Surface Hardening of HVOF Sprayed Stainless Steel Coatings by Plasma Nitriding and Nitrocarburizing

This study compares the effects of plasma nitriding and nitrocarburizing treatments on HVOF sprayed stainless steel coatings with different crystal structure. The treatments were conducted at 550 °C for 10 h in a gas mixture of N₂ and H₂ for nitriding and N₂, H₂, and C₂H₂ for nitrocarburizing. Optical microscopy, SEM-EDS, and XRD show that the treatments produced thick nitride layers consisting of a compound layer and a nitrogen diffusion layer. The treatments increased not only the surface hardness, but also the load bearing capacity of the coatings due to the formation of CrN, Fe₃N, and Fe₄N phases. Plasma nitrocarburized 410 stainless steel had the highest microhardness and load bearing capacity because of the precipitation of Cr₂₃C₆ on the surface.