Elsevier

Tribology International

Volume 131, March 2019, Pages 94-101
Tribology International

Microstructure and tribological behaviour of alumina composites reinforced with SiC-graphene core-shell nanoparticles

https://doi.org/10.1016/j.triboint.2018.10.009Get rights and content

Highlights

  • The composites showed excellent wear-resistant and self-lubricating performance.

  • The reduction effect of the nanofiller on friction tends to be more significant under higher loads.

  • The enhancement is attributed to the improved microstructure, mechanical properties and GNSs-rich tribofilms.

Abstract

SiC-graphene nanosheets (GNSs) core-shell nanoparticles were incorporated into alumina matrix composites. The composites showed refined grain sizes, increased toughness and hardness. Ball-on-disk wear tests were conducted and the results demonstrated that the coefficient of friction was reduced by 29.4%, and the wear rate was reduced by 90.1% for the composite containing 5 vol % SiC and 1 vol % GNSs at 90 N in comparison to monolithic alumina. These results are superior to Al2O3-graphene composites documented in the literature. The microstructure and topography of the wear scars were examined to understand the wear behaviour. GNSs-rich tribofilms were observed on the wear scar. The contributing mechanism of the self-lubricating and wear-resistance performance was explored.

Introduction

Wear-resistant and self-lubricating materials are highly desirable in various industrial practices, such as mineral processing, metallurgy and material processing. The mechanical working parts, such as bearings, seats and dies, should be able to prevent or reduce wear loss for the purpose of ensuring adequate equipment running and also improving serving performance in contact with abrasive environment [1]. Ceramics are suited candidate materials for these applications owing to their strong chemical bonds, superior wear resistance, good chemical inertness and corrosion resistance [[1], [2], [3]].

Alumina is one of the most widely used wear-resistant ceramics because of its abundance and economical manufacturing process. However, engineering alumina is basically characterised with low toughness and medium hardness in the ceramic community, which can result in grain pull-out, micro-cracking and weakened surfaces under abrasive sliding and impingement [2,4,5]. The tribological performance of ceramics can be optimised in terms of grain refinement, hardness, grain-boundary toughness, internal residual stresses and surface lubrication [[1], [2], [3],6,7]. It is a common strategy to introduce a second phase (e.g. particle or fibre) to refine the grain size and toughen the matrix of the material [8]. Alumina-SiC composites have been frequently studied. It was shown that the addition of 2 vol% SiC reduced the area fraction of pull-out and wear rate by factors of 2.2 and 2.3 respectively, and these two values were insensitive to the SiC particle size [9]. The addition of 12–90 nm SiC nanoparticles brought about smaller grain size and higher hardness, but no obvious change of fracture toughness in the alumina matrix [10]. Graphene nanosheets (GNSs) have been recently used as a nanofiller to improve the toughness and wear-resistant performance of ceramics as well [3,[11], [12], [13], [14]]. Crack bridging, crack deflection and GNS pull-out are recognised as the main contributing mechanisms of GNSs to toughen ceramics [12,15]. The GNSs-rich tribofilm formed at grain boundaries and worn tracks can work as solid lubricants, which are believed to be the main GNS-induced wear mechanism [3,12]. There are also a few studies which employed more than one fillers to produce hybrid ceramic matrix composites with combined properties [16,17]. For example, the hardening effect of SiC nanoparticle and toughening effect of GNSs were used in the study of Liu et al. [18] to produce advanced alumina composites with increase in hardness, flexural strength and fracture toughness. Hybrid Al2O3single bondSiC-CNT composites were also developed with enhanced mechanical properties [19]. However, SiC, GNSs and CNT fillers were usually incorporated separately and the tribological properties of the composites have been paid less attention.

There are several advantages to utilise nanoparticle-GNSs core-shell nanofillers in composite materials. Firstly, nanoparticles can serve as carriers for GNSs and thus minimise the high surface energy of GNSs in the core-shell structure, which benefits their uniform dispersion in the matrix powder [20]. Secondly, it has been demonstrated that core-shell nanostructures enable a good combination of the properties of constituent components [21,22]. More importantly, multi-layered nanoparticles (e.g. fullerene-like nanoparticles) have been proved to be a kind of excellent lubricant additives [23,24] due to the self-lubricating shell. In this study, SiC-GNSs core-shell nanoparticles are incorporated into alumina composites as a novel type of nanofillers. The core-shell nanofillers show a good potential to boost the wear-resistant and self-lubricating performance of the alumina composites. The contributing mechanisms are investigated as well.

Section snippets

Fabrication of alumina composite materials

SiC-GNSs nanoparticles (as shown in Fig. 1a) were produced using a wet ball milling process, which was detailed in our previous study [25]. Briefly, graphite flakes were wet ball milled with SiC nanoparticles and exfoliated into thin GNSs. The freshly exfoliated GNSs are unstable and tend to attach and scroll on the SiC nanoparticle with the help of shearing movement in the ball milling. The produced SiC-GNSs powder was then mixed with 300 nm sized α-Al2O3 powder (as shown in Fig. 1b) to

Microstructure and mechanical properties

Table 1 summarises the measured density, electrical conductivity, mean grain size, Vickers hardness and fracture toughness of the produced samples. It is shown that all produced samples are nearly fully densified and the relative densities reach more than 99%. However, there are still pores between the grain boundaries of alumina as shown in Fig. 2. The dispersion of the nanofiller is critical for the properties of the composites. It is noted that the incorporation of the core-shell nanofillers

Conclusions

SiC-GNSs core-shell nanoparticles were incorporated into the alumina matrix to fabricate hybrid ceramic matrix composites. Both the microstructure and wear behaviour of the ceramics were investigated and the following conclusions can be drawn in the present study.

  • 1)

    The SiC-GNSs nanofillers have been well dispersed into the ceramic matrix and the electrical conductivity is significantly improved.

  • 2)

    The incorporation of SiC or SiC-GNSs nanoparticles refines the grain size of the alumina composites. A

Conflicts of interest

None.

Acknowledgements

The authors acknowledge use of the facilities at the UOW Electron Microscopy Centre. Thanks for the editoral suggestion from JECS for reexamination and refining of the experimental design and results. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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