Effects of pre-heat conditions on diffusion hardening of pure titanium by vacuum rapid nitriding

The pre-heat treatment in vacuum rapid nitriding process was introduced to improve the surface hardening characteristics such as thick depth, high hardness and particularly to reduce the processing time. The pre-heat treatment was maintained for 30 min with an interval of 10 °C at the temperatures of 700- 800 °C under a high vacuum to allow the absorption of oxygen atoms on the titanium surface into the inner side. The following nitriding processes were equally performed at the same condition as 800 °C for 1 h. As a result, the depth of a hardened layer in the sample, which was treated at 800 °C for 1 h with the pre-heat treatment at 760 °C for 30 min, was similar to that of the treated sample at 800 °C for 3 h without pre-heat treatment and the depth was about 40 μm. The processing time can be largely reduced by introducing the pre-heat treatment process. The pre-heated specimens showed a longer wear lifetime by more than three times and higher friction resistance than that of PVD or non-nitrided specimens. The surface gradient-hardened layers exhibited excellent wear resistance and could prevent separation or debonding between matrix and the coated layer effectively.