Polymer Testing 22 (2003) 875–882www.elsevier.com/locate/polytest

Material Behaviour

Investigation of tribological properties of Al2O3-polyimidenanocomposites

Hui Cai, Fengyuan Yan∗, Qunji Xue, Weimin LiuState Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou

730000, PR China

Received 12 December 2002; accepted 17 February 2003

Abstract

Polyimide (PI) nanocomposites with different proportions of nanoparticle Al2O3 were made by compression moldingat elevated temperature. The mechanical and tribological properties of the resulting PI-based nanocomposites wereinvestigated. The bending strength and microhardness of the nanocomposite specimens were determined, and the tribol-ogical behavior of the nanocomposite blocks in dry sliding against a plain carbon steel ring was evaluated on an M-2000 friction and wear tester. The morphologies of the worn nanocomposite surfaces and transfer films on the counter-part steel ring were observed on a scanning electron microscope. Results indicated that the PI-based nanocompositeswith appropriate proportions of nanometer Al2O3 exhibited lower friction coefficient and wear volume loss than PIunder the same testing conditions. The nanocomposite containing 3.0wt.%–4.0wt.% nanometer Al2O3 registered thelowest wear volume loss under a relatively high load. The differences in the friction and wear behaviors of PI and PI–Al2O3 nanocomposites were attributed to the differences in their worn surface morphologies, transfer film characteristics,and wear debris features. The agglomerated abrasives on the worn composite and transfer film surfaces contributed toincrease the wear volume loss of the nanocomposites of higher mass fractions of nanometer Al2O3. 2003 Elsevier Ltd. All rights reserved.

Keywords: Polyimide; Nanoparticle Al2O3; Nanocomposites; Tribological behavior

1. Introduction

It is usually more cost-effective to reach a new/uniqueset of property/performance through a combination ofmaterials, based on the tailoring of existing materials tocomposites, which generally causes a lower technicalrisk than developing a new kind of material. This is whya lot of attention has been paid to the preparation ofmany inorganic/polymer composites by rational selectionof the raw materials and the preparative approaches, soas to achieve various properties required in differentapplications[1–10]. Of those composites, the ones made

∗ Corresponding author. Tel.:+86-931-827-4661; fax:+86-931-827-7088.

E-mail address: fyyan@ns.lzb.ac.cn (F. Yan).

0142-9418/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0142-9418(03)00024-2

of polymers incorporated with nanometer inorganic par-ticulates have been largely focused on, because theyshowed much better mechanical, tribological, and multi-functional behavior than the polymer matrices and thecomposites made of the polymer matrices and conven-tional micrometer inorganic particulates[11, 12], owingto the effect of the unique nature of the ultrafine particu-lates on the bulk properties of polymer-based nanocom-posites[11–14]. As a typical example, extensive rese-arches have been conducted concerning the developmentof novel polyimide-based nanocomposites, becausepolyimides (PI) show outstanding thermal, mechanical,and electrical properties as well as resistance to solventand radiation, as one of the most important super-engin-eering plastics[15–17]. The research on the tribologicalbehavior of PI-based nanocomposites, however, is wait-ing to be broadened and deepened[12,18], although the

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effect of inorganic nanometer filler on the tribologicalproperties of polymer matrix composites and the mech-anisms of various fillers have been roughly recognized[12,19].

Accordingly, the tribological properties of PI-basednanocomposites incorporating different proportions ofnanometer Al2O3 particles are dealt with in the presentwork, by sliding the PI nanocomposite blocks against aplain carbon steel ring.

2. Experimental

2.1. Materials and specimens

4,4’ -oxydianiline (ODA) and diphenylether-3,3’ ,4,4’ -tetracarboxylic dianhydride (OPDA) were purchasedfrom commercial sources and dried before use. Nano-meter Al2O3 was also commercially obtained from a nan-ometer material corporation in Shandong Province ofChina. The other compounds including triethylamine,pyridine, N-methyl-2-pyrrolidone (NMP) etc., were com-mercial analytical grade reagents and used withoutfurther purification, except that NMP was dried overmolecular sieves before use.

