Surface engineering of wrought and additive layer manufactured Ti-6Al-4V alloy for enhanced load bearing and bio-tribocorrosion applications

The beneficial effect of surface engineering on the wear and corrosion performance of Ti-6Al-4V alloy for biomedical purposes has recently gained a lot of interest. To date, researchers have shown TiN ceramic coatings to be an effective strategy to improve the poor tribocorrosion properties of Ti-based alloys. However, coating degradation and adhesions remains a major hurdle to overcome for successful clinical translation. Recently, a duplex TPON + TiN treatment process on Ti-alloy has been suggested for applications involving with high contact loads. For the first time, this technique was extended to the Additive Layer Manufactured (ALM) Ti-6Al4V alloys in an attempt to enable load bearing patient personalised implants. The bio-tribology and corrosion resistance of the coated ALM materials were compared with that of the coatings on conventional wrought manufactured alloy for orthopaedic applications. XRD analysis showed that the coatings on both substrates are primarily composed of TiN. The Knoop microhardness technique proved a tribologically effective diffusion layer with a case depth of 35-45 mu m. The LC2 and LC3 values were measured above 40 N and 60 N which is an excellent cohesive and adhesive strength for these types of the coatings. Electrochemical measurements in both static and sliding conditions showed a quick recovery capability of the protective layer in 25% Foetal Bovine Serum (FBS) diluted in Phosphate Buffered Saline (PBS) electrolyte. The static electrochemical measurements also showed reduced corrosion current densities when compared to that of the bulk Ti-alloy. Coating on both substrates showed an excellent wear resistance which is correlated to the enhanced load bearing capacity of the coated surfaces. While the coating thickness was 3-6 mu m, the wear depth was only 0.3 mu m after 2 h of reciprocating sliding wear test.