Characterization and effect of pre-oxidation on D.C. plasma nitriding for AISI4140 steel

Pre-oxidation was carried out in plain air prior to plasma nitriding for AISI4140 steel. The pre-oxidized surfaces of samples were characterized using X-ray diffraction, scanning electron microscopy, optical microscopy and Vickers microhardness tester. The results showed that iron oxide was formed on the surface during pre-oxidation and the surface free energy (SFE) was significantly increased, and it was found that pre-oxidation at 300 °C and 30 min provided the surface with a uniformly distributed nano iron oxide particles and of the highest SFE, which had optimum enhancement effect on the followed plasma nitriding, and oxide was easier to be transformed to nitride. The thickness of compound layer was more than twice thicker than that without pre-oxidation. The enhancement mechanism was contributed to the formation of a uniformly nano iron oxide particles along with some nano cracks and pores on the surface during pre-oxidation.