1 Introduction

Recently, polymer matrix composites were found ever-rising tribological applications such as gears, bearings, seals and artificial joints instead of metals due to their advantages such as low density, easy manufacturability, superior shock, high vibration and self-lubrication [1, 2]. UHMWPE has been widely used for artificial joints due to its clinical performances in the last two decades [3, 4]. On the other hand, UHMWPE has low Young’s modulus, low load bearing and anti-fatigue capacity [5]. Therefore, efforts have been put on to improve tribological performances of UHMWPE. Incorporation of fillers to overcome shortcomings of the UHMWPE matrix is a promising solution [6]. For this purpose, composite materials were produced using inorganic fillers such as kaolin, zirconium, nano zinc oxide and also, various organic fillers, including carbon fiber, carbon black and carbon nanotubes have been investigated in UHMWPE matrix due to their good surface adherence and better solid lubrication properties. However, these fillers limited their application at UHMWPE composites because of high cost and inadequate performance of composites in artificial joints [5]. Graphene has attracted as an ideal filler recently because of its special properties graphene has potential applications in many fields such as composite materials [7]. Commonly, graphene derivatives such as graphene oxide (GO), reduced graphene oxide (RGO), graphene nanoplates (GNP) and multi-layer graphene (MLG) are widely used as fillers for polymer composite materials [8]. GO is prepared from oxidation of graphite powder by Hummers method and RGO is synthesized from the reduction process of GO. Usually, GO and RGO are more preferred than other graphene derivatives due to their hydrophilicity, ease of formation of stable colloidal suspensions and easy-inexpensive synthesis [9]. At the same time, they have antibacterial properties and they were often used for biomedical applications recently [10]. There are studies to investigate tribological properties of UHMWPE composites by using GO [5, 11,12,13] and GNP [14, 15] but to the best of our knowledge, there are no report on the green synthesis of RGO for UHMWPE and their effects on the tribological properties of UHMWPE biocomposites, in the literature. Bhattacharyya et al. reported that UHMWPE nanocomposite films was produced with RGO using two different process routes. But they used phenylhydrazine as reduction agent. Phenylhydrazine that hydrazine derivative is highly toxic and instability. Therefore, it is not suitable for biomaterial application areas whether excess hydrazine is not removed [16]. In this paper, RGO filler was produced by green synthesis with vitamin C. RGO filled UHMWPE biocomposites were prepared, and the effect of the RGO content of the biocomposites on the structural and tribological properties under distilled water lubrication conditions were investigated. As a consequence, tribological and mechanical properties of UHMWPE can be altered by changing loading content of RGO. Moreover, the synthesized RGO may be a good candidate for UHMWPE based biomaterials.

2 Experimental methods

Firstly, GO was prepared by oxidizing the graphite powder according to a modified Hummers’ method [8, 17]. The detailed information about the reduction process of GO was described in previous literature [18]. UHMWPE/RGO biocomposites of different weight percentage of RGO to UHMWPE were prepared as follows: In brief, as-prepared powder RGO were sonicated for 30 min in ethyl alcohol using a homogenizer to form a well dispersed suspension. After that, UHMWPE powders were added into the suspension and the mixture was stirred for 30 min and then sonicated for 1 h. Then the ethyl alcohol was removed at 60–70 °C in an oil bath and the biocomposite powders were dried in an oven at 60 °C. Finally, the unfilled UHMWPE and biocomposite powders were molded by hot-pressing at 180 °C under a 10 MPa pressure and holding at this pressure for 30 min. In order to investigate the effects of RGO on the tribological properties of the biocomposites with the RGO content of unfilled, 0.7 and 3.0 wt% RGO were prepared. The codes of unfilled UHMWPE and these two biocomposites were UHMWPE, UHMWPE/RGO-0.7 and UHMWPE/RGO-3.0. The crystallinity of the biocomposites were represented by X-ray diffraction (XRD) patterns acquired by a PAN analytical, Empyrean diffractometer using Cu Kα radiation in the angle range 2θ = 5–30°. The molecular structure of the biocomposites were characterized by Fourier transfer infrared spectroscopy (FTIR) spectra which is recorded by a Spectrum 100, Perkin Elmer between 400 and 4000 cm−1. Shimadzu microhardness tester was used to measure the hardness of the biocomposites prepared. The hardness values were calculated based on the Vickers method with a load of 25 g. At least ten successive measurements were performed for each condition. The scanning electron microscope (SEM) (Zeiss, Supra 40VP) and energy-dispersive X-ray spectrometry (EDS) were used to observe morphology and worn surfaces of the biocomposites. A ball-on-disc reciprocating tribometer was used for all friction and wear tests. Wear tests were performed in a reciprocating mode with a 1.7 cm s−1 sliding rate under 5 N applied load for 45 min. The counter body was an Al2O3 ball with 10 mm diameter. Following the wear tests, the Al2O3 counterface surfaces were examined under an optical microscope (OM) in order to investigate the wear mechanisms.

