Tribological properties of the modified layers prepared on 38CrMoAl steel by plasma nitrocarburized and post-sulfurized method

The tribological properties of the different modified layers of plasma nitrocarburized and post-sulfurized on 38CrMoAl steel under oil lubrication were investigated, and the wear mechanism was proposed. The modified layers were prepared by the plasma nitrocarburized at 520 °C for 5 h and post-sulfurized at 160 °C for 1, 3, and 5 h. The morphology, phase structures, and composition were characterized by optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, three-dimensional white light interferometry, X-ray diffraction, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. The sliding-wear characteristics were investigated by ball-on-disk wear tests against GCr15 steel counterparts. The results show that the nitrocarburized layer mainly comprises ε-Fe2-3(N, C) and γ ′-Fe4(N, C), with a surface hardness of 1250 HV0.1. The nitrocarburized treatment significantly improved the wear resistance of the 38