The corrosion–wear behaviour of thermally oxidised CP-Ti and Ti–6Al–4V

The use of commercial purity titanium (CP-Ti) and Ti–6Al–4V alloys in bio-medical implant applications has been limited by their poor resistance to surface degradation processes. In this paper the corrosion–wear behaviour of untreated and thermally oxidised CP-Ti and Ti–6Al–4V have been compared. Oxidation of both alloys at 625 °C for 36 h resulted in the formation of an exterior layer of TiO2 (rutile) that had a hardness ∼1000 HV. Corrosion–wear tests were made in reciprocation sliding contact with an α-Al2O3 ball immersed in physiological saline (0.89% NaCl) at room temperature. The oxidation treatment retarded the corrosion–wear of both CP-Ti and Ti–6Al–4V. For the untreated alloys, surface damage was dominated by micro-asperity shearing which resulted in rapid wear. Corrosion–wear of the oxidised materials was slower but more complex. The exterior TiO2 layer formed on the oxidised Ti–6Al–4V alloy provided little protection, it was rapidly removed during the first 60 min of testing, by a process involving interfacial fracture. Conversely, the TiO2 layer, albeit thinner, provided protection for the oxidised CP-Ti. Here, the layer becomes smoothly worn by a process that is proposed to be caused by the mechanical dissociation of the TiO2-layer. For both oxidised titanium alloys the hardened oxygen diffusion zone (ODZ), formed beneath the TiO2 layer, provided good protection from corrosion–wear. In both cases the ODZ was smoothly worn by a combination of abrasion and corrosion–wear processes. The latter process, termed Type I corrosion–wear, involves the repetitive mechanical degradation of the passive film that forms through aqueous corrosion. However, this is a relatively slow process.