Surface modification of titanium alloy orthopaedic implants via novel powder immersion reaction assisted coating nitriding method

There is increasing interest in using surface modification technology to improve the wear properties of titanium alloys used in total joint replacements. In the present work, a simple and original Power Immersed Reaction Assisted Coating (PIRAC) nitriding method suitable for surface modification of large complex shape orthopaedic implants has been developed. CP Ti and Ti-6Al-4V alloy samples were annealed at 850–1100°C in sealed stainless steel containers that allow selective diffusion of nitrogen atoms from the atmosphere. The relationship between the microstructure, nitrogen concentration and microhardness was studied employing X-ray diffraction (XRD), scanning electron microscopy (SEM)/electron probe X-ray microanalysis (EPMA) and high resolution SEM (HRSEM). PIRAC nitrided surfaces were found to have a layered structure with a TiN/Ti2N coating followed by nitrogen-stabilized α-Ti. In contrast to previous investigations of surface nitrided Ti-6Al-4V alloy, a Ti3Al intermetallic phase was detected at the Ti2N/α–Ti interface acting as a barrier for nitrogen diffusion. Importantly for biomedical applications, no toxic Al or V were detected in the surface layer of PIRAC nitrided Ti-6Al-4V alloy.