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 625degreesC for 3 6 h resulted in the formation of an exterior layer of TiO2 (rutile) that had a hardness similar to1000 HV. Corrosion-wear tests were made in reciprocation sliding contact with an alpha-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. (C) 2003 Elsevier B.V. All rights reserved.