Impact of nitrogen fraction in N2-H2 plasma nitrocarburizing on mechanical, tribological, and corrosion performance of AISI 316L

This study investigates how the nitrogen fraction ( f N ) in N₂-H₂ feed gas affects the microstructure, mechanical, wear and corrosion properties of AISI 316L stainless steel treated at 460 °C for 5 h by active screen plasma nitrocarburizing (ASPNC) using a plasma-activated carbon screen as the carbon source. Investigation includes glow discharge optical emission spectroscopy (GDOES), X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) to characterize the elemental composition, phase composition, and surface topography of the expanded austenite layers across five different nitrogen fractions (0 ≤ f N ≤ 1). A transitional regime at f N = 0.5 showed maximum nitrogen uptake, minimal carbon content, and the thickest expanded austenite layer, though accompanied by highest defect density. Mechanical testing indicate that hardness and wear resistance reach a peak at f N = 0.5 (Martens hardness HM = 3.27 GPa), while higher nitrogen fractions ( f N ≥ 0.9) lead to decreased hardness due to nitride-induced brittleness. Electrochemical polarization in 0.05 M H₂SO₄ reveal that corrosion resistance deteriorates with increasing f N , particularly at f N = 0.5, where nitride precipitates, grain boundary defects, and chromium depletion impair passive film stability. Treatments at low nitrogen fraction ( f N ≤ 0.1) offer an optimal balance between corrosion resistance and mechanical performance, suitable for applications requiring both wear and corrosion protection. In contrast, high nitrogen conditions ( f N ≥ 0.5) enhance wear resistance but are susceptible to corrosion, emphasizing the importance of tailoring plasma parameters to optimize AISI 316L performance for specific industrial applications.