Poly(amic acid) solution was obtained by adding anequimolar amount of OPDA to an ODA/NMP solution.The homogeneous suspensions of Al2O3/NMP with dif-ferent Al2O3 proportions were added into PAA solutionunder vigorous agitation. The homogeneous suspensionof Al2O3/PAA/NMP was precipitated by introduction oftriethylamine and pyridine. After drying to allow thermalimidization, PI/Al2O3 powders were compressed andheated to 340° C in a mold at a rate of 8° C/min. Thepressure was held at 10 MPa below 315° C and raisedto 25 MPa for the rest of the heating cycle, to ensuregood compacting of the resultant composite specimens.The compressed composite was held at 340° C for 10min and then cooled to 100° C in the mold. After releas-ing from the mold, the resultant block specimens wereused for friction and wear tests.

2.2. Characterization

The microhardness of the specimens was determinedby conducting five replicate measurements on an MH-5microhardness meter, at a load of 2 N and a loading timeof 5 s. The average of the five replicate measurementsof the hardness values is cited as the hardness of thespecimen.

The bending strength of the PI/Al2O3 nanocompositeswas measured using an AG-10TA universal material tes-ter (Shimadzu Corporation of Japan) in accordance withnational standard GB9341-88.

Fig. 1. Contact schematic diagram for the frictional couple.

2.3. Friction and wear test

The friction and wear tests were conducted on an M-2000 model friction and wear tester. The contact sche-matic diagram of the frictional couple is shown in Fig.1. A plain carbon steel ring (HRC 48–50) with diameterof 40 mm was used as the counterpart. Sliding was per-formed under ambient conditions over a period of 1.5 hat sliding velocities of 0.431 m/s and 0.862 m/s, normalload of 50 N, 100 N, 200 N, and 290 N. The ambienttemperature was roughly 25° C and the relative humidityabout 50%±10%. Before each test, the surfaces of theblock specimens and counterpart ring were abraded withNo. 900 water-abrasive paper. Then, the steel ring wascleaned with acetone-dipped cotton and the PI or its nan-ocomposite blocks were cleaned with acetone, followedby drying. The friction force was measured using atorque shaft equipped with strain gauges, and the frictioncoefficient calculated by taking into account the normalload applied and the friction force measured. At the endof each test, the width of the wear scar on the blockspecimens was measured with a digital-reading micro-scope, then the wear volume loss V of the block speci-men calculated from the relationship:

V � B·�pR2

180·arcsin� b

2R��b2�R2�

b2

4 �,where V refers to the volume loss (mm3), B to the widthof the block specimen (mm), R to the radius of the steelring (mm), and b to the width of the wear scar (mm).Three replicate friction and wear tests were carried out

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so as to minimize data scattering, and the average of thethree replicate test results is reported. The relative errorof the replicate friction and wear test data was ±10%.

2.4. Scanning electron microscope studies

The worn surfaces of the nanocomposite blocks andof the transfer films on the counterpart steel rings wereion plated with Au to become conductive and thenobserved on a JSM-5600LV scanning electron micro-scope (SEM).

3. Results and discussion

3.1. Bending strength and microhardness of PInanocomposites

The microhardness and bending strength of thePI/Al2O3 nanocomposite as a function of Al2O3 contentare shown in Fig. 2. It is seen that the microhardness ofPI/Al2O3 nanocomposite increases with increasing Al2O3

content. Thus, it would be rational to infer that nano-meter Al2O3 as the reinforcing agent might effectivelyincrease the load-carrying capacity of PI/Al2O3 nanoc-omposite. Contrary to the above, the bending strength ofthe nanocomposite stayed almost unchanged at a massfraction of the nanometer Al2O3 below 4.0%, and it evendecreases at a mass fraction of the nanometer Al2O3 con-tent above 4.0%. This implies that the interfacial interac-tion between the polymer matrix and the nanometerAl2O3 reinforcing agent is weakened with increasingcontent of the filler, which is also expected to affect thetribological behavior of the nanocomposites.

