Preparation and tribological properties of graphene/poly(etherether ketone) nanocomposites

HaoJie Song • Na Li • Yanjie Li • Chunying Min •

Zhen Wang

Received: 12 April 2012 / Accepted: 10 May 2012 / Published online: 24 May 2012

� Springer Science+Business Media, LLC 2012

Abstract The tribological behavior of poly(ether ether

ketone) (PEEK) composites was investigated using a uni-

versal micro-tribotester under dry friction conditions. We

studied the effect of addition concentration, applied load, and

sliding speed on the friction and wear behavior of composites

filled with multi-walled carbon nanotubes (MWNTs),

graphene oxide (GO) nanosheets, and c-aminopropyl tri-

methoxysilane-modified graphene oxide (GO-Si) nano-

sheets. The results showed that the friction reduction and

anti-wear performance of PEEK composites filled with

GO-Si is the most effective, that of the composites filled with

GO is the next, while that of the composites filled with

MWNTs is the worst. GO-Si-filled PEEK nanocomposites

have the best friction reduction and anti-wear properties

when the applied load and the sliding speed are 2.94 N and

0.0628 ms-1, respectively. Furthermore, scanning electron

microscope (SEM) investigation showed that GO-Si-filled

PEEK composites had smooth worn surface under given

applied load and sliding speed, and well-dispersed GO-Si in

PEEK matrix provided a large surface area available for

interaction between polymer molecules and GO-Si, which

helped to reduce the wear of PEEK composites.

Introduction

With the rapid development of technology in the areas

of aviation, aerospace, automobile, and machinery, the

high-performance polymers and their composites are

substituting the metallic materials as promising rubbing

materials for their excellent tribological properties.

Poly(ether ether ketone) (PEEK) is a semicrystalline ther-

moplastic with outstanding thermal stability, good resis-

tance to solvent and wear, and high-glass transition and

mechanical strength. As the matrix of high-performance

composites, PEEK is widely used in the areas of aerospace

and nuclear industries [1–4]. However, it is difficult to use

PEEK under harsh conditions because of its relatively

high-friction coefficient [5].

Recently, studies have focused on the tribological

behavior of PEEK composites [6–8]. Tang et al. [9] reported

that the sliding velocity played a significant role on the

tribological characteristics of carbon fiber-reinforced

PEEK. Xie et al. [10] demonstrated that the incorporation of

PTW into PEEK would achieve high-wear resistance and

low-friction coefficient under low load. Greco et al. [11]

studied the effect of reinforcement morphology on high-

speed sliding friction and wear of PEEK polymers, and

found that the material with long woven reinforcement

exhibited the lowest wear rate and lowest friction at high-

sliding speed.

Much of the current interest in carbon materials was ini-

tiated by the discovery of carbon nanotubes (CNTs) and later

the graphene, which are promising for many applications

[12–14]. Owing to their unique structures, CNTs and

graphene have excellent thermal, electrical, and mechanical

properties [15–17]. Therefore, CNTs and graphene are also

used as fillers in the base-lubricant materials to improve their

friction and wear properties [18–20]. Polymer composites

reinforced with CNTs [21–25] and graphene [26, 27] have

attracted much attention in research in the lubricating and

mechanical properties. For instance, Men et al. [28] reported

the functionalization of CNTs to improve the tribological

H. Song (&) � N. Li � Y. Li � C. Min � Z. Wang

School of Materials Science and Engineering, Jiangsu

University, Zhenjiang 212013, Jiangsu,

People’s Republic of China

e-mail: songhj@ujs.edu.cn

123

J Mater Sci (2012) 47:6436–6443

DOI 10.1007/s10853-012-6574-0

properties of poly(furfuryl alcohol) composite coatings, and

the results showed that the functionalization led to an

improvement in the tribological properties of composite

coatings. Dong et al. [29] explored the influence of multi-

walled carbon nanotubes (MWNTs) on the friction and wear

behaviors of the nanocomposites and found that MWNTs

could dramatically reduce the friction and improve the wear

resistance behaviors of the nanocomposites. Li et al. [30]

studied the preparation and tribological properties of

graphene oxide/nitrile rubber nanocomposites, and the fric-

tion and wear mechanisms of GO/NBR nanocomposites

were finally proposed. Liu et al. [31] researched the tribo-

logical properties of thermosetting polyimide/graphene

oxide nanocomposites. Experimental results showed that the

addition of graphene oxide (GO) evidently improved the

friction and wear properties of PI, which were considered to

be the result of the formation of uniform transfer film and the

increasing of load-carrying capacity. However, the friction

and wear behavior of GO nanosheet-filled PEEK composites

has not previously been investigated. It might be feasible to

develop high-performance polymer composites by rein-

forcing polymers with GO nanosheets.

