Metallic Structures and Tribological Properties of Ti-15mass%Nb Alloy After Gas Nitriding and Quenching Process

This study aimed to experimentally investigate the differences in metallic structures owing to the gas nitriding and quenching process (GNQP) temperature of the Ti-15mass%Nb alloy and differences in the tribological properties of the surface layer. The GNQP heating temperature was 1023 K or 1223 K, and the holding time was set to 1 h. In the X-ray diffraction profiles, the diffraction peak of the (101¯1) plane of the hexagonal close-packed phase exhibited a shift toward lower angles, following the sequence AN:α, AQ:α′, and GNQP:α-TiN0.3. In both the 1023 K and 1223 K GNQP specimens, the α″ phase exhibited lower values than the α′ phase; nonetheless, it still exhibited larger values than the annealed α phase. Based on transmission electron microscopy observations, the high core hardness of the 1223 K GNQP specimen was attributed to solid-solution strengthening caused by nitrogen diffusion or to strain hardening associated with the diffusion and was not attributed to the influence of precipitation phases, such as the ω phase. In the friction and wear tests, both the 1023 K and 1223 K GNQP specimens exhibited narrower wear track widths, clearly demonstrating that the GNQP enhanced the wear resistance. Moreover, the TiO2 layer was effective in maintaining a low coefficient of friction.