It was recalled that thermal or ionic nitridation could not normally be achieved at temperatures below 550C because of a marked decrease in N diffusivity. It was shown here that improved N transport could be obtained after nitridation in NH3 plasma without any cathodic bias of the samples. The present conditions permitted the Fe matrix to be nitrided, to depths ranging from 100 to 400μm, at temperatures as low as 350C. When using this process, no growth of compound layers was observed at the surface. The resultant absence of diffusion barriers was clearly beneficial to the improvement in N transport. The high nitridation efficiency at low temperatures was attributed to enhanced grain boundary diffusion, due to defects which were generated by hydrogenous radicals in the plasma.

Improved Nitrogen Transport in Fe-C Alloys during NH3 Plasma Nitridation. L.Marot, L.Pichon, M.Drouet, A.Straboni: Materials Letters, 2000, 44[1], 35-8