulates varied with the types of thllers, which largely accounted for the differences in the tribological properties of the glass fabric composites filled with differenoreover, the transferred layers of varied features formed on the counterpart steel pins also partly accounted for the different fear behaviors of the unfilled glass fabric and the composites. In a practical viewpoint, 10% PTFE-filled glass fabric compositeuitable to tribological applications at moderately elevated temperature.2004 Elsevier B.V. All rights reserved.

eywords:Glass fabric composite; Solid lubricant; Nanoparticulates; Filler; Friction and wear behavior

. Introduction

Polymers and their composites form a very important classf tribo-engineering materials[1–4]. It has been found thatany inorganic particulate fillers are effective to modify thehysical and mechanical properties and hence to reduce theear of the polymers as well. The modification and wear-

educing effects of the inorganic particulate fillers are highlyependent on the formation and characteristics of the trans-

er films generated on the counterface surface by way of the

∗ Corresponding author. Tel.: + 86 931 4968098; fax.: +86 931 4968098.E-mail address:zzzhang@ns.lzb.ac.cn (Z.-Z. Zhang).

mechanical and tribochemical interactions among the pmer matrices, the fillers, the counterface metal, and thmospheric oxygen, during the sliding process[5]. Moreoverit has also been extensively reported that the reinforceof various matrices by fibers functions to increase the wresistance significantly[6–8]. For example, glass fiber hbeen attracting much attention in the modification and rforcement of polytetrafluoroethylene (PTFE), owing togood thermal stability and relatively low cost[12,13]. How-ever, glass fiber as the reinforcing agent is usually insuffito increase the wear-resistance of many polymers, sincliable to brittle fracture during the sliding process and heto cause severe scuffing of the polymeric matrix[13]. This

921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2004.09.036

360 F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365

shortcoming of the glass fiber could be overcome to someextent by introducing glass fabric of increased load-carryingcapacity[14]. Unfortunately, the use of glass fabric in bear-ing industry is still limited owing to the unsatisfactory wear-resistance and load-carrying capacity. Therefore, it is imper-ative to seek for the effective ways to decrease the brittlenessof the glass fabric and glass fibers, so as to increase its ap-plicability in the bearing industry where the integration andmulti-functionalization of the bearings made of various com-posites are of particular interest[15]. Various surface modifi-cation techniques, such as surface chemical modification, sur-face physical modification, painting, and coating, have beentried to modify the glass fabric in this respect[9–11]. Withthis perspective in mind, we selected solid lubricants PTFEand MoS2, and inorganic nano-TiO2 and nano-CaCO3 widelyused as fillers[16] to fill the glass fabric in the presence ofphenolic adhesive resin, so as to endow the glass fabric withgood self-lubricity and increased mechanical strengths andwear-resistance.

This article deals with the preparation of the glass fabriccomposites modified with various solid lubricant particulatesand inorganic nano-particles. The friction and wear behaviorsof the composites and their dependence on the environmen-tal temperature are also investigated. The present work isexpected to broad the application of glass fabric compositesi

2

ofG dhe-s nt ofC yL )a Sp idC ace-t in,f ath.T me-t pol-i s ofR no-p massf ul-t them ture.T andt 60w glassf i-n nlesss red at1 Sa ed

Fig. 1. The picture of pin-on-disk friction and wear tester. P—applied load;1—the counterpart pin (diameter 3 mm); 2—the disk specimen; 3—electricfurnace; 4-themocouple.

as 10 and 3%, respectively, according to a series of screeningtests.

The friction and wear behavior of the glass fabric compos-ites supported on the stainless steel sliding against AISI-1045steel pin of a diameter 3mm at various temperatures wereevaluated on a Xuanwu-III test rig as shown inFig. 1. Prior tothe tests, the pin was sequentially polished with 350#, 700#,and 900# sand paper, to a surface roughnessRa = 0.15�m,and then cleaned with acetone. The sliding was performedunder ambient condition at a sliding speed of 0.252 m/s, a nor-mal load within 156.8–313.6 N, a temperature of 25–240◦C,and over a period of 2 h, except for otherwise indication. Atthe end of each test, the disk was cleaned and dried, then itswear volume loss (V) was obtained by measuring the wearscar area and depth on a micrometer (±0.001 mm). The wearrate was obtained from dividing the wear volume loss by thenormal load and sliding distance. The friction coefficientswere obtained from the frictional torque measured by a loadcell sensor. Three replicate friction and wear tests were car-ried out for each specimen to minimize data scattering and theaverages of the three replicate test results are reported in thiswork (The relative errors to measure the friction coefficientand wear volume loss are±10 and±5%, respectively). Themorphologies of the worn composite surfaces and the transferfilms on the counterpart steel pin surface were analyzed on aJ pedw

3

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fab-r insttc b-r est

n dry-sliding bearings.

