Fretting wear evolution mechanisms of Ti6Al4V substrate and micro-arc oxidation-treated Ti6Al4V for femoral stems of modular artificial hip joints

Fretting wear of Ti6Al4V femoral stems is a major contributor to hypersensitivity, implant loosening, and even failure. Micro-arc oxidation (MAO) has been demonstrated to improve the short-term fretting wear resistance of Ti6Al4V. However, the wear-evolution characteristics of MAO-treated Ti6Al4V remain unclear. Therefore, this study conducted fretting wear experiments under the service conditions of artificial hip joints. The fretting tribological behaviors of the Ti6Al4V substrate and MAO-treated Ti6Al4V were systematically analyzed across different numbers of fretting cycles, and their fretting wear evolution mechanisms were elucidated. The results indicated that, in partial slip regime (PSR), the fretting wear evolution mechanisms of the Ti6Al4V substrate and MAO-treated Ti6Al4V were characterized by “micro-cutting → abrasive/oxidative wear” and “compression of pores,” respectively. In mixed fretting regime (MFR), they were characterized by “abrasive/oxidative wear → fatigue/oxidative wear” and “pore collapse → abrasive wear → fatigue wear,” respectively. In gross slip regime (GSR), the corresponding mechanisms were characterized by “progressively intensified fatigue/oxidative wear” and “abrasive wear → third-body wear → fatigue/oxidative wear,” respectively. Notably, the Ti6Al4V substrate developed significant cracking and delamination even during short-term fretting. By contrast, owing to increased surface hardness and the formation of third-body debris layers, the MAO coating effectively suppressed the premature onset of fatigue damage in Ti6Al4V, markedly reduced the friction coefficient and wear rate, and enhanced the fretting wear resistance of the substrate. Across different fretting regimes and cycle numbers, MAO treatment enhanced the resistance to fretting wear of substrate by approximately 2.2–4.3 times.