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

Abstract

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 TiO 2 (rutile) that had a hardness ∼1000 HV. Corrosion–wear tests were made in reciprocation sliding contact with an α-Al 2O 3 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 TiO 2 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 TiO 2 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 TiO 2-layer. For both oxidised titanium alloys the hardened oxygen diffusion zone (ODZ), formed beneath the TiO 2 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.