Research On The Properties And Mechanism Of Plasma Nitrided Medical 316L Stainless Steel And CoCrMo Alloy

Biomedical metallic materials have become the most important and most extensively used bearing implant materials in clinical application.However,the wear particles and corrosion products,as well as negative body reactions in long-term applications,may have significant effect on the normal exertion of biological properties after the biomedical metallic materials have been implanted in the human body.In order to improve the combining wear and corrosion resistance of metal surface,and to enhance the biocompatibility,biosafety and biofunctionability in physiologic environments,low temperature plasma nitrided technology has been widely used in the recent years.In the present study,according to the existing problems of the susceptibility to pitting corrosion,the low surface hardness and poor wear resistance in body fluids,medical forged CoCrMo alloy and 316L austenitic stainless steel have been respectively treated by DC and RF plasma nitrided process under pure nitrogen.The effects of plasma nitrided processes on the mechanical property,corrosion resistance and wetting property of biomedical metallic materials have been investigated.Based on experimental results,the influence of plasma nitrided process parameters on producing of active species and plasma nitrided mechanisms have been discussed,meanwhile,several models for explaining the nitrided mechanisms have also been reviewed.In this paper,the medical forged CoCrMo alloy has been treated by high-voltage DC plasma nitrided process to alter its surface characteristics and improve comprehensive properties.The significant effects of process parameters as well as ion beam flux density influenced by DC voltage,temperature,working pressure and deposition time on the nitriding behavior of CoCrMo alloy have been studied.The active species in the nitrogen plasma,ion beam flux density,phase compositions,microstructures,surface microhardness,and wettability are characterized by means of OES,milliammeter,XRD,SEM,microhardness tester and optical dynamic/static contact angle meter.A ball-on-disc tribotester is used to perform dry wear tests.The corrosion resistance is evaluated by electrochemical polarization measurement in Tyrode's solution with pH=7.The experimental analysis confirmed that plasma nitrided process is promising in producing thicker,harder,highly hydrophilic,more wear resistance and corrosion resistance layers on CoCrMo alloy for biomedical applications due to the formation of hard compounds of ?-CoCr,CrN and Cr2N.Compared with untreated CoCrMo alloy,all nitrided specimens show an important reduction in wear and corrosion rate,and the nitrided specimen at 200Pa-650V-8h-800? exhibits the best film quality.As a low temperature,low pressure nitriding approach,RF-CCP technology has been used to nitride medical 316L austenitic stainless steel.After the surface nitriding treatment,a nitrided layer ?N forms on the 316L stainless steel,the surface properties including microhardness and dry-sliding wear resistance can be obviously improved.The effects of plasma nitrided ion beam flux density and deposition time period on the characterisation of 316L stainless steel have been investigated.The results show that a single high nitrogen face-centered cubic(f.c.c)phase identified as ?N(111)forms on the 316L stainless steel after low temperature nitriding treatment,and there is no evidence show the precipitation of CrN phase.As a thermodynamic metastable phase,?N phase can not only improve the hardness and wear resistance without corrosion resistance reduction,but also influence the variation of contact angles with nitrided specimens at different process parameters.In this work,the nitrided 316L stainless steel specimen with the highest nitrogen concentration of about 37.3 at.%at 10Pa-500V-4h,has the lowest wear rate,the smallest corrosion current density and the lowest contact angle.Through the evaluation of the effects of plasma nitrided process parameters and ion beam flux density on the characterisation of biomedical metallic materials,it should be recognized that longer deposition time period,higher ion beam flux density and temperature can obviously alter the surface of biomedical metallic materials and enhance the nitriding efficiency.Moreover,the research results suggest that active species like excited neutral molecules N2*?atoms N*and excited molecule ions N2+ have important influence on the nitriding kinetics.In this work,nitrided layer with good comprehensive properties have been deposited by different plasma techniques.Combined with experimental and theoretical studies,it can be obtained that the research results play important roles in the development of the environmental-friendly and safety nitriding treatment,and provide necessary experimental support for the development of the low temperature plasma technology in the surface modification of biomedical metallic materials.