Effect of process time on structural and tribological properties of ferritic plasma nitrocarburized AISI 4140 steel

AISI 4140 steel was plasma nitrocarburized at a gas mixture of 49%N2 + 49%H2 + 2%CO2, for different process times (1, 2, 4, 8 and 12 h), at a temperature of 570 °C. The structural, mechanical and tribological properties of nitrocarburized steel were analyzed using a X-ray diffraction, microhardness tester, scanning electron microscopy, optical microscopy and pin-on-disk tribotester. The results have shown that the compound layer was composed of the ε and γ′ iron carbonitrides. In addition, the compound layer included pores having a columnar structure. These pores are open to the surface. The nitrocarburizing process increases surface hardness, roughness and friction coefficient. The wear rate improves after plasma nitrocarburizing, and decreases with increasing surface hardness.