Ultrathin graphene tribofilm formation during wear of Al2O3–graphene composites

Abstract

Graphene nanoplatelet (GNP)-reinforced aluminum oxide (Al2O3) composites were sintered by spark plasma sintering in three different compositions (0, 0·5, 5 vol.% GNPs). To investigate the effects of graphene addition on the composites’ wear resistance, ball-on-disk wear tests were conducted under very high normal load (40 N) by using a 3-mm-dia. ceramic counterface. Aluminum oxide–0·5 vol.% GNP exhibited 65% improvement in the wear resistance, while aluminum oxide–5 vol.% GNP displayed 53% poorer wear resistance as compared with aluminum oxide. The coefficient of friction was 0·45 for aluminum oxide–0·5 vol.% GNP, 0·40 for aluminum oxide–5 vol.% GNP and 0·60 for aluminum oxide. The highest wear resistance of aluminum oxide–0·5 vol.% GNP is attributed to formation of a continuous, protective and ultrathin graphene tribofilm on the wear surface. Tribofilm formation occurs due to the high shear forces induced by countersurface movement and localized heating, which causes GNP’s delamination, overlap and welding together. In the case of aluminum oxide–5 vol.% GNP, poor dispersion and agglomeration of GNP results in a thick and discontinuous graphene tribofilm, which does not protect from the brittle fracture of aluminum oxide grains during wear.