Kinetics and enhancement mechanism of plasma oxynitriding for AISI 1045 steel

Plasma oxynitriding (PON) with air addition of 0.2L/min was carried out at temperatures ranging from 783K to 843K and holding time ranging from 30min to 240min in the plasma nitriding furnace for AISI 1045 steel, and plasma nitriding (PN) without air addition was also conducted for comparison. The microstructure, surface morphology and phase compositions of compound layers were investigated by means of optical microscopy, SEM and X-ray diffraction, and then the kinetics for forming compound layer was analyzed. The results show that air addition can promote the diffusion of active nitrogen atoms toward the substrate, thus enhance the nitriding efficiency. In other words, plasma oxynitriding can shorten the holding time and decrease the treating temperature to get the expected compound layer thickness comparing with normal plasma nitriding (PN) due to a loose oxide layer formed on the outmost surface. Kinetics analysis found that the diffusion coefficient of nitrogen in PON is much higher, and the activation energy for nitrogen atoms diffusion is reduced from 227.25kJ/mol to 138.57kJ/mol in PON. Kinetics of PON with air addition of 0.2L/min at temperatures ranging from 783K to 843K was obtained as the following formula: d=4.16×10−3exp−1666.67/Tt.