Fig. 2. Effect of the nanometer Al2O3 content on themicrohardness and bending strength of the nanocomposite.

3.2. Friction and wear properties of thenanocomposite

Fig. 3 shows the variations of the friction coefficientand wear volume loss of PI/Al2O3 nanocomposites withthe mass fraction of nanometer Al2O3. The lowest fric-tion coefficient at a load of 200 N is recorded when themass fraction of the nanometer Al2O3 in the nano-composite reaches 3.0%, and so is the smallest wear vol-ume loss, Fig. 3(a). In combination with the frictioncoefficient and wear volume loss in this case, it is there-fore rational to recommend the optimal Al2O3 content inthe PI/Al2O3 nanocomposite as 3.0%. Similarly, the fric-tion coefficient and wear volume loss of PI/Al2O3 nano-

Fig. 3. Effect of the nanometer Al2O3 content on the frictioncoefficient and wear volume loss of the PI/Al2O3 nanocompos-ite at (a) 200 N and (b) 290 N (sliding speed: 0.431 m/s).

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Fig. 4. The friction coefficient (a) and wear volume loss (b)under various loads as functions of nanometer Al2O3 content(sliding speed: 0.431 m/s).

Fig. 5. Variations of friction coefficient (a) and wear volumeloss (b) with nanometer Al2O3 content at sliding speeds of 0.431m/s and 0.862 m/s.

composite at a load of 290 N show the same tendencieswith increasing nanometer Al2O3 content, except that thelowest friction coefficient is recorded at a nanometerAl2O3 mass fraction of 3.0% while the smallest wear vol-ume loss registered at a nanometer Al2O3 mass fractionof 4.0%, Fig. 3(b). This observation conforms to the vari-ation of the microhardness and bending strength of thenanocomposite with the nanometer Al2O3 content. Inother words, higher mass fraction of nanometer Al2O3

(above 4.0%) is detrimental to the friction-reducing andantiwear ability of the PI/Al2O3 nanocomposite.

The friction coefficient and wear volume loss of thePI/Al2O3 nanocomposites under various loads as func-tions of Al2O3 content are shown in Fig. 4. The frictioncoefficient for pure PI increases with increasing load,

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while that for the PI-based nanocomposites varies in acomplicated manner with increasing load. All the nano-composites register relatively larger friction coefficientsthan PI under loads of 50 N and 100 N, except that theone containing 3.0% nanometer Al2O3 gives a slightlylowered friction coefficient than the bare PI, while thenanocomposites record sharply decreased friction coef-ficients with mass fractions of nanometer Al2O3 below5.0% under loads of 200 N and 290 N (see Fig. 4(a)).On the other hand, the incorporation of the nanometerAl2O3 in PI almost has no effect on the wear resistanceat relatively low load (50 N and 100 N), while the PI–Al2O3 nanocomposites with proper proportions of nano-meter Al2O3 show better wear resistance than PI at loadsof 200 N and 290 N, Fig. 4(b). This implies that thenanometer Al2O3 as the reinforcing agent functions tocarry load and resist wear at a relatively higher load(above 200 N). However, the nanocomposites with largermass fractions of nanometer Al2O3 give significantlyincreased wear volume losses compared to the bare PI.This might be attributed to the abrasion action of thenanometer Al2O3 particles agglomerated on the wornsurfaces in the sliding of the nanocomposites of highernanometer Al2O3 content against the steel.