In this study, GO nanosheet-filled PEEK composites

have been prepared by the method of solution blending.

Tribological performances of the composites under dry

sliding conditions were evaluated using a universal micro-

tribotester. Anti-wear and friction reduction mechanism

have also been discussed. To the best of our knowledge,

this is the first report on the tribological behavior of

graphene/PEEK composites.

Experiments

Materials

The PEEK matrix (PEEK 450G) was purchased from the

Victrex High-performance Materials Trading Co., Ltd of

Shanghai. Natural flake graphite, ethanol, hydrogen perox-

ide, sulfuric acid, hydrochloric acid, and potassium per-

manganate were purchased from East Instrument Chemical

Glass Co., Ltd, China. DB-551 (c-aminopropyl trimethox-

ysilane) was commercially provided by the Diamond New

Material of Chemical Inc., China. The oxidized MWNTs

(Purity C 95 wt%, 20–30 nm outer diameters, 10–30 lm

length, and 1.23 wt% COOH content) were purchased from

the Chengdu Organic Chemicals Co., Ltd, Chinese Academy

of Science.

Preparation and surface treatment of GO nanosheets

Graphene oxide nanosheets were prepared from purified

natural flake graphite through the method reported by

Daniela et al. [32]. Typically, concentrated H2SO4 (69 mL)

was added to a mixture of graphite flakes (3.0 g) and

NaNO3 (1.5 g), and the mixture was cooled using an ice

bath to 0 �C. KMnO4 (9.0 g) was added slowly in portions

to keep the reaction temperature below 20 �C. The reaction

mixture was warmed to 35 �C and stirred for 7 h. Addi-

tional KMnO4 (9.0 g) was added in one portion, and the

reaction was stirred for 12 h at 35 �C. The reaction mixture

was cooled to room temperature and poured onto ice

(400 mL) with 30% H2O2 (3 mL). The solid product was

separated by centrifugation, washed repeatedly with 5%

HCl solution until sulfate could not be detected with BaCl2,

then the suspension was dried in a vacuum oven at 50 �C

for 48 h to obtain GO nanosheets.

Figure 1 shows the fabrication procedure of GO-Si

nanosheets. Typically, 0.5 g of GO nanosheets were added

into a single-neck flask with 50.0 mL ethanol and dispersed

through ultrasonication (in a water bath) for 30 min. Then,

2.0 g of DB-551 was added and stirred with long-drawn

reflex condensing for 24 h at 75 �C for silanization. After

the reaction, GO-Si nanosheets were obtained by filtration

of the solution and washing with ethanol and distilled

water, and finally dried in a vacuum oven at 50 �C for 48 h.

Composites preparation

Three kinds of carbon materials were used as nanofillers

for the preparation of composites: (i) oxidized MWNTs;

(ii) GO nanosheets; and (iii) GO-Si nanosheets. Firstly,

2.0 g of PEEK powders were dispersed in a large beaker

with 20 mL ethanol, ultrasonic treated for 10 min, and

stirred for 30 min. Secondly, a certain amount of nanofil-

lers were added into a small beaker with 10 mL ethanol

and dispersed through ultrasonication for 30 min. Thirdly,

mixed nanofiller dispersion and PEEK dispersion together

and stirred to dry, and finally dried in a vacuum oven at

100 �C for 24 h, leading to desired samples with different

contents of PEEK composites. The blending powders were

molded into sheets of 1 g and then annealed in a tube

furnace at 320 �C for 12 h.

Evaluation of the tribological behavior

A universal micro-tribotester (UMT-2, Center for Tribol-

ogy Inc., USA) was used to evaluate the friction and wear

behavior of PEEK composites. Friction and wear tests were

conducted at a rotating speed of 0.0314–0.0942 ms-1 and

under a constant load of 1.96–3.92 N, with the test duration

of 20 min. Before each test, steel balls were cleaned with

acetone followed by drying. All the friction and wear tests

were carried out at 20–25 �C and a relative humidity of

40–60%. The friction coefficient was recorded automati-

cally with a strain gauge equipped with a tester. Wear

J Mater Sci (2012) 47:6436–6443 6437

123

behavior was evaluated by the resultant wear scar diameter

(WSD) of composite sheets after friction. The WSD value

was obtained from images taken by a Leica DM 2500M

optical microscope.

