Plasma-radical nitriding was performed to harden the surface of SKD 61 steel for 1-10 hours at a temperature range of 450-550 degrees C. This process involved the use of NH3 gas instead of N-2 gas which is employed for the well-established plasma-nitriding method. An NH radical, which played a key role to produce a nitrogen diffusion layer without the formation of the brittle compound layer, was generated in a gas mixture of NH3 and H-2. One of the main advantages of the plasma-radical nitriding is to improve the surface hardness by maintaining the roughness of the initial polished surface. The microstructures and material properties of the radical-nitrided layer were characterized in order to investigate the effects of various radical-nitriding processing parameters. In addtion, radical nitriding produces better surface roughness, compared with conventional ion nitriding.

Keywords: plasma radical nitriding; diffusion layer; compound layer