The variations of the friction coefficients and wearvolume losses of pure PI and PI/Al2O3 nanocompositeswith sliding speed at a load of 200 N are shown in Fig.5. It is seen that the nanocomposites with mass fractionsof nanometer Al2O3 below 4.0% give slightly decreasedfriction coefficients at both low and high sliding speedscompared to the bare PI, Fig. 5(a), while they registerlarger friction coefficients at a mass fraction of nano-meter Al2O3 above 4.0%, which is similar to what wasobserved in Fig. 4(a). In addition, a relatively small fric-tion coefficient is registered for the nanocomposites at ahigher sliding speed, and the PI/Al2O3 nanocompositeswith smaller mass fractions of nanometer Al2O3 showwear-resistance comparable to or slightly better than thebare PI at a lower sliding speed, but they give muchbetter wear-resistance at a higher sliding speed, Fig. 5(b).Contrary to the above, the nanocomposites with largermass fraction of nanometer Al2O3 give poorer wear-resistance than the bare PI at both low (with the massfraction of Al2O3 above 5.0%) and high sliding speed(with the mass fraction of Al2O3 above 7.0%). Thus thePI/Al2O3 nanocomposites are more suitable to the work-ing condition of relatively high load and large slidingspeed.

It could be rational to infer that the sliding surfacetemperature of PI/Al2O3 nanocomposite increased withsliding speed, which resulted in micro-melting of thenanocomposite surface. Such a kind of micro-meltingcould be speeded at a relatively larger sliding speed,owing to the extended sliding distance. Therefore itmight be rational to deduce that the formation of a trans-fer film of the nanocomposite on the counterpart steel

surface was speeded as well at a relatively larger slidingspeed. Subsequently, a decreased friction coefficient andwear volume loss was observed with the change fromthe sliding of the steel against the nanocomposite to thatof the nanocomposite against its transfer film on thecounterpart steel surface. Such a decreased friction andwear was dependent on the mass fraction of nanometerAl2O3 in the nanocomposite, i.e. the nanocomposites oftoo large mass fractions of the nanometer Al2O3 had evenpoorer wear-resistance than the bare PI, because of theabrasion action of the agglomerated Al2O3 particulatesin the wear debris, which led to damage to the transferfilm. On the other hand, the interfacial strengtheningaction of the nanometer Al2O3 in the nanocompositesmight also account for their increased wear-resistanceover bare PI. In other words, nanometer Al2O3 of anappropriate proportion was homogeneously dispersed inthe PI matrix, and the interfacial adhesion between thematrix and the nanoparticle was hence strong owing tothe high specific surface area and reactivity of the nano-particles [11–17]. Subsequently, the molecular chains ofPI attained unchanged or even increased interaction ata proper mass fraction of the nanometer Al2O3, whichcontributed to improve the wear-resistance of the nanoc-omposites. However, the interfacial interaction betweenthe PI matrix and the inorganic nanophase would beworsened at an extended mass fraction of the nanometerAl2O3, which accounted for the poorer wear-resistanceof the PI–Al2O3 nanocomposites in this case as well.

In a similar manner, the mechanical strength of thenanocomposite was related to the tribological propertiesto some extent. As shown in the present article, whenthe nanometer Al2O3 mass fraction was above 4.0%, thebending strength of the nanocomposite decreases, whichcorresponds to the decrease of the wear-resistance. Thisindicates that the mechanical strength played animportant role in determining the tribological behaviorof the nanocomposites, though no evident correlationbetween the friction coefficient or wear volume loss andthe microhardness was found.

3.3. Scanning electron microscopy studies

In order to understand the effect of the nanometerAl2O3 on the friction and wear behavior of the PI/Al2O3

nanocomposites, the morphologies of the nanocompos-ites worn surfaces and the counterpart steel rings as wellas the wear debris were studied by SEM. Fig. 6 showsthe SEM pictures of the worn surfaces of pure PI andnanocomposites blocks, the corresponding transfer filmson the steel counterpart surface and the wear debris. Theworn surface of pure PI, Fig. 6(a), shows signs ofscuffing and adhesion which is obviously abated on theworn surface of PI/3.0%Al2O3 nanocomposite, Fig. 6(b),while that of PI/12.0%Al2O3 shows signs of agglomer-ated abrasives, Fig. 6(c), which accounts for the