. Experimental

The glass fabric was provided by Nanjing Academylass Fibers in China. The adhesive resin (204 phenolic aive) was provided by Shanghai Xingguang Chemical Plahina. Irradiated PTFE powders (<20�m) were provided banzhou Irradiation Center of China. Nano-TiO2 (20–30 nmnd nano-CaCO3 (30–50 nm) were prepared at our lab. Mo2articulates (<38�m) were produced by Shanghai Collohemical Plant of China. The glass fabric was dipped in

one for 24 h and then boiled in distilled water for 10 mollowed by cleaning with acetone in an ultrasonic bhe surface of the 1Cr18Ni9Ti stainless steel disk (dia

er 22.5 mm) to be coated with the fabric composite wasshed with 280# and 350# sand paper to a surface roughnesa = 0.45�m. The solid lubricant particulates and the naarticles were evenly dispersed in the adhesive at proper

ractions with the assistance of magnetically stirring andrasonic stirring. Then the glass fabric was immersed inixed adhesive to allow the coating by the adhesive mixhe immersing of the glass fabric in the mixed adhesive

he successive drying of the coated glass fabric around◦Cere repeated until the adhesive was used up and the

abric composite about 400–450�m thick was obtained. Fally, the glass fabric composites were affixed on the staiteel surface using the phenolic resin adhesive and cu80◦C for 2 h. The contents (mass fraction) of micro-Mo2nd nano-CaCO3 in the glass fabric composites were fix

SM-5600LV scanning electron microscope (SEM) equipith an energy dispersive X-ray analyzer (EDXA).

. Results and discussion

.1. The friction and wear behaviors of the glass fabricomposites

The friction coefficients and wear rates of the glassic composites reinforced with different fillers sliding agahe AISI-1045 steel pin at room temperature and 150◦C areomparatively shown inFig. 2. It is seen that the glass faic composites filled with irradiated PTFE show the b

F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365 361

friction-reducing and antiwear ability at room temperatureand 150◦C. The lowest friction coefficient is recorded for the10% PTFE-filled glass fabric composite at 150◦C, which issupposed to be related to the easier shearing and flowing of thethermoplastic PTFE at elevated temperature. The other threekinds of fillers have minor effects on the friction-reducingbehavior of the glass fabric composite at 150◦C, while thenano-CaCO3 as the filler leads to a significant increase inthe friction coefficient of the glass fabric composite at roomtemperature. Interestingly, the glass fabric composites filledwith various fillers record smaller friction coefficients at ele-vated temperature than at room temperature, which could beattributed to the enhanced thermal effect thereat that helps toreduce the shearing stress by strengthening the plastic defor-mation and flowing. Besides, the incorporation of the PTFEparticulates contributes to dramatically decrease the wear rateof the glass fabric at both room temperature and 150◦C by afactor up to 3. Nano-TiO2 and MoS2 as the fillers effectivelydecrease the wear rate of the glass fabric at elevated tempera-ture, but nano-CaCO3 is harmful to the wear-resistance of theglass fabric at both room and elevated temperatures. Specif-ically, MoS2 is effective in increasing the wear-resistance ofthe glass fabric only at elevated temperature. Namely, MoS2even causes a damage to the wear-resistance of the glass fabricat 25◦C, though it is one of the best traditional solid lubri-cd

ionc bricc ticu-l cienta f the

fabric

Fig. 3. Effect of PTFE content on the friction and wear behavior of PTFE-filled glass fabric composite sliding against steel at 313.6 N and room tem-perature.

PTFE-filled glass fabric composite assumes a slight decreasewith increasing PTFE content from 5 to 20%. The wear ratekeeps almost unchanged within the PTFE content from 5 to15%, with respect to the relative error to measure the wearrate. However, the composite containing 20% PTFE has amuch larger wear rate, which could be attributed to the weak-ened adhesion between the incorporated particulates and theglass fabric matrix in the presence of an excessive amount ofPTFE particulates.

