Materials Science and Engineering A364 (2004) 94–100

Investigation of tribological properties of polyimide/carbonnanotube nanocomposites

Hui Cai∗, Fengyuan Yan, Qunji Xue

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

Received 29 March 2003; received in revised form 28 July 2003

Abstract

Polyimide/carbon nanotube (PI/CNT) nanocomposites with different proportions of CNT were fabricated by in situ process. The bendingstrength and microhardness of the PI/CNT nanocomposites were measured. The friction and wear behavior of the nanocomposites was evaluatedon an M-2000 friction and wear tester. As the results, the bending strength and microhardness of the PI/CNT nanocomposites increased withincreasing CNT content and reached stable values at a certain content of CNT. CNT could effectively enhance the friction-reduction andantiwear capacity of the nanocomposite because it increased the load capacity and mechanical strength of the CNT/PI. The variables such asapplied load and sliding speed had a significant influence on friction and wear performance.© 2003 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotube; Polyimide; Nanocomposite; Tribological behavior

1. Introduction

Polymer matrix composites are widely used in auto-motive, air, and railway transport systems for tribologicalapplications[1–3]. Along with the extensive application ofpolymers for tribological purposes, the understanding ofpolymer tribology is becoming increasingly important. Theimportance of the tribological properties is convincing manyresearchers to study it and to improve the wear-resistanceof polymers and polymer composites[4,5]. Composite ma-terials consist of resin and reinforcement chosen accordingto desired mechanical properties and the application. Manyresearchers have largely focused on the mechanism of fillerin reducing wear and found that different inorganic fillersshow distinct effect on the friction and wear behaviors ofpolymer composites[2,3,5].

Nanometer materials posses many special physical andchemical properties, such as quantum size effect, small sizeeffect, surface and interface effects. Since the interactionbetween the polymer chains and the surface of the particlescan evidently alter the chain kinetics in the region imme-diately surrounding the particle due to the presence of the

∗ Corresponding author. Tel.:+86-931-8274661;fax: +86-931-8277088.

E-mail address: c hui9999@sina.com (H. Cai).

interface, it is thus rational to anticipate that nanometerparticulates as additives will provide a well-bonded inter-face which will enable the polymer-based nanocomposite topossess high performance[6–10]. As a matter of fact, evenlow filler volume fractions provide an enormous amountof interfacial area through which the bulk properties of thepolymer can be altered, if the filler is well dispersed in thepolymer matrix. Accordingly, it will be possible to developnovel high-performance polymer-based nanocompositesby proper combination of polymer matrix with nanometerreinforcement. With this perspective in mind, polyimide(PI) and carbon nanotube have been selected in the presentwork, because polyimide has excellent mechanical, elec-trical (insulating) and thermal properties and resistanceto most of the solvents, radiation[11], while carbon nan-otube (CNT) has superior mechanical properties and is apotential excellent reinforcing agent for polymers[12,13].However, few report have been available on the tribologi-cal behavior of PI/CNT nanocomposite prepared by in situprocess.

This article deals with the preparation of the PI/CNTnanocomposites by in situ process and the effect of CNTreinforcing on the tribological properties of the nanocom-posites. The wear mechanisms of the nanocomposites in drysliding against a plain carbon steel counterpart are also dis-cussed.

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0921-5093(03)00669-5

H. Cai et al. / Materials Science and Engineering A364 (2004) 94–100 95

2. Experimental

2.1. Materials and specimens

4,4′-Oxydianiline (ODA) and diphenylether-3,3′,4,4′-tetracarboxylic dianhydride (OPDA) were purchased fromcommercial sources and dried before use. CNT with a diam-eter about 10–50 nm was provided by Tsinghua University.The other compounds including triethylamine, pyridine, andN-methyl-2-pyrrolidone (NMP) were commercial analyticalgrade reagents and used without further purification, exceptthat NMP was dried over molecular sieves before use.

Poly(amic acid) solution (coded as PAA solution) was ob-tained by adding an equimolar amount of OPDA to a solutionof ODA/NMP. The homogeneous suspensions of CNT/NMPwith different CNT proportions were added into PAA so-lution under vigorous agitating. The homogeneous suspen-sion of CNT/PAA/NMP was precipitated by introduction oftriethylamine and pyridine. After drying to allow thermalimidization, PI/CNT powders were compressed and heatedto 340◦C in a mold at a rate of 8◦C/min. The pressure washeld at 10 MPa below 315◦C and raised to 25 MPa for therest of the heating cycle, to ensure good compactness of theresultant composite specimens. The compressed compositewas held at 340◦C for 10 min and then cooled to 100◦C inthe mold. After releasing from the mold, the resultant blockspecimens were used for friction and wear tests. The chem-ical structure of the PI matrix was shown inFig. 1.

2.2. Characterization

The microhardness of the specimens was determined byconducting five replicate measurements on an MH-5 mi-crohardness meter (made by Shanghai Hengyi ElectronicsMeasurement and Testing Corporation Limited of China) ata load of 2 N and a loading time of 5 s. The average of thefive replicate measurements of the hardness values is citedas the hardness of the specimen in this article.

