Improved Wear Performance of Nitrided AISI-304 Steel by Plasma Post-oxidation at Various Conditions

Abstract

The aim of this study is to enhance the wear performance of nitrided austenitic stainless steel AISI 304 by plasma post-oxidation at various temperatures and times. The surface hardness of untreated steel (186 HV_0.01) is upgraded to 1200 HV_0.01 by plasma nitriding; this is reduced to 870-920 HV_0.01 by plasma post-oxidation treatment. On nitrided steel, an expanded austenite (nitrogen supersaturated) phase is obtained, while in the post-oxidized samples, the magnetite phase (Fe₃O₄) is attained without the appearance of a hematite phase (Fe₂O₃). The appearance of the magnetite phase is more beneficial in minimizing the friction coefficient and thus improving the sliding wear performance. Through plasma nitriding, a nitrided layer with a thickness of 4.7 μm is formed, while the thickness further increases to 6.9-12.7 μm by post-oxidation, and an oxidized layer with a maximum thickness of 1.12 μm is attained at 440 °C and 60 minutes. The wear volume dramatically decreases by 98% (0.21-0.003 mm³) when plasma nitriding and post-oxidation are combined, while the friction coefficient is reduced by 33% (untreated friction coefficient from 0.93 to 0.62 for the duplex-treated sample). The results reveal that the post-oxidation layer acts as a protective layer, enhancing wear resistance and decreasing the friction coefficient by forming the sole magnetite phase. This combination of treatments, i.e., plasma nitriding and post-oxidation, is applied in the same system (only gases are changed). Thus, no additional equipment is involved, except for a small increase in the amount of time for oxidation treatment. In addition, the plasma processing system is well-suited for industrial-scale applications, and thus, it can be applied to the large-scale processing of steel.