Fig. 4shows the effect of the test temperature on the fric-tion coefficients and wear rates of the 10% PTFE-filled glassfabric composites. It is seen that the friction coefficient con-siderably decreases with increasing temperature up to 100◦C,then it increases gradually with further increase of the tem-perature up to 240◦C. This is attributed to the strengthenedadhesion of the PTFE at relatively higher testing tempera-ture. Besides, the wear rate at 50◦C is a little bit smaller than

ants. This implies that the solid lubricity of MoS2 is highlyependent on the environmental conditions[17].

Fig. 3 shows the effect of PTFE content on the frictoefficients and wear rates of the PTFE-filled glass faomposites. It is seen that the inclusion of the PTFE parates leads to a significant decrease in the friction coeffind wear rate of the glass fabric. The friction coefficient o

Fig. 2. Comparison of friction coefficients and wear rate of glass

composites modified with different fillers at 25 and 150◦C (235.2 N, 2 h).

362 F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365

Fig. 4. Effect of temperature on the friction and wear behavior of 10% PTFE-filled glass fabric composite sliding against steel at 313.6 N.

that at 25◦C, but it assumes a significant rise with increasingtemperature up to 240◦C, which is also determined by theenhanced adhesion and plastic deformation at excessively el-evated temperature. Thus it is concluded that the PTFE-filledglass fabric composite could be suitable to the tribological ap-plications at ambient and moderately elevated temperatures.

The effect of the filler content on the friction and wear be-havior of nano-TiO2 filled glass fabric composite is similarto that of the PTFE-filled one, except that a higher contentof nano-TiO2 above 5% leads to a minor increase in the fric-tion coefficient and a larger increase in the wear rate (seeFig. 5) as compared with the PTFE particulates. At the sametime, as shown inFig. 6, the friction coefficient of the 5%nano-TiO2/glass fabric composite decreases slightly with in-creasing temperature up to 200◦C and then it assumes a mi-nor increase at 240◦C. And the wear rate gradually increaseswith increasing temperature up to 200◦C but it sharply risesas the test temperature increases from 200 to 240◦C (seeFig. 6). Therefore, the glass fabric composite filled with thenano-TiO2 is not suitable to high temperature tribologicalapplication.

Fig. 7 show the effect of load on the friction and wearbehaviors of the glass fabric composites filled with differentfillers at room temperature. It is seen that the glass fabric com-posite filled with PTFE shows the best friction-reducing and

F ofn roomt

Fig. 6. Effect of temperature on the friction and wear behavior of 5% nano-TiO2-filled glass fabric composite sliding against steel at 274.4 N.

antiwear ability, and its wear-resistance is almost irrespec-tive of the load up to 274.4 N. The glass fabric compositefilled with 3% nano-TiO2 is next to the PTFE filled one interms of the friction-reducing and antiwear ability. Moreover,nano-CaCO3 and micro-MoS2 as the fillers cause increasesin the friction coefficients and wear rates of the composites.Thus nano-CaCO3 and micro-MoS2 are unsuitable for thetribological modification of the glass fabric.

3.2. SEM investigation of worn surfaces

Fig. 8shows the SEM morphologies of the worn surfacesof the glass fabric composites filled with various fillers slid-ing against the steel pin at 235.2 N and room temperaturefor 2 h. It is seen that some glass fibers are pulled out fromthe fabric matrix on the worn surface of the unfilled glassfabric characterized by severe adhesion (seeFig. 8A), whichindicates that the glass fabric in this case undertakes a largecontact stress. Contrary to the above, though the adhesionsigns are still visible on the worn surface of the glass fabriccomposite filled with 10% PTFE, no pulled out and exposedglass fibers are seen thereon (seeFig. 8B). This indicatesthat the PTFE particulates incorporated in the glass fabriccomposite effectively act to restrain the direct contact be-tween the brittle glass fibers and the hard counterpart steel,t wingt Thew 5%n onea stc site.Dg abriccc ands ,t omt n ow-i orer

ig. 5. Effect of nano-TiO2 content on the friction and wear behaviorano-TiO2/glass fabric composite sliding against steel at 274.4 N and

emperature.

