Effects of graphene and α-Al₂O₃ nano-additives on the fretting wear behavior of MAO coatings on Ti6Al4V alloy: a comparison of self-lubrication and hardness-enhancement strategies

Fretting wear of Ti6Al4V alloy for femoral stems in artificial hip joints is a major contributor to prosthesis loosening and subsequent revision surgeries. Micro-arc oxidation (MAO) emerges as a feasible and highly promising surface modification technique to mitigate this problem. In recent years, two key optimization strategies have been proposed to enhance wear resistance of MAO coatings: incorporating self-lubricating and high-hardness nanomaterials. However, existing studies have focused on sliding wear, while the effects of such nano-additives on fretting wear behavior of MAO coatings remain unclear. In this study, self-lubricating graphene (G) and high-hardness alumina (Al₂O₃) nanomaterials were incorporated into MAO coatings on Ti6Al4V alloy. The fretting wear behaviors of conventional, G-reinforced, and Al₂O₃-reinforced coatings under various fretting regimes were systematically investigated, and optimization mechanisms were elucidated. The results revealed that both graphene and alumina significantly enhanced coatings’ fretting wear resistance. The optimization mechanism of graphene was primarily manifested in promoting lubrication. During debris generation, pore compression, and coating damage, graphene progressively transferred and dispersed throughout the wear region, forming a smooth and uniform lubricating film on the coating surface. In contrast, alumina markedly improved coating hardness by bearing load, reinforcing bonding, and filling pores, thereby enhancing wear resistance. Notably, under the mixed fretting regime (MFR) and gross slip regime (GSR), G-reinforced coatings exhibited the lowest friction coefficient due to excellent lubricating ability. Nevertheless, across all fretting regimes, Al₂O₃-reinforced coatings exhibited the lowest wear rate due to superior fatigue resistance. Overall, alumina demonstrated more pronounced improvement and greater potential than graphene.