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Fig. 6. SEM pictures of (a) worn surface of pure PI, (b) worn surface of PI/3.0% Al2O3, (c) worn surface of PI–12.0% Al2O3, (d)transfer film of PI, (e) transfer film of PI–3.0% Al2O3, (f) transfer film of PI–12.0% Al2O3, (g) wear debris of PI, (h) wear debrisof PI–3.0% Al2O3, and (i) wear debris of PI–12.0% Al2O3. (sliding speed: 0.431 m/s; load: 200 N).

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Fig. 7. SEM pictures of (a) worn surface of the nanocompositePI/Al2O3(3.0wt%); (b) worn surface of the nanocompositePI/Al2O3(12.0 wt%) (sliding speed: 0.862 m/s; load: 200 N).

increased wear of the nanocomposite with higher massfraction of nanometer Al2O3 [20]. A thick and lumpytransfer film is formed on the counterpart steel ring sur-face sliding against pure PI, Fig. 6(d), while that of thePI/3.0%Al2O3 nanocomposite is relatively uniform andcompact, Fig. 6(e), and the agglomerated abrasives onthe transfer film of PI/12.0%Al2O3, Fig. 6(f), also con-tributes to the larger wear volume loss of the nanocom-posites with higher mass fraction of nanometer Al2O3.Moreover, the wear debris of the pure PI, Fig. 6(g) islarger than that of the PI/Al2O3 nanocomposites andcharacterized by large-flakes peeling off. Such a kind ofpeeling off is significantly restrained for thePI/3.0%Al2O3 and PI/12%Al2O3 nanocomposites, Fig.6(h) and (i). Therefore, it can be concluded that the dif-ferences in the worn surface morphologies, transfer filmcharacteristics, and wear debris characteristics of thePI/Al2O3 nanocomposites accounted for their different

friction and wear behaviors in sliding against the plaincarbon steel.

Fig. 7 shows the worn surfaces of PI/3.0%Al2O3 andPI/12.0%Al2O3 at a relatively high sliding speed of 0.862m/s. More scuffing, ploughing and plastic deformationmarks appear on the worn surface of PI/12.0%Al2O3, ascompared with that of PI/3.0%Al2O3. This indicates thatPI/Al2O3 nanocomposite of higher mass fraction of nano-meter Al2O3 experienced more severe plastic defor-mation and weakening of the molecular chain interac-tion, which is related to the poorer wear-resistance of thenanocomposites with higher mass fraction of nanometerAl2O3 as well.

4. Conclusion

The incorporation of appropriate content of nanometerAl2O3 into PI improved the tribological behavior sig-nificantly. This is especially so at an extended load andsliding speed. The differences in the friction and wearbehaviors of PI and PI–Al2O3 nanocomposites were attri-buted to the differences in their worn surface morpho-logies, transfer films characteristics, and wear debris fea-tures. The increased wear volume losses of thenanocomposites of higher mass fractions of nanometerAl2O3 were attributed to the abrasion action of theagglomerated abrasives on the worn composite andtransfer film surfaces.

References

[1] B.A. Cook, V.E. Yudin, J.U. Otaigbe, Thermal propertiesof polyimide foam composites, Journal of MaterialsScience Letters 19 (21) (2000) 1971–1973.

[2] M. Yoshida, N. Deepak Kumar, P.N. Prasad, TiO2 nano-particle-dispersed polyimide composite optical waveguidematerials through reverse micelles, Journal of MaterialsScience 32 (15) (1997) 4047–4051.

[3] Yu.N. Sazanov, Applied significance of polyimides, Rus-sian Journal of Applied Chemistry 74 (8) (2001) 1253–1269.

