The influence of surface microcracks and temperature gradients on the rf plasma nitriding rate

Rf plasma nitriding processes have been used recently for surface treatment. 304 Austenitic stainless steel has been nitrided using an inductively coupled rf plasma. High rates of nitriding and relatively high microhardness have been achieved. The effect of plasma power and plasma processing time on the rate of nitriding and the microstructure was investigated. A high nitriding rate of 3 μm²/s and a microhardness value of 1800 HV0.1 were achieved under optimum plasma conditions. The interpretation is based on a new approach that the microcracks in the surface, at a certain ambient temperature and plasma, can create traps for the active nitrogen species. The nitrogen concentration and temperature gradient can also draw some of the nitrogen species towards the bulk side. The microhardness can be explained in terms of the high concentration of nitrided phases created under the surface of the treated samples.