hus, the friction and wear is considerably decreased, oo their easy shearing and good solid-lubricating action.orn surface of the glass fabric composite filled withano-TiO2 has similar features as that of the PTFE-fillednd is somewhat smoother (seeFig. 8C), which conform

o the next-to-the-best effectiveness of the nano-TiO2 in de-reasing the friction and wear of the glass fabric compoifferent from those of the PTFE-filled and nano-TiO2-filledlass fabric composites, the worn surfaces of the glass fomposites filled with 10% MoS2 and 3% nano-CaCO3 areharacterized by obvious pulling out of the glass fibersevere adhesion and pitting (seeFig. 8D and E). Especiallyhe nano-CaCO3 particulates are almost totally detached frhe glass fabric matrix in the absence of the adhesive resing to severe wear, which agrees well with the much po

F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365 363

Fig. 7. Variation of friction coefficients and wear rate of glass fabric composites filled with different fillers with load at room temperature. (a) Friction coefficientvs. load. (b) Wear rate vs. load.

wear-resistance of the nano-CaCO3/glass fabric compositeeven than the unfilled glass fabric.

The PTFE-filled glass fabric composites of varied PTFEmass fractions were used as the examples to investigate theeffect of the filler content and test temperature on the wornsurface morphological features. As shown inFig. 9, the wornsurface of the unfilled glass fabric at 25◦C shows signs ofsevere adhesion and segregation of the adhesive resin, whilea great amount of glass fibers are pulled out and exposed(seeFig. 9A). Contrary to the above, the worn surface ofthe glass fabric composite filled with 10% PTFE at 25◦C ischaracterized by mild plastic deformation, and the glass fiberswell adhered to the PTFE particulates in the presence of theevenly distributed adhesive resin are not pulled out or exposedthereon (seeFig. 9B). However, when the PTFE content risesto 20%, the pulling out and exposing of the glass fibers onthe worn surface become much more severe (seeFig. 9C),and it seems that in this case some metallic components are

F reinforc era2 ass fab

enriched at some locations of the worn composite surface (seethe bright zones inFig. 9C). This implies that the glass fabriccomposite filled with 20% PTFE was liable to be abraded bythe counterpart steel during the sliding process in which someof the peeled off metallic wear debris could be embeddedonto the worn composite surface owing to the abrasion bythe exposed or detached glass fibers.

It is interesting to note that the worn surface of the 10%PTFE-glass fabric composite at 150◦C (seeFig. 9D) has sim-ilar features as that of the same composite at 25◦C, whichimplies that this composite would assume a minor differencein the wear-resistance at 25 and 150◦C (seeFig. 4). However,the worn surface of this composite at 240◦C shows a greatamount of pulled out and exposed glass fibers (seeFig. 9E),similar as what is shown inFig. 9(A); which, unsurpris-ingly, conforms to the significantly increased wear rate of thiscomposite at 240◦C, as observed inFig. 4. Since PTFE haspoor mechanical strength and wear-resistance, it is natural to

ig. 8. SEM pictures of the worn surfaces of glass fabric compositesh: (A) pure glass fabric, (B) 10% PTFE-glass fabric, (C) 5% nano-TiO2/gl

ed with different fillers sliding against steel at 235.2 N and room tempture forric, (D) 10% MoS2-glass fabric, and (E) 3% nano-CaCO3/glass fabric.

364 F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365

Fig. 9. SEM pictures of the worn surfaces of glass fabric composites filled with PTFE of varied mass fractions sliding against steel at 313.6 N and varioustemperatures: (A) unfilled glass fabric at 25◦C, (B) 10% PTFE-glass fabric at 25◦C, (C) 20% PTFE-glass fabric at 25◦C, (D) 10% PTFE-glass fabric at 150◦C,and (E) 10% PTFE-glass fabric at 240◦C.

observe that the glass fabric containing 20% PTFE showsmuch poorer wear-resistance than the one containing 10%PTFE. In addition, the temperature at which the adhesive resinsolidifies is around 180◦C, thus the 10% PTFE-glass fabriccomposite has slightly increased wear rate with increasingtest temperature up to 150◦C. However, as the test temper-ature rises to 240◦C, the adhesive resin would be degradedor decomposed to lose its ability to bind the glass fabric ma-trix and the particulate fillers, hence the mechanical strengthand wear-resistance of the composite at 240◦C dramaticallydecrease, which is accompanied by the pulling out and ex-posure of the glass fabric matrix and the detachment of theparticulate fillers from the glass fabric matrix. Moreover, thedifferences in the binding states between the glass fabric ma-trix and the adhesive resin of the unfilled glass fabric and thecomposite filled with 5% nano-TiO2 can be further clearlyseen from the magnified SEM pictures of the correspondingworn surfaces (seeFig. 10).

