Improved nitrogen transport in Fe-C alloys during NH3 plasma nitridation

In the thermochemical process of Fe-C alloy nitridation, improvements of me chanical properties are governed by the way the nitrogen diffusion profiles extend into the material. Up to now, thermal or ionic nitridation cannot b e achieved at temperature lower than 550 degrees C because of the strong lo wering of nitrogen diffusivity. The present study shows that improved nitro gen transport can be obtained after nitridation in NH3 plasma without catho dic bias on the samples. Such cold conditions allow the iron matrix to be n itrided in a depth range of 100-400 mu m at a temperature as low as 350 deg rees C. The top surface hardness was shown to be improved by a factor of 3. This surface strengthening, markedly superior to that obtained with the cl assical treatments, is a well-known consequence of the temperature lowering that avoids coarsening of nitride microprecipitates. In addition, using th is particular process, no growth of compound layer was observed at the surf ace. This absence of diffusion barrier is clearly beneficial to the improve ment of the nitrogen transport. The high nitridation efficiency obtained at low temperature may be explained by an enhanced grain boundary diffusion d ue to defects generated by hydrogenous radicals produced in the plasma. (C) 2000 Elsevier Science B.V. All rights reserved.