Improved nitridation efficiency and mechanical property of stainless steel surface after N2-H2 plasma nitridation at low temperature

In the thermochemical process of stainless steel nitridation, the improvement of mechanical properties is governed by the way the nitrogen diffusion profiles extend into the material. The efficiency of conventional thermal or ionic nitridation is substantially reduced at temperatures lower than 550 °C because of low nitrogen diffusivity. The present study shows that improved nitrogen transport can be obtained after nitridation in N 2 -H 2 plasma at floating potential, that is without cathodic bias on the samples. Such cold conditions allow for the iron matrix to be nitrided in a depth range of 10 μm at a temperature as low as 430 °C. Under these conditions, the top surface hardness was increased by a factor of three. Using the nanoindentation technique on cross-sections prepared on plasma nitrided samples, we could determine the profiles of hardness and Young modulus. Unexpectedly, the Young modulus was shown to be almost unaffected by the treatment. Furthermore, hardness profiles correlated very well with the nitrogen chemical profile and the results showed that the interface between the treated layer and the untreated part of the sample was very abrupt. The high level of surface strengthening and the increased nitridation efficiency at low temperature are thought to be a consequence of an activation of surface transport and diffusion using plasma (cold) conditions at floating potential.