3.3. The Anti-wear mechanism of PTFE and Nano-TiO2

The SEM morphologies of the worn surfaces of the coun-terpart steel pins sliding against unfilled glass fabric, 10%PTFE-filled glass fabric composite, and 5% nano-TiO2-filedglass fabric composite, respectively, at room temperature, ares ingaT orns 10%Pfi sur-f gies

of the worn surfaces of unfilled glass fabric and 5% nano-TiO2-filled glass fabric are also shown inFig. 10to illustratethe differences in the binding states between the glass fab-ric matrix and the adhesive resin of the unfilled glass fab-ric and the composite filled with 5% nano-TiO2. It is seenthat the worn surfaces of the counterpart steel pins slidingagainst unfilled glass fabric, 10% PTFE-filled glass fabriccomposite, and 5% nano-TiO2-glass fabric composite seemto be polished to different degrees, with the formation ofthe transferred layers with varied thickness and uniformitythereon. This could partly account for the different wear-resistance of the glass fabric composites filled with differentfillers sliding against a same steel pin. Namely, the coun-terpart worn surface sliding against the unfilled glass fabricis relatively rougher and seems to have a thicker transferredlayer (seeFig. 10(A)), while those sliding against the 10%PTFE-glass fabric composite and 5% nano-TiO2-glass fabriccomposite look smother and seem to have thinner transferredlayers (seeFig. 10B and C). The formation of the transferlayer on the counterpart steel surface sliding against 10%PTFE-glass fabric composite is confirmed by the existenceof F thereon, as shown inFig. 10(F). Moreover, as shownin Fig. 10(D), the glass fibers are loosely bonded with theadhesive and many cracks are produced at the matrix-binderinterface, which accounts for the poor wear-resistance of theu , thefi dedi cksa (seeF nceo ledg

hown inFig. 10, in an attempt to reveal the friction-reducnd anti-wear mechanisms of the PTFE and nano-TiO2 fillers.he EDXA image of the plane distribution of F on the wurface of the counterpart steel pin sliding against theTFE-glass fabric composite is also given inFig. 10to con-rm the transfer of the PTFE onto the counterpart steelace. At the same time, the magnified SEM morpholo

nfilled glass fabric. Contrary to the above, the adhesivelled particulates, and the glass fabric matrix are well bonn 5% nano-TiO2-filled glass fabric composite, and no crare visible at the filler-adhesive-glass fiber interfacesig. 10E), which conforms to the increased wear-resistaf the 5% nano-TiO2/glass fabric composite than the unfillass fabric.

F.-H. Su et al. / Materials Science and Engineering A 392 (2005) 359–365 365

Fig. 10. SEM morphologies of the worn steel surfaces sliding against (A) unfilled glass fabric, (B) 10% PTFE-filled glass fabric and (C) 5% nano-TiO2-filledglass fabric; the magnified SEM pictures of the worn surfaces of (D) pure glass fabric and (E) 5% nano- TiO2-filled glass fabric, and (F) EDXA image of F on(B) (274.4 N, room temperature).

4. Conclusions

The anti-wear and friction-reducing properties of the glassfabric composite was greatly improved by incorporationwith irradiated PTFE and nano-TiO2 particulates, but nano-CaCO3 and micro-MoS2 as the fillers led to damage to thefriction and wear behavior of the glass fabric.

The environmental temperature had an important effect onthe friction and wear properties of the glass fabric compositefilled with irradiated PTFE and nano-TiO2. The wear rates ofthe two kinds of composites assumed a minor increase withincreasing temperature up to 150◦C, but they rose signifi-cantly at 240◦C, which was attributed to the degradation anddecomposition of the adhesive resin at excessively elevatedtemperature.

The differences in the friction and wear behaviors of theunfilled glass fabric and its composites filled with differentparticulate fillers were mainly attributed to the different bond-ing strengths between the interfaces of the glass fabric, theadhesive resin, and the incorporated particulates.

Transferred layers of varied features were formed on thecounterpart steel pin surfaces sliding against unfilled glassfabric and its composites filled with 10% PTFE and 5%nano-TiO2. This partly accounted for the different frictionand wear behaviors of the unfilled glass fabric and the filledc

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