Study On The Catalytic Nitriding Mechanism Of Rare Earth Perovskite Oxides

Nitriding is a chemical heat treatment process in which nitrogen atoms penetrate the surface of the workpiece under specific conditions,and mainly used to improve the surface hardness,wear resistance,corrosion resistance and extend the lifetime of the workpiece.The traditional nitriding process has the disadvantages of slow nitriding speed and low efficiency.In order to improve the efficiency of nitriding,various catalytic nitriding technologies have been continuously developed and applied,all of which have achieved good results.Relatively speaking,the rare earth catalytic nitriding technology not only has obvious effect of nitriding,but also can refine the surface microstructure and significantly improve the surface performance,its research has been widely concerned.The catalytic performance of perovskite-type rare earth oxide LaFeO₃ in gas nitriding has been confirmed.However,its catalytic efficiency is sensitive to nitriding parameters.In order to clarify the catalytic conditions of LaFeO₃,the low-pressure gas nitriding was introduced in this paper.The effects of air pressure and ammonia flow rate on the catalytic performance of LaFeO₃ are discussed,and the catalytic characteristics and conditions of LaFeO₃ are determined.Rare earth Perovskite oxides are often used as catalysts because of their special structure.Doping and substitution of the ions in perovskite structure will change their catalytic performance.Therefore,the influence of ions doping and substitution in perovskite structure on the nitridating catalytic performance of LaFeO₃ perovskite oxide was also studied.The microstructure and phase composition of nitrided layer were analyzed by OM,TEM,SEM and XRD.The effective thickness of the hardened layer was measured and characterized by microhardness tester to evaluate the catalytic efficiency of rare earth Perovskite oxides.The X-ray photoelectron spectroscopy was used to identify the catalytic mechanism of the LaFeO₃by analyzing the chemical state and elemental distribution on the nitrided surface.The results show that LaFeO₃presents an excellent catalytic activity during low pressure gas nitriding,which can improve surface hardness,the depth of effective hardening layer and the formation of iron nitrogen phase.The catalytic efficiency of LaFeO₃ increases with the decrease of pressure and ammonia flow rate.According to the nitriding parameter range selected in this experiment:0.01?0.1MPa and 0.1?1L/min NH₃ flow rate,LaFeO₃presents the best nitriding efficiency under the conditions of 0.01MPa pressure and 0.1L/min ammonia flow,while the effect of pressure is more significant than that of ammonia flow rate.The research results also show that the doping of Ce,Cr,Ni ions improves the nitriding catalytic performance of LaFeO₃,Ion doping and substitution enhance the catalytic activity of nitriding by changing the chemical state of La atom and increasing the oxygen vacancy concentration.The LaFeO₃ oxide performed the catalytic activity on the nitrided surface by accelerating the dissociation rate of ammonia and the diffusion rate of active nitrogen atoms.The gas pressure and ammonia gas flow mainly affect the catalysis of LaFeO₃ by changing the dehydrogenation reaction rate.Therefore,LaFeO₃ catalyst shows more excellent catalytic activity under low pressure and low flow.At the same time,the nitriding surface can be kept rough due to the low pressure and ammonia flow,which is conducive to the adsorption of ammonia molecules and the increase of nitrogen potential.The catalytic activity of LaFeO₃can be sustained under low pressure and low ammonia gas flow.