Enhanced surface performance of Ti-6Al-4V alloy using a novel duplex process combining PVD-Al coating and triode plasma oxidation

Abstract

Despite the well-known advantageous properties of the titanium alloy Ti-6Al-4V, in terms of (for example) its strength-to-weight ratio and corrosion resistance, its inadequate tribological performance still inhibits the range of applications in which it can potentially be used. The use of surface engineering treatments to enhance the tribological behaviour can extend that range, but industry (especially the aerospace sector) is still seeking further improvements in the load-bearing and tribological performance of titanium alloy surfaces. In this paper, a novel duplex approach to further improve the tribological behaviour of titanium alloy is presented, which involves the physical vapour deposition (PVD) of aluminium onto the surface of Ti-6Al-4V and subsequent triode plasma oxidation (TPO). The aim of this work was to establish the effect on the surface layer structure, composition, hardness and sliding wear performance of varying the TPO treatment temperature, duration and substrate negative electrical bias – and to determine whether electron-beam plasma-assisted physical vapour deposition (EB-PAPVD), followed by TPO, could be a suitable low temperature duplex treatment to improve the surface properties of Ti-6Al-4V. Films oxidised at a temperature of 600°C (and substrate negative bias of up to −800V) did indeed exhibit a distinct improvement in performance compared to the untreated alloy. The hardness of the aluminium-coated and oxidised Ti alloy increased to a maximum of ~11GPa, compared to around 4.4GPa for the untreated material. Film adhesion was found to be satisfactory with nano-scratch test loads reaching 300mN prior to failure and ball-on-plate reciprocating-sliding wear tests indicated an improvement in tribological performance, with a reduction in measured wear rate of around three orders of magnitude. This improvement was achieved due to the more precise control of surface compound layer growth provided by this new processing route, thereby enhancing both chemical and mechanical properties of the surface.