[4] A.O. Pozdnyakov, K. Friedrich, V.V. Kudryavtsev,Effects of thermal treatment on the wear behavior of polyi-mide-fullerene (C60) composite coatings, Journal ofMaterials Science Letters 20 (22) (2001) 2071–2075.

[5] E. Totu, E. Segal, A.K. Covington, On the thermal behav-iour of some polyimide membranes, Journal of ThermalAnalysis and Calorimetry 52 (2) (1998) 383–391.

[6] L. Augh, J.W. Gillespie Jr., B.K. Fink, Degradation ofcontinuous carbon fiber reinforced polyetherimide com-posites during induction heating, Journal of ThermoplasticComposite Materials 14 (2001) 96–115.

[7] J. Bijwe, J. Indumathi, J.J. Rajesh, M. Fahim, Friction andwear behaviour of polyetherimide composites in variouswear modes, Wear 249 (2001) 715–726.

[8] P. Filip, Z. Weiss, D. Rafaja, On friction layer formation

882 H. Cai et al. / Polymer Testing 22 (2003) 875–882

in polymer matrix composite materials for brake appli-cation, Wear 252 (2002) 189–198.

[9] H. Pihtili, N. Tosun, Investigation of the wear behaviourof a glass fibre reinforced composite and plain polyesterresin, Composites Science and Technology 62 (2002)367–370.

[10] F. Li, F.Y. Yan, L.G. Yu, W.M. Liu, The tribologicalbehaviours of copper-coated graphite filled PTFE com-posites, Wear 237 (2000) 33–38.

[11] J.C. Huang, X.F. Qian, J. Yin, Z.K. Zhu, H.J. Xu, Prep-aration of soluble polyimide–silver nanocomposites by aconvenient ultraviolet irradiation technique, MaterialsChemistry and Physics 69 (2001) 172–175.

[12] L.G. Yu, S.R. Yang, H.T. Wang, Q.J. Xue, An investi-gation of the friction and wear behaviors of micrometercopper particle- and nanometer copper particle-filledpolyoxymethylene composites, Journal of Applied Poly-mer Science 77 (2000) 2404–2410.

[13] Z.K. Zhu, Y. Yang, J. Yin, Z.N. Qi, Preparation andproperties of organosoluble polyimide/silica hybridmaterials by sol–gel process, Journal of Applied PolymerScience 73 (1999) 2977–2984.

[14] T. Agag, T. Koga, T. Takeichi, Studies on the thermal andmechanical properties of polyimide–clay nanocomposites,Polymer 42 (2001) 3399–3408.

[15] A. Morikawa, H. Yamaguchi, M. Kakimoto, Y. Imai, For-mation of interconnected globular structure of silica phasein polyimide–silica hybrid films prepared by the sol–gelprocess, Chemistry of Materials 6 (1994) 913–917.

[16] Y. Echigo, Y. Iwaya, I. Tomioka, H. Yamada, Solventeffects in thermal curing of poly(4,4’ -oxybis(phenylenepyromellitamic acid)), Macromolecules28 (1995) 4861–4865.

[17] L. Liu, Q.H. Lu, J. Yin, X.F. Qian, W.K. Wang, Z.K. Zhu,Z.G. Wang, Photosensitive polyimide (PSPI) materialscontaining inorganic nanoparticles (I) PSPI/TiO2 hybridmaterials by sol–gel process, Materials Chemistry andPhysics 74 (2002) 210–213.

[18] A. Sidorenko, H.S. Ahn, D.I. Kim, H. Yang, V.V. Tsu-kruk, Wear stability of polymer nanocomposite coatingswith trilayer architecture, Wear 252 (2002) 946–955.

[19] Q.H. Wang, Q.J. Xue, W.M. Liu, J.M. Chen, The frictionand wear characteristics of nanometer SiC and PTFE filledPEEK, Wear 243 (2000) 140–146.

[20] A.P. Harsha, U.S. Tewari, Tribo performance of polyary-letherketone composites, Polymer Testing 21 (2002)697–709.