Oxidation of Ti6Al4V and TiZrNbHfTa and its influence on tribological properties

Because of its ductility, low Young’s modulus, high compressive yield strength, good fatigue behavior, and biocompatibility, TiZrNbHfTa is in discussion as implant material in comparison to Ti6Al4V. To improve the wear resistance, as required for use in artificial joints, we deploy single-step and two-step thermal oxidation as a surface hardening method. Single-step thermal oxidation at 600 °C of cold-rolled, single-phase bcc, nanocrystalline TiZrNbHfTa leads to the formation of an adherent, μm-sized, vitreous oxide layer. Underneath this oxide layer, EPMA, XRD, XPS, and NRA-analysis indicate, that the bcc TiZrNbHfTa completely decomposes into another bcc and a hcp-phase during thermal oxidation. Selective internal oxidation of hafnium and zirconium occurs upon oxygen inward diffusion, raising the surface hardness by four times to 1522±64 HV 0.5. However, equal thermal oxidation treatment of coarser grained TiZrNbHfTa leads to catastrophic oxidation. The successful achievement of an adherent oxide layer with internal oxides in an oxygen diffusion zone underneath in case of nanocrystalline TiZrNbHfTa allows to add an additional heat treatment step at 1200 °C under vacuum in order to reduce the internal oxides formed during the single-step process. According to SEM, EDS, XRD, XPS, and APT-analysis, μm-sized oxygen-rich hcp precipitates in a bcc matrix form in the subsurface region. With this precipitates in the subsurface region, a gradual Martens-microhardness decrease from 5 GPa near the surface to 4.2 GPa in the substrate can be achieved. In contrast to the single-step process, the substrate itself is again single-phase bcc due to the heat treatment at 1200 °C.