| Owing to their excellent resistance to corrosion and environmental degradation in a wide variety of media,stainless steel are the materials of choice for a diverse range of applications,from critical components in nuclear reactors,components for the chemical and petrochemical processing,through to biomedical prostheses in human bodies,to the equipment for food processing,marine engineering and kitchen ware.However,austenitic stainless steels are characterised by low hardness and poor wear properties,which have impeded their wider tribological and tribochemical applications.Ion nitriding has been proved to be an effective means to enhance the surface hardness and tribological properties of austenitic stainless steel.In order to avoid precipitation of Cr nitrides and the depletion of Cr in the matrix,nitriding temperature is generally set at below 450℃.And the nitriding efficiency at this temperature is very low,it is difficult to meet the needs of practical application.Low pressure arc plasma nitriding(LPA-PN)is a new kind of ion nitriding technology,which uses gas arc discharge plasma,under high vacuum condition(as low as 0.4-4 Pa).This ion nitriding technology featured as high plasma density and high nitriding efficiency,but it is lack of systematic and deep research on the structure and performance of the nitrided layers on austenitic stainless steel surface.In this paper,we have nitrided the austenitic stainless steel to enhance their surface hardness via LPA-PN technology,and systematically studied their microstructure,tribological and corrosion properties,which will provide theoretical support for the application of this technology in the nitriding treatment of austenitic stainless steel.The effects of the nitriding parameters on the phase structure,nitriding kinetics and microhardness of the nitrided layer were studied.The results show that LPA-PN technology can obtain two kinds of nitrided layer,one is the nitrides phase layer formed at the temperature exceeded 430℃ and their components are iron nitride and a small amount of chromium nitride;Another is expanded austenite phase(γN)formed below 430 ℃ with a high N content of 20 at.%.The microhardness of the γN layer can reach 1000 HV,which is more than 4 times of the hardness of substrate.And the nitriding effect is very high,the thickness of γN layer can reach 30 μm in 1h.The hardened γN layer has a double layer structure composed of a nanocrystalline outer layer and a coarse inner layer.The nitriding atmosphere(argon and nitrogen ratio)and arc current had no direct effect on the phase composition of nitrided layer.The wear resistance of nitrided layer was synthetical analyzed.The results show that the nitriding layer can greatly improve the wear resistance of austenitic stainless steel.The oxidation reaction occurred in the process of friction,and the wear mechanism of γN layer was abrasive wear and oxidation wear.The corrosion behavior of nitrided layer was studied by means of electrochemical and XPS analysis.Potentiodynamic polarization curves of γN layer in 3.5%NaCl solution underwent a typical transition course from spontaneous passivation to transpassive dissolution process without pitting corrosion.The Nuquist plots for EIS showed that the austenitic stainless steel had one typical capacitive arc,and γN layer had double capacitive arcs.And the dameter of the capactive arc for γN layer was bigger than the original austenitic stainless steel.The impedance value of γN layer was larger than that of stainless steel,and the γN layer had two phase platform on the Bode plots.The nanocrystalline sublayer of γN layer promotes the growth of the passive film and induces the dissolve in the NaCl solution,which is the key to the corrosion resistance of the modified austentic stainless steel. |