3 Results and discussion

Figure 1 showed the XRD diffractograms of unfilled UHMWPE and UHMWPE/RGO biocomposites. The two peaks at 2θ° = 21.56 and 2θ° = 23.92, that appear in all diffractograms, correspond to the (110) and (200) planes of the orthorhombic crystal [19]. Inclusion of 0.7 wt% and 3.0 wt% RGO filling reduced the intensity of both UHMWPE peaks, which could be attributed to modification of matrix crystallinity [20]. The values of crystallite size of unfilled UHMWPE and UHMWPE/RGO biocomposites for 2θ = 23.92°, were shown in Fig. 1. It is observed that 0.7 wt% RGO contents results in increase of crystallite size achieving 100%, for comparison biocomposite with 3.0 wt% RGO and unfilled polymer. The increase of crystalline size in the UHMWPE/RGO-0.7 biocomposite and subsequently will affect the wear behavior of the biocomposite. Because the low RGO amount was act as nucleation centers and not disruptive reorganization and chain folding during crystallization process [21]. Additionaly, there is no new diffraction peak were observed in the patterns of biocomposites except the orthorhombic crystal peaks of unfilled polymer. This clearly indicated that RGO was exfoliated in the UHMWPE matrix [4, 22].

Fig. 1
figure 1

XRD patterns of unfilled UHMWPE and UHMWPE/RGO biocomposites

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Figure 2 showed the FTIR spectra of the UHMWPE and UHMWPE/RGO biocomposites. The peaks of 2915.92 cm−1, 2848.69 cm−1, 1463.04 cm−1 and 718.47 cm−1 were attributed to the CH asymmetric vibration, the CH symmetric vibration, the CH2 the bending vibration and CH2 rocking vibration, respectively, for the unfilled UHMWPE. The strength of the peaks at 2915.92 cm−1, 2848.69 cm−1, 1463.04 cm−1 and 718.47 cm−1 enhanced with low and high amounts of RGO, which possibly indicated that more interaction between RGO and UHMWPE matrix [23].

Fig. 2
figure 2

FTIR spectrum of unfilled UHMWPE and UHMWPE/RGO biocomposites

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Figure 3 showed the SEM images of the surfaces of UHMWPE/RGO biocomposites with different amounts of RGO and EDS elemental map of carbon and oxygen. It could be seen that the surface of UHMWPE/RGO-0.7 was relatively flat but also it had uneven regions. The image of this biocomposite showed RGO were embedded into the UHMWPE matrix so that good interfacial bonding strength exhibited between RGO and UHMWPE [14]. Furthermore, as the content of RGO increased to 3.0 wt%, the morphology of the surface was totally different. The UHMWPE/RGO-3.0 biocomposite exhibited an obviously rough and deformed morphology. As a result, the reorganization and chain folding of the polymer was hindered by the increasing content of RGO. In the XRD analysis section discussed the low amount of the RGO that caused nucleation centers by using crystallite size data. The EDS elemental mapping of biocomposites confirms that oxygen was uniformly distributed in the biocomposites. The dispersion of oxygen is very important because only RGO have oxygen containing functional groups. The results obtained from the EDS are compatible with XRD results.