The bending strength of the PI/CNT nanocomposites wasmeasured using an AG-10TA universal material tester (Shi-madzu Corporation of Japan) in accordance with nationalstandard GB9341-88.

2.3. Friction and wear test

The friction and wear tests were conducted on an M-2000model friction and wear tester (made by Xuanhua Tester

O

O

O

O

N

O

N O

n

Fig. 1. Chemical structure of the PI matrix.

Fig. 2. Contact schematic diagram for the frictional couple.

Factory of China). The contact schematic diagram of thefrictional couple is shown inFig. 2. The plain carbon steelring (HRC 48–50) in a diameter of 40 mm was used as thecounterpart. Sliding was performed under ambient condi-tions over a period of 1.5 h (or 45 min) at sliding velocityof 0.431 m/s (or 0.862 m/s), normal load of 50, 100, 200,and 290 N. The ambient temperature was roughly 25◦C andthe relative humidity about 50± 10%. Before each test,the surfaces of the block specimens and the counterpartring were abraded with No. 900 water-abrasive paper (Ra:0.2–0.52�m). Then the steel ring was cleaned with acetone-dipped cotton and the PI or its nanocomposite blocks werecleaned with acetone, followed by drying. The friction forcewas measured using a torque shaft equipped with straingauges, and the friction coefficient calculated by taking intoaccount the normal load applied and the friction force mea-sured. At the end of each test, the width of the wear scar onthe block specimens was measured with an optical micro-scope, then the wear volume lossV of the block specimencalculated from the relationship

V = B

πR2

180arcsin

(b

2R

)− b

2

√R2 − b2

4

whereV refers to the volume loss (mm3), B to the widthof the block specimen (mm),R to the radius of the steelring (mm), andb to the width of the wear scar (mm). Threereplicate friction and wear tests were carried out so as tominimize data scattering, and the average of the three repli-cate test results is reported in this article. The relative errorof the replicate friction and wear test data was±10%.

96 H. Cai et al. / Materials Science and Engineering A364 (2004) 94–100

0 5 10 15 20 25 30

185

190

195

200

CNT content (wt.%)

Ben

ding

str

engt

h (M

Pa)

Bending strength

25

30

35

40

45

Mic

roha

rdne

ss (

HV

)

Microhardness

Fig. 3. Variation of the bending strength and microhardness of PI/CNT nanocomposites with CNT content.

0 5 10 15 20 25 30

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

50N 100N200N 290N

Fric

tion

coef

ficie

nt

CNT content (wt.%)(a)

0 5 10 15 20 25 300.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

50N 100N200N 290N

Wea

r vo

lum

e lo

ss (

mm

3 )

CNT content (wt.%)(b)

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

H. Cai et al. / Materials Science and Engineering A364 (2004) 94–100 97

2.4. Worn surface observation with a scanning electronmicroscope

The worn surfaces of the nanocomposite blocks and ofthe transfer films on the counterpart steel rings were ionplated with Au to become conductive and then observed ona JSM-5600LV scanning electron microscope (SEM).

3. Results and discussion

3.1. Bending strength and microhardness of PI/CNTnanocomposites

Fig. 3gives the bending strength and microhardness of thePI/CNT nanocomposite as a function of CNT content. Themicrohardness increases with increasing CNT and reachesalmost unchanged value when the CNT content is above8.0 wt.%. The bending strength of the nanocomposites shows

0 5 10 15 20 25 300.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.431m/s 0.862m/s

Fric

tion

coef

ficie

nt

CNT content (wt.%)(a)

0 5 10 15 20 25 300

5

10

15

20

25

0.431m/s0.862m/s

Wea

r vo

lum

e lo

ss (

mm

3 )

CNT content (wt.%)(b)

Fig. 5. Variations of friction coefficient (a) and wear volume loss (b) with CNT content at sliding speeds of 0.431 and 0.862 m/s (sliding distance:2327 m; load: 200 N).

the same changing tendency with the variation of the CNTcontent. It is thus inferred that the inclusion of CNT as areinforcing agent helps to increase the load-carrying capacityand mechanical properties of PI and improve the tribologicalbehavior as well.

3.2. Friction and wear properties of the PI/CNTnanocomposites

Fig. 4 shows the variations of the friction coefficient andwear volume loss of PI and the PI/CNT nanocomposites un-der various loads with CNT content. For pure PI, the frictioncoefficient increases with increasing load, which is supposedto be dependent on the enhanced plastic deformation and ad-hesion of the PI matrix under an extended load. The PI/CNTnanocomposites register sharply decreased friction coeffi-cients in dry sliding against the steel counterpart at variousloads when the CNT content is below 8.0%, then the varia-tion of the friction coefficients at various loads becomes mar-

98 H. Cai et al. / Materials Science and Engineering A364 (2004) 94–100

gin when the CNT content surpasses 8.0 wt.%. On the otherhand, the incorporation of CNT contributes to decrease thewear volume loss of PI significantly. Similar as above, whenthe CNT content in the nanocomposites surpasses 8.0 wt.%,the nanocomposites show almost unchanged wear volumeloss with further increasing CNT content. The above obser-vations agree well with the results of the microhardness andbending strength of the nanocomposites. In other words, theincorporation of CNT in PI helps to increase the microhard-ness and bending strength considerably, hence the PI/CNTnanocomposites show much better wear-resistance than neatPI. Since the strengthening effect of CNT as a reinforcingagent comes to the maximum at a mass fraction of 8.0%,thus the changing tendency of the mechanical and tribolog-ical properties of the nanocomposites comes to a criticalturning point at this filler mass fraction.