Results and discussion

Morphology of GO nanosheets

Figure 2 shows the scanning electron microscope [SEM,

(a)] and transmission electron microscopy [TEM, (b)]

images of GO nanosheets. In Fig. 2a, we can clearly see

that the surface of GO nanosheets is rough and has lost the

metallic luster of graphite. The morphology of the edge of

GO nanosheets is ladder-like and the nanosheets are of

different sizes. In Fig. 2b, it is clearly seen that GO

nanosheets are highly transparent but the transparency of

various parts is different, suggesting that the thickness of

GO nanosheets is very small and the nanosheets are not

completely the single-layered structure for some are the

superposition of several single layers. We can also observe

some dark lines with large contrast, which clearly illus-

trates that GO nanosheets are wrinkled or folded. It is

supposed that the wrinkled morphology can reduce the

surface energy of GO nanosheets and thereby make the

nanosheets stably exist.

FTIR analysis

Figure 3 shows the FTIR spectra of GO and GO-Si nano-

sheets. From the spectra of GO nanosheets, we observe a

broad and intense absorption at 3360 cm-1 (O–H stretching

vibration) and the bands at 1731 cm-1 (C=O stretching

vibrations from COOH groups situated at edges of GO

nanosheets), 1627 cm-1 (C=C stretching vibrations from

unoxidized graphitic domains), 1400 cm-1 (C–OH stretch-

ing vibrations), 1227 cm-1 (C–O–C stretching vibrations),

Fig. 1 The fabrication

procedure of GO-Si nanosheets

Fig. 2 SEM (a) and TEM (b) images of GO nanosheets

6438 J Mater Sci (2012) 47:6436–6443

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and 1043 cm-1 (C–O stretching vibrations). The presence of

different types of oxygen-containing functional groups

illustrates that the graphite has been oxidized.

From the spectra of GO-Si nanosheets, we observe an

obvious absorption at 3421 cm-1, which is attributed to the

N–H stretching vibration. Significantly, the absorption at

1731 cm-1 that reflects C=O structure becomes weaker and

wider after surface treatment. This is possibly due to the

effect of the N–H bending vibration. The weak absorption

appears at 669 cm-1 is also ascribed to the N–H stretching

vibration. The additional vibrational band appears at around

1087 cm-1 after surface treatment can be assigned to Si–O

stretching vibrations. It indicates that DB-551 has been

successfully grafted onto the surface of GO nanosheets.

Friction and wear behaviors

Figure 4 shows typical results from the friction and wear

test, in which the friction coefficient and the WSD of

PEEK composites mixed with MWNTs, GO, and GO-Si

are compared. It is obvious that all additives resulted in the

improvement of friction reduction and anti-wear abilities.

Compared with MWNTs and GO-reinforced PEEK com-

posites, GO-Si as additive holds the lower friction coeffi-

cient and the higher wear life. It is worth to note that when

the content of GO-Si is at 0.1 wt%, the friction coefficient

of the composites is the lowest.

Poly(ether ether ketone) composites filled with 0.3 wt%

MWNTs, 0.7 wt% GO, and 0.1 wt% GO-Si were selected

to investigate the effect of applied load on the friction and

wear properties at a speed of 0.0628 m s-1 and at a load of

1.96–3.92 N in Fig. 5. It can be seen that both the friction

coefficient and the WSD of PEEK composites of various

additives decrease up to approximately 2.94 N and then

increase with further increasing applied load. Compared

with MWNTs and GO-filled PEEK composites, GO-Si

treated one appears to have better friction reduction and

anti-wear abilities under different applied loads. Due to an

increased active group resulting from the grafting DB-551,

it is believed that this is beneficial for GO/matrix misci-

bility and hence enhances GO/matrix adhesion by chain

entanglement and chemical bonding between the grafting

DB-551 and matrix resin.