Fig. 3
figure 3

SEM micrographs of unfilled UHMWPE and UHMWPE/RGO biocomposites; EDS elemental map of Carbon and Oxygen

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Table 1 showed that the microhardness, wear rate and friction coefficient values of unfilled UHMWPE and biocomposites under distilled water lubricating condition. Hardness of the biocomposites increased at all RGO loading content and when the loading content is 0.7 wt%, biocomposite had the best microhardness value in contrast to that of unfilled UHMWPE. The distribution of RGO in polymer matrix, as discussed in EDS analysis and XRD sections, may result in an increase of resistance to indentation [18]. It could be seen that the wear rate of all the biocomposites filled with RGO were lower than that of unfilled UHMWPE. Also, it was observed that the low amount of RGO that resulted in the maximum wear resistance. These results may be attributed to the excellent mechanical properties and high specific surface area of RGO, which facilitates good load transfer to the RGO network [12]. It could be seen that after adding the content of RGO into UHMWPE matrix, the friction coefficient of biocomposites decreased significantly. Both RGO and distilled water displayed lubricant properties because of homogeneous dispersion of RGO in the UHMWPE matrix and good interaction of filler and polymer matrix according to XRD, EDS and FTIR analysis results [15, 24].

Table 1 Microhardness, wear rate and friction coefficient values of UHMWPE and UHMWPE/RGO biocomposites
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Figure 4 showed the effects of different amount of RGO on the worn surface of biocomposites. In the low magnified image, it could be seen that the worn surface of unfilled UHMWPE were thin and superficial grooves under deionized water. The fatigue wear was found dominant where the cracked surface layer of the unfilled UHMWPE in the high magnified image. The low and high magnified images of UHMWPE/RGO-0.7 showed that the wear marks due to the grooves were disappeared and lead to severe adhesive wear. When the RGO filler loading was increased to 3.0 wt% the worn surfaces seemed to be severe adhesive wear in the low magnified image. Additionally, from the high magnified image shown in Fig. 4, it is clear that adhesive wear tracks and significant fatigue tear increased on surface of biocomposite. As a result, with the decrease in friction surface temperature, plastic deformation was not observed on the surface of biocomposites in the distilled water condition [25].

Fig. 4
figure 4

SEM micrographs of worn surfaces of the unfilled UHMWPE and biocomposites

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OM analyses of the morphologies of Al2O3 counterface under distilled water lubricated conditions were evaluated in Fig. 5a, c. Interfacial interactions between the polymer and filler played an important role in tribofilms formation [26]. Unfilled UHMWPE have a thin and uniform transfer film on the Al2O3 ball (Fig. 5a). When sliding occurred under distilled water lubricated condition, a robust tribofilm generated from UHMWPE/RGO-3.0 (Fig. 5c) and a patchy tribofilm produced from UHMWPE/RGO-0.7 were observed (Fig. 5b), which led to a lower wear rate of biocomposite with 0.7 wt% of RGO than that of biocomposite with 3.0 wt% of RGO. The strongly interfacial interactions between UHMWPE with RGO and high crystallite size amount of UHMWPE/RGO-0.7 biocomposite under distilled water condition was prone to destroy the tribofilm formed on Al2O3 surface.

Fig. 5
figure 5

OM images of the Al2O3 balls sliding against the a unfilled UHMWPE, b UHMWPE/RGO-0.7 and c UHMWPE/RGO-3.0 biocomposites

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4 Conclusions

RGO filled UHMWPE biocomposites were successfully fabricated and assessed in terms of tribological performance under distilled water lubrication condition. The following conclusions can be obtained from above studies.

  • FTIR results showed that there was interaction between the RGO and UHMWPE. It could be affirmed homogenous dispersion of the RGO in the UHMWPE matrix and crystallite size achieving 100% in UHMWPE/RGO-0.7 biocomposite as confirmed by XRD analysis.

  • SEM images showed that RGO were embedded into the UHMWPE matrix so that good interfacial bonding strength exhibited between RGO and UHMWPE in UHMWPE/RGO-0.7 biocomposite. The EDS elemental mapping of biocomposites confirms that oxygen was uniformly distributed in the all biocomposites.

  • The addition of RGO with small amounts was obviously increased the microhardness and the biocomposite with 0.7 wt% RGO had the best microhardness value.

  • The fatigue wear tracks and wear rate were significantly reduced when RGO was added up to 0.7 wt%.

  • With the RGO content increases, the frictional coefficient of the biocomposites were decreased. It is due to an efficient load transfer from the matrix to the filler.