Microstructure, mechanical properties and fretting corrosion behavior of micro-arc oxide coatings on biomedical Ti6Al4V alloy for modular hip implant: Effect of Na₂SiO₃/KOH electrolyte concentration

In this study, micro-arc oxidation (MAO) coatings were prepared on biomedical Ti6Al4V alloy using electrolytes with varying concentrations of KOH and Na₂SiO₃. Subsequently, based on the actual service conditions of the artificial hip joint head-neck interface, fretting corrosion experiments were performed. Effects of Na₂SiO₃/KOH concentrations on growth characteristics, microstructure, mechanical properties, and fretting corrosion behavior of the coatings were systematically investigated. The results indicated that KOH predominantly modulated the microstructure and properties of coatings by altering discharge behavior, whereas Na₂SiO₃ exerted its influence primarily through facilitating SiO₂ formation. Elevated KOH concentrations decreased the breakdown voltage and promoted coating growth, but also increased porosity and surface roughness, leading to diminished mechanical properties. Moderate increases in Na₂SiO₃ concentration enhanced coating density and mechanical properties. However, excessive Na₂SiO₃ resulted in microcracking and irregular porosity, which degraded the performance of coatings. Following fretting corrosion, abrasive wear was identified as the principal damage mechanism for all coatings. Fatigue wear also contributed to the fretting damage mechanism at high KOH concentrations. Furthermore, excessive Na₂SiO₃ concentrations shifted dominant damage mechanisms to severe fatigue and three-body wear. Appropriately increasing Na₂SiO₃ improved both the wear and corrosion resistance of coatings. Notably, when the Na₂SiO₃/KOH concentration ratio was 2:1 (4 g/L: 2 g/L), the resulting coatings exhibited optimal fretting corrosion resistance, enhancing the fretting corrosion resistance of the Ti6Al4V alloy by approximately 4.5-fold. The results provide a reference for the surface protection and MAO coating design of biomedical titanium alloys.