Variations of the friction coefficient and the WSD of

PEEK composites filled with 0.3 wt% MWNTs, 0.7 wt%

GO, and 0.1 wt% GO-Si with sliding speed under 2.94 N

are shown in Fig. 6. It can be seen that the friction coef-

ficient and the WSD slightly decrease when sliding speed is

below 0.0628 m s-1 and then increase as the sliding speed

rises from 0.0628 to 0.0942 m s-1. Under the same con-

dition, the friction coefficient and the WSD of GO-Si-filled

PEEK composites are slightly smaller than that of MWNTs

and GO-filled PEEK composites. Accordingly, GO-Si-

filled PEEK composites have the best friction reduction

and anti-wear abilities at a speed of 0.0628 m s-1. It is

supposed that the variations in the friction coefficient and

the WSD with sliding speed are closely related to the

friction-induced heat which played a larger role at the

frictional interface sliding at a higher speed than at a lower

speed. Thereby, the degradation and decomposition of

PEEK resin matrix and various fillers would be accelerated,

Fig. 3 FTIR absorption spectra of GO and GO-Si nanosheets

Fig. 4 Friction coefficient

(a) and WSD (b) as a function

of concentration of MWNTs,

GO, GO-Si (2.94 N,

0.0628 m s-1, 20 min)

J Mater Sci (2012) 47:6436–6443 6439

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and the wear of PEEK composites becomes more severe at

higher sliding speed.

Thermal analysis

The effect of MWNTs, GO, and GO-Si on the thermal

stability of PEEK composites was estimated by TGA and

DSC. The curves were obtained by heating the sample up

to 1000 �C at a rate of 10 �C/min with N2 gas purging. As

shown in Fig. 7a, the unfilled PEEK began to degrade at

approximately 560 �C and was completely decomposed at

800 �C; the decomposition curves of MWNTs and GO-

filled PEEK composites were nearly the same for which

began at 580 and 570 �C and completely at 850 �C,

respectively. While GO-Si-filled PEEK composites began

to degrade at approximately 590 �C and were completely

decomposed at 860 �C, it is clear that from Fig. 7b that the

unfilled, 1.0 wt% MWNTs, GO, and GO-Si-filled PEEK

composites began to melt at 340, 348, 353, and 355 �C,

respectively. Significantly, the addition of MWNTs, GO,

and GO-Si enhance the thermal stability of the matrix.

Furthermore, compared with the unfilled PEEK, the ther-

mal stability of GO-Si-filled PEEK composites slightly

decreased when the temperature was below 600 �C due to

the decomposition of GO nanosheets. However, the ther-

mal stability of 1.0 wt% GO-Si-filled PEEK composites

was higher than that of the unfilled one when the temper-

ature was above 600 �C. The reason is possibly because the

Fig. 5 Effect of applied load on

the friction coefficient and WSD

of the PEEK composites filled

with 0.3 wt% MWNTs,

0.7 wt% GO, and 0.1 wt%

GO-Si under 0.0628 m s-1

Fig. 6 Effect of sliding speed

on the friction coefficient and

WSD of the PEEK composites

filled with 0.3 wt% MWNTs,

0.7 wt% GO, and 0.1 wt%

GO-Si under 2.94 N

Fig. 7 TGA (a) and DSC

(b) curves of the unfilled and

1.0 wt% MWNTs, 1.0 wt% GO,

and 1.0 wt% GO-Si-filled

PEEK composites

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residues after the decomposition of GO which has good

thermal stability protect the decomposition products of

PEEK composites and therefore improve the thermal

stability.

XRD analysis

Figure 8 shows the XRD patterns of the unfilled, 1.0 wt%

MWNTs, 1.0 wt% GO, and 1.0 wt% GO-Si-filled PEEK

composites. The patterns of PEEK composites reinforced

by various additives are very similar to that of unfilled

PEEK. PEEK shows some amorphous peaks around

2h = 20�, revealing its amorphous nature. After the addi-

tives were incorporated into PEEK matrix, the XRD pat-

terns of the composites are almost the same as unfilled

PEEK but the peak intensity of the composites becomes

stronger than that of unfilled PEEK. The possible reason is

that the ratio of the amorphous carbon increases after

adding additives and thereby enhances the diffraction

intensity. Moreover, the peak intensity of 1.0 wt% GO-Si-

filled PEEK composites is the strongest. The reason is

possibly because that GO-Si nanosheets disperse well in

PEEK matrix and there are no evident agglomerates. No

crystalline peaks of graphite were detected. It is supposed

that the orientation of MWNTs and GO in PEEK matrix

may occur during the preparation, which increases the

directionality and decreases the intensity of the crystalline

peaks; thereby we can hardly detect it.

