Abstract
AISI 4140 steel was initially plasma nitrocarburized at 570 and 700 °C for 4 h in a gas mixture of 60%N₂+37%H₂+3%CO₂. After nitrocarburizing, post-oxidation treatment was performed at 450°C for 1 h using oxidation gas mixture ratios of 50%H₂+50%O₂ and pure oxygen. The structural, mechanical, tribological, and corrosion properties of nitrocarburized and post-oxidized steel were analysed using a X-Ray diffraction, microhardness tester, scanning electron microscopy, pin-on-disc tribotester, and electrochemical polarization. The experimental results showed that oxide layer included Fe₂O₃ and Fe₃O₄ phases while the compound layer formed after both ferritic and austenitic nitrocarburizing consisted of Fe_2-3(N, C) and Fe₄(N, C) phases. Post-oxidation treatment improved further tribological and corrosion properties in consequence of nitrocarburizing. The contribution of post-oxidation treatment made after the ferritic nitrocarburizing was much more than the contribution of post-oxidation treatment made after the austenitic nitrocarburizing. Furthermore, it was seen that using hydrogen in oxidation gas mixture had useful effects such as better tribological properties and corrosion resistance.

Key words
Plasma nitrocarburizing, post-oxidizing, tribology, corrosion

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