Response of a molybdenum alloy to plasma nitriding

A molybdenum alloy (TZM: 0.50 wt% titanium, 0.08 wt% zirconium and 0.02 wt% carbon with balanced molybdenum) was used to investigate its response to plasma nitriding in terms of layer formation and hardening at temperatures between 500 and 760 °C using various times. A thin hard nitride case plus a shallow diffusion zone was formed after plasma nitriding. The thickness of nitride layer increased with the treatment temperature and time. The nitrided surface of molybdenum alloy had a low coefficient of friction against the Al2O3 counterpart ball at ambient atmosphere and elevated temperatures, and the wear resistance of the surface was greatly improved. Molybdenum had a tendency to react with nitrogen to form a mixed nitride phases at the tested temperature range, and the TZM alloy had an activation energy of 330 kJ/mol.