Effect of different active screen hole sizes on the surface characteristic of plasma nitrided steel

Active screen plasma nitriding (ASPN) was performed on tempered 42CrMo4 low alloy steel samples. The effects of two technological parameters, namely 1) the hole size of the screen and 2) the open area ratio were investigated on the properties of the developed nitride layer. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and Vickers microhardness testing were used for the characterization of the surface. The thickness of the nitride layer, the microhardness and the length of the nitride diffusion zone and surface areal parameters like surface roughness, skewness, grain diameter and area were measured and correlated with the screen hole size and open area ratio. It was found that these two major technological parameters influence different aspects of the developed nitride layer. The layer thickness and surface skewness (connected to either a balanced surface with zero skewness or the appearance of hill-like complex structures with positive skewness) is more sensitive to the open area ratio, while the surface roughness is primarily a function of the hole size. The maximum surface hardness, the length of the nitride diffusion zone or the size (diameter and surface area) of the nitride grains did not show a strong correlation with either of these two parameters. Keywords: Active screen plasma nitriding, Hole size, Surface characterisation, AF