Characterisation of the structure and the tribological behaviour of PN/PVD duplex processed Ti6Al4V

A duplex process including plasma nitriding (PN) and physical vapour deposition (PVD) of TiN was carried out for improvement of the tribological properties of Ti6Al4V alloy. Samples were characterised by optical microscope, field emission scanning electron microscope, atomic force microscope, energy dispersive spectroscopy, X-ray diffraction and microhardness. The sharp interface of PVD coating led to the quick failure of TiN, while the PN process distributed hardness profile gradually, but decreased surface quality. The duplex process optimised surface hardness, interface hardness gradient and coefficient of friction by four regions. The outer region (I) was -TiN, which was formed by PVD. Region II consisted of delta-TiN+epsilon-Ti2N', regions III was Al-enriched, and region IV showed a diffusion zone produced during PN. These structures led to high surface hardness, gradual hardness profile, reasonable surface quality, optimum friction behaviour and superior wear resistance. Overall, the duplex process improved the tribological properties of Ti6Al4V alloy significantly, as compared to the single stage PVD and the separate PN treatment.