Oxide layer formed on AISI 5140 steel by plasma nitriding and post-oxidation in a mixture of air and ammonia

Abstract

AISI 5140 low alloy steel has been treated by plasma nitriding at 540 °C for 4 h, followed by post-oxidation at 450 °C in plain air and a mixture containing air and ammonia with various ratios for 1 h respectively. The microstructure, phase composition, corrosion resistance and tribological characteristics of the nitrided and nitrided + post-oxidation samples are examined. The results show a thin, continuous and dense oxide layer with the thickness ranging from 1.6 to 1.8 μm was formed on the top of nitride layer after plasma post-oxidation. The oxide layer is mainly composed of Fe3O4 phase accompanied by a small amount of Fe2O3 phase for post-oxidation in plain air. Only Fe3O4 phase appears in oxide layer after post-oxidation in 25% air – 75% NH3, 50% air – 50% NH3 gas mixture. Addition of ammonia into air favors the formation of Fe3O4 which is responsible for the excellent corrosion resistance of treated samples. The friction coefficients of the nitrided samples are significantly decreased due to oxide layer formation by plasma post-oxidation in plain air and mixture of air and ammonia.