SEM analysis

Figure 9 shows SEM images of the surfaces of the unfilled,

1.0 wt% MWNTs, 1.0 wt% GO, and 1.0 wt% GO-Si-filled

PEEK composites. It can be seen from Fig. 9a that the

structure of unfilled PEEK is relatively loose, and there

were some obvious defects or voids in the surface. The

dispersion of oxidized MWNTs in PEEK matrix was

observed in Fig. 9b. Large areas with MWNTs agglomer-

ation can also be seen on an oxidized MWNTs composite

sample. In Fig. 9c, we find agglomerated GO particles in

the PEEK matrix. The oxidized MWNTs and GO were no

longer loose powders as during processing (mainly while

drying), the nanoparticles became clustered together, and it

was extremely difficult to break them apart even during

extrusion. The oxidized fillers are supposed to make better

linkage with the polymer matrix due to increased adher-

ence but on account of their initial state of agglomeration,

and it was not possible to achieve composites with well-

dispersed nanofillers. After modified by DB-551, the dis-

persion of GO-Si sheets in PEEK matrix was observed by

SEM. As shown in Fig. 9d, one could find that no obvious

GO-Si aggregates were observed from the surface of the

nanocomposites, which revealed that GO-Si sheets were

homogeneously dispersed in PEEK matrix.

Figure 10 shows the SEM images of the worn surfaces

of the unfilled, 1.0 wt% MWNTs, 1.0 wt% GO, and

1.0 wt% GO-Si-filled PEEK composites at a normal load

of 2.94 N. The worn surface of the unfilled PEEK shows

signs of adhesion and abrasive wear (Fig. 10a). The cor-

responding surface is very rough, displaying plucked and

plowed marks indicative of adhesive wear and plowing.

This phenomenon corresponds to the relatively poorer wear

resistance of the unfilled PEEK. As oxidized MWNTs, GO,

and GO-Si are added into the PEEK matrix, the damage of

the friction decreases because the stress is transferred by

the inorganic fillers (Fig. 10b, c). In contrast, the surface of

GO-Si/PEEK composites gets smoother than the other

samples and shows no surface defects, but only a smooth

and very shallow wear track (Fig. 10d).

Figure 11 presents the tribological model of GO-Si

nanosheets under dry sliding. Because of high-aspect ratio

of GO-Si, well-dispersed GO-Si in PEEK provided a large

surface area available for interaction between the polymer

molecules and GO-Si, which facilitates good load transfer

to the GO-Si network and, thus, resulted in improved

wear properties of GO-Si/PEEK composites. Furthermore,

GO-Si dispersed uniformly in PEEK composites can

prevent the close touch of the two contact surfaces

between the steel counter face and the coating surface,

which slows the wear rate and reduces the friction coef-

ficient. Finally, during the course of friction and wear,

GO-Si are released from PEEK composites and trans-

ferred to the interface between the composites and the

steel counter face. The self-lubricate properties of GO-Si

result in the reduction of the wear rate and the friction

coefficient.Fig. 8 XRD patterns of the unfilled, 1.0 wt% MWNTs, 1.0 wt% GO,

and 1.0 wt% GO-Si-filled PEEK composites

J Mater Sci (2012) 47:6436–6443 6441

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Fig. 9 SEM images of the unfilled (a), 1.0 wt% MWNTs (b), 1.0 wt% GO (c), and 1.0 wt% GO-Si (d)-filled PEEK composites

Fig. 10 SEM micrographs of the worn surfaces of the unfilled (a), 1.0 wt% MWNTs (b), 1.0 wt% GO, (c), and 1.0 wt% GO-Si (d)-filled PEEK

composites

6442 J Mater Sci (2012) 47:6436–6443

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Conclusions

As grafting DB-551 onto GO nanosheets which increase

the interfacial interaction between the GO nanosheets and

the PEEK matrix through chemical bonding improve the

dispersion of GO in PEEK, the friction reduction and anti-

wear abilities of GO-Si as lubricant additive are enhanced.

The friction reduction and anti-wear performance of the

PEEK composite filled with GO-Si is the most effective.

The applied load and sliding speed have great effect on

PEEK composites. It appears that PEEK composites have

the best performance of friction reduction and anti-wear

when the applied load and the sliding speed are 2.94 N and

0.0628 ms-1, respectively. The characterization performed

after friction tests indicated that GO-Si nanosheets are

released from PEEK composites and transferred to the

interface between the composites and the steel counter

face. Therefore, the self-lubricate properties of GO-Si

nanosheets result in reduction of the wear rate and the

friction coefficient.

Acknowledgements This work has been supported by the Natural

Science Foundation (50903040, 51103065).

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Fig. 11 The tribological model of GO-Si nanosheets under dry

sliding

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