Ultrathin graphene tribofilm formation during wear of Al2O3-graphene nanoplatelet composites

Abstract

Graphene Nanoplatelets (GNP) reinforced aluminum oxide (Al2O3) composites were sintered by spark plasma sintering (SPS) in threedifferent compositions (0, 0.5, 5 vol. %GNP). To investigate the effects of graphene addition on composites’ wear resistance, ball-on-disc wear tests are conductedunder very high normal load (40N)using a 3 mm diameter ceramic counterface.Al2O3-0.5vol. %GNP exhibited 65% improvement in thewear resistance while Al2O3-5 vol. %GNP displayed 53% poorer wear resistanceas compared to Al2O3. The coefficient of friction (COF) was 0.45 for Al2O3-0.5 vol.% GNP, 0.40for Al2O3-5 vol.% GNP and 0.60 for Al2O3. The highest wear resistance of Al2O3-0.5vol.% GNP is attributed toformation of a continuous, protective and ultrathin graphene tribofilm on the wear surface. Tribofilm formation occurs due to high shear forces induced by counter surface movement and localized heating, which causes GNP’s delamination, overlap and weld together. Ultrathin graphene film serves both as a solid lubricant and a protecting film which can provide toughness and containswear damage. In the case of Al2O3-5 vol.% GNP, poor dispersion and agglomeration of GNPresults in thick and discontinuous graphene tribofilm, which does not protect brittle fracture of Al2O3grainsduring wear.