Kinetics and enhancement mechanism of plasma oxynitriding for AISI 1045 steel

Plasma oxynitriding (PON) with air addition of 02 L/min was carried out at temperatures ranging from 783 K to 843 K and holding time ranging from 30 min to 240 min 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 227.25 kJ/mol to 138.57 kJ/mol in PON. Kinetics of PON with air addition of 0.2 L/min at temperatures ranging from 783 K to 843 K was obtained as the following formula: d = 4.16 x 10(-3) root exp(-1666.67/T)t. Finally, kinetics verification results confirmed that the obtained kinetics is in good agreement with the tested results; therefore, it can be used in real applications to predict the compound layer thickness for given process parameters of temperature and time; or to determine the parameters of temperature and time for a designed compound layer thickness. (C) 2016 Elsevier B.V. All rights reserved.