Effects of nitriding temperature on microstructures and vacuum tribological properties of plasma-nitrided titanium

Facing the outer space application of titanium and its alloys in drilling pipes and drilling robots, it is necessary to find an optimized surface engineering method to improve their vacuum tribological properties. In order to find an optimized nitriding temperature, a series of plasma nitriding experiments were carried out for commercial titanium (UNS R50400/Gr. 2/TA2/CP3) at various temperatures ranging from 650 to 950°C for 8h to investigate the effects of nitriding temperature on microstructures and mechanical properties. Vacuum tribological properties of the nitrided and untreated samples were examined using a ball-on-disc vacuum tribo-meter. The results show that the wear resistance of titanium tested under vacuum condition is effectively enhanced, due to the formation of the hard nitrided layer. The surface roughness, thickness and hardness of the nitrided layer increase with the increase of nitriding temperature. Nevertheless, the load bearing capacities of the samples nitrided at 900 and 950°C are much lower than those of the samples nitrided at 800 and 850°C. Therefore, the wear volume keeps dropping at the nitriding temperature from 650 to 850°C, whereas, it converts to rise up when the nitriding temperature is above 850°C. In general, it is an optimized process to be nitrided at the temperature of 850°C for 8h to improve the vacuum tribological properties of titanium.