Thermal oxidation of a porous Ti[sbnd]23Nb alloy for wear related biomedical applications: Effect of oxidation duration

In this study, an attempt has been made for tailoring the surface features of an 18 vol% porosity containing Ti[sbnd]23Nb alloy manufactured via powder metallurgy method. The aim was enhancing the wear resistance without sacrificing the mechanical properties. For this purpose, porous Ti[sbnd]23Nb alloys having hardness of 280 HV_0.025 were subjected to thermal oxidation (TO) at 600 °C in air for two different holding times. TO duration of 6 h, which introduced a ~ 0.5 μm thick TiO₂-rutile type exterior oxide layer (OL), provided surface hardness of 620 HV_0.025. Extension of TO duration to 60 h caused covering of the surfaces with a ~ 3.0 μm thick OL and increase of surface hardness to 980 HV_0.025. As the result of the increased surface hardness, TO'ed. alloys showed superior sliding wear resistance coupled with a reduction in friction coefficient (COF) against alumina ball in 1.5× simulated body fluid (SBF) when compared to the as-sintered state. Despite having the highest surface hardness, the alloy TO'ed. for 60 h exhibited slightly higher wear loss and COF than the alloy TO'ed. for 6 h. More important than this, the alloy TO'ed. for 60 h collapsed at a compressive stress well below the yield strength of the alloy TO'ed. for 6 h, which exhibited almost similar compressive stress-strain curve with that of the as-sintered alloy. From this perspective 6 h appeared as a promising TO duration for the examined porous alloy to be used in biomedical applications as it preserves the mechanical properties while providing remarkable enhancement in the wear resistance.