Study On The Effect Of Lanthanum Iron Oxide On Gas Nitriding For 42CrMo Steel And The Catalytic Mechanism

As one of the most widely applied materials in the modern society,42CrMo steel is vulnerable to corrosion and wear.As a result,its service life will strongly decrease.Gas nitriding is a technology improving the corrosion and wear resistance properties for steels.However,the nitriding efficiency is so low that the production cost will increase.Therefore,more and more researches focus on improving the nitriding efficiency.Lanthanum iron oxide（LaFeO₃）is a type of the rare earth perovskike oxide and owns a fiercely potential application in the catalytic field.In this paper,42CrMo steel is used as the substrate.The LaFeO₃ film is pre-fabricated on the substrate surface by sol-gel method.And it is the first time to introduce the LaFeO₃into gas nitriding.In addition,we also study on the effects of various parameters on structure as well as property for nitrided layers,such as the thickness of the thin films,nitriding temperatures and mixed gas atmosphere.The structure and morphology on the outermost surface and in the cross section can be observed by optical microscope（OM）and scanning electron microscope（SEM）.The phase and chemistry components can be characterized by X-ray diffractometer（XRD）and energy dispersive spectrometer（EDS）,respectively.The micro-hardness tester is used for measuring the micro-mechanical property in the nitrided layer and the thickness of effective hardened layer.The corrosion and wear resistance properties can be tested through the electrochemical station and pin-on-desk tribometer,respectively.In order to determine the catalytic mechanism for LaFeO₃ during the gas nitriding,chemistry and bonding state on the surface can be characterized by X-ray photoelectron spectroscopy（XPS）and the micro-section can be observed by transmission electron microscopy（TEM）.Results show that the pre-fabrication of a LaFeO₃ film on the surface can effectively promote the enhancement of the compound layer and effective hardened layer.The LaFeO₃film specimen deposited by atomization method has the thickest compound layer and effective layer with 15.29μm and 305.8μm respectively,after nitrided at 550℃for 4h.In addition,the increase of surface nitrogen content also significantly improve the surface hardness,and the maximum value is 910.5HV_0.1.in the electrochemical corrosion test,the self-corrosion potential(E_corr)and self-corrosion(I_corr)current density are-0.03V and 1.8×10^-6A/cm2 respectively.And the breakdown potential(E_pit)can be obviously increased to+1.79V.In the friction and wear test,The wear volume is also apparently reduced to 0.75×10^-2mm².Therefore,the LaFeO₃ film sample deposited by atomization method owns the best corrosion and wear resistance nitrided at 550°C for 4 h.Compatative studies have shown that LaFeO₃can apparently catalyze on the surface under the low nitrogen potential.However,the catalytic effect is not obvious under the high nitrogen potential and it even suppresses the nitriding process.Generally,the catalytic mechanism can be attributed to as follows.Firstly,LaFeO₃ is able to promote ammonia dehydrogenation reaction and increase the concentration of active nitrogen atoms on the surface during the nitriding.Besides,the position of the oxygen vacancy defect can also become the diffusion of active nitrogen atoms,resulting in improving the nitriding efficiency.Therefore,this paper verifies the feasibility of using LaFeO₃ as a catalyst to improve gas nitriding efficiency.