Modification of Ion Nitriding of Ti-6Al-4V for Simultaneous Improvement of Wear and Fatigue Properties

Extensive application of titanium alloys in the aerospace sector is restricted due to their poor tribological properties especially when contact of surfaces is inevitable. Over the past few decades, many coating deposition and thermochemical treatments have been developed to improve the wear resistance of titanium alloys such as plasma (ion-) nitriding. A typical ion nitrided microstructure in titanium alloys consists of a thin surface layer composed of TiN and Ti2N titanium nitrides (compound layer), a region of nitrogen-stabilized α-titanium (α-case), and a nitrogen diffusion zone. However, similar to other nitriding processes, the ion nitriding treatment involves high temperatures (750-1100°C) and results in brittle surface features and substrate microstructural changes that lead to deterioration of fatigue strength. In this research, a modified plasma nitriding treatment was developed to achieve simultaneous improvements of wear resistance and fatigue strength by optimization of the microstructure. The findings revealed that utilizing a low temperature of 600°C during the nitriding treatment inhibited the formation of brittle surface features (α-case) and bulk microstructural changes and increased the resistance of ion nitrided surfaces to surface crack initiation and propagation. The nitrided alloys exhibited a higher fatigue strength compared with those reported in the literature. Furthermore, at least 48% reduction in the coefficient of friction compared to the untreated alloy and considerable improvements in wear resistance were obtained by the formation of a thin (\u3c 2 µm) compound layer on the surface supported by a 40 µm deep nitrogen diffusion zone. It was found that further improvements in wear resistance can be achieved by alteration of the surface microstructure prior to the plasma nitriding treatment. As such, a novel pretreatment step was developed to increase the nitriding kinetics at low temperatures via introducing a severe plastic deformation (SPD) surface layer and subsurface microstructural defects, such as twins, in the near-surface region. This pretreatment step resulted in a 50% deeper diffusion zone and the formation of a nanocrystalline TiN surface layer after plasma nitriding. Overall, the research performed in this dissertation provides a new insight into the microstructural evolution during plasma nitriding of titanium alloys