On Processing Conditions And Properties For Plasma Source Nitriding AISI316Austenitic Stainless Steel

AISI316austenitic stainless steel was nitrided by plasma source nitriding technology. The relationships between the process parameters and composition, phase structure, wear and corrosion properties of the nitrided layer were investigated. A high nitrogen face-centered-cubic (γn) phase modified layer formed on AISI316austenitic stainless steel with the optimizing processiong conditions. The abrasive wear mechanism of friction strain induced martensite phase transformation and the transformation condition were discovered. A wear map of the γn phase was constructed to summarize the wear volumes and wear regime. The electrochemical corrosion properties and semiconductor characteristics of the γn phase layer were studied. The corrosion resistance mechanism of the passive film on the γn phase layer attributing to the two regions structure was revealed.Plasma source nitriding equipment the metal screen was supplied to a pulsed negative potential, the pulsed plasma formed in the plasma source which was constructed between the anode furnace wall and the cathod metal screen. The workpiece was biased to a DC negative potential or was placed in a floating potential. The processing parameters of plasma source nitrided AISI316austenitic stainless steel at various temperatures from350to500℃under a working pressure of300-500Pa and a sample bias of a floating potential or-200～-400V with the distance of20-200mm between the screen and the sample. The optimized processing parameters range of plasma source nitrided AISI316austenitic stainless steel for forming a single ynγn phase on the AISI316austenitic stainless steel surface using response surface methodology were obtained as a nitriding temperature of450℃, a working pressure of250-350Pa, a distance of20～100mm between the screen and the sample, a sample bias of-200～-300V. The thickness of the γn phase layer varied from15to18μm, the surface roughness (Ra) of the γn phase layer varied from0.111to0.158μm, the peak nitrogen concentration of the γnhase layer varied from20to25at.%and the microhardness of the γn phase layer varied from HV0.1N14.6GPa to HV0.1N15.6GPa.Plasma source nitriding on the AISI316austenitic stainless steel at temperature of450℃under a working pressure of300Pa with the distance of100mm between screen and sample for a nitriding time of6h produced a γn phase layer. Friction and wear behavior of the γn phase layer against an Si3N4ceramic ball was studied using a ball-on-disc tribometer under a normal load varying from2to8N and a sliding speed varying from0.15to0.29m/s. Specific wear rate of the γN phase nitrided layer decreased from4.3×10-5mm3/Nm to4,7×10-6mm3/Nm, relative to that of the original austenitic stainless steel. It is shown that the the wear resistance of γN phase nitrided layer remarkable improved compared with that of the original austenitic stainless steel. The transition of the wear mechanisms from oxidative to abrasive wear was found with the applied load increasing, which was derived from the appearance of the worn surfaces with smooth and covered a dark oxide film transformation the heavily scratched surface and the distinct plastic grooves. The crystal structure of the oxidative and abrasive wear debris was identified as a ε-Fe2O3phase and a nitrogen-containing h.c.p. martensite phase (εN’) respectively, which was implied that an abrasive wear mechanism of friction strain induced martensite phase transformation occuring during the wear tests. The transition of the oxidation reaction to h.c.p. martensite phase transformation of the γN phase during the wear testing was satisfied with the thermodynamics as ΔG°γN→εN’gΔG°oxid. A wear map of the γN phase was constructed to summarize the measured wear volumes and wear regime transitions. The friction coefficient and the specific wear rate of the γNhase decreased slightly in oxidative wear regime having wear volume loss in the range2×10-3mm3to3×10-2mm3with the increasing of the normal load It suggested that the oxides on worn surface acted as a protective layer and helped reducing both specific wear rate and friction coefficient. The friction coefficient and the specific wear rate of the γN phase decreased in abrasive wear regime having wear volume loss in the range4×10-2mm3to1×10-1mm3with the increasing of sliding speed. It was probable that a higher sliding speed made it more difficult for the wear debris to be entrapped, resulting in the severe ploughing process not occurring.Plasma source nitriding on the AISI316austenitic stainless steel at temperature of450℃under a working pressure of300Pa with the distance of100mm between screen and sample for a nitriding time of6h produced a yN phase layer. The anodic polarization curve of the γN phase layer in3.5%NaCl solution underwent a typical transition course from spontaneous passivation-pitting breakdown process into spontaneous passivation-transpassive dissolution process without pitting corrosion. Compare with the original AISI316stainless steel, the self-corrosion potential of the γN phase layer increases224mV(SCE) and the passivation current density deduced one order magnitude, which exhibited remarkable performance of corrosion resistance. The Nyquist plots for EIS of the original AISI316austenitic stainless steel and the γN phase layer shown typical capacitive arc, the diameter of the capacitive arc increased than the original AISI316austenitic stainless steel. The passive film resistance of the γNphase layer was about1.377×106Ω cm2, which increased one order magnitude compared with the original AISI316stainless steel. The passive film on the7n phase nitrided layer in3.5%NaCl solution was characteristics of w-type and p-type semiconductors, which corresponded to the two regions structure:the iron hydroxide/chromium hydroxide/iron oxides in the outer region and the chromium oxides in the inner region. The donor densities of the passive film decreased approximately from2.4×021cm-3to2.2×1020cm-3, relative to that of the original austenitic stainless steel. The more homogeneous and denser passive film with a nearly pure capacitance characteristic on the γn phase nitrided layer can restrict the migration of space charges and inhibit the electrochemical dissolution.