The variations of friction coefficients and wear volumelosses for neat PI and PI/CNT nanocomposites with slid-ing speed are shown inFig. 5. The nanocomposite recordsconsiderably decreased friction coefficient with increasingCNT content at both small and large sliding speed, prior toreaching a relatively stable value (Fig. 5a). Moreover, thenanocomposite shows better wear-resistance at a larger slid-ing speed (Fig. 5b), and the wear-resistance of the nanocom-posite becomes insensitive to the increasing content of CNT

Fig. 6. SEM micrographs of (a) worn surface of pure PI; (b) corresponding transfer film of pure PI formed on the steel ring surface; (c) worn surfaceof PI/8.0%CNT; (d) corresponding transfer film of PI/8.0%CNT formed on the steel ring surface (load: 200 N; sliding velocity: 0.431 m/s; test duration:90 min).

when the mass fraction of CNT surpasses 8.0 wt.%, whichis the same as what has been mentioned above.

From Figs. 4b and 5b, it is also noted that the wear vol-ume losses of the neat PI and nanocomposite increase withincreasing applied load and sliding speed. It is thus supposedthat the temperature of the specimen will increase with in-creasing applied load and sliding speed, which results inmicromelting in the specimen surface and makes the trans-fer film rubbed off and specimens worn away easily, hencethe wear volume losses of the neat PI and nanocompositeincrease.

3.3. SEM analysis of the worn surfaces

The SEM micrographs of the worn surfaces and transferfilms of PI and PI/CNT nanocomposites under differenttesting conditions are shown inFigs. 6 and 7, respectively.The worn surface of neat PI shows signs of adhesion andabrasive wear (Fig. 6a), while the corresponding trans-fer film on the steel counterpart is lumpy and incoherent(Fig. 6b), which corresponds to the relatively poorer wear-resistance of the neat PI in sliding against the steel. Con-trary to the above, the scuffing and adhesion on the wornsurface of the PI/8.0%CNT nanocomposite is considerablyabated (Fig. 6c), while a thin, uniform, and compact trans-

H. Cai et al. / Materials Science and Engineering A364 (2004) 94–100 99

Fig. 7. SEM micrographs of (a) worn surface of pure PI; (b) corresponding transfer film of pure PI formed on the steel ring surface; (c) worn surface ofPI/8.0%CNT; (d) corresponding transfer film of PI/8.0%CNT formed on the steel ring surface (load: 200 N; sliding velocity: 0.862 m/s; sliding distance:2327 m).

fer film is formed on the counterpart steel surface in thiscase (Fig. 6d), which agrees well with the considerablyincreased wear-resistance of the PI/CNT nanocomposite.Therefore, it can be deduced that the incorporation of CNTcontributes to restrain the scuffing and adhesion of the PImatrix in sliding against the steel and to generate transferfilm of better quality on the counterpart steel surface, sub-sequently the PI/CNT nanocomposites show much betterwear-resistance than the neat PI.

The severe adhesion signs on the worn surface of PI andthe slight abrasion on that of PI/8.0%CNT correspond well tothe corresponding loose and incoherent transfer film for neatPI and to the thin and compact transfer film for PI/8.0%CNT,respectively (Fig. 7). This agrees well with their differentfriction and wear behaviors under larger sliding speed. Fur-thermore, it can be seen that more obvious ploughed fur-rows appear on the worn surface of the polymer block andthe transfer film under a larger sliding speed as comparedto that under a smaller sliding speed (Figs. 6 and 7), whichindicates that the damage to the worn surface and transferfilm is more severe under a larger sliding speed than that un-der a smaller sliding speed. It is therefore inferred that themicromelting on the specimen surface becomes more severeat a larger sliding speed, which contributes to increasing thewear volume losses of the CNT/PI and pure PI.

4. Conclusions

The PI/CNT nanocomposites register lower friction co-efficients and wear volume losses than the neat PI underdry sliding. CNT as a reinforcing agent also contributes torestrain the adhesion and scuffing of the PI matrix and en-hance the formation of the transfer films with better qual-ity on the counterpart steel surface, which together with theload-carrying capacity of the CNT contributes to improvethe friction-reduction and wear-resistance of the PI/CNTnanocomposites.

Acknowledgements

The authors are thankful to Tsinghua University for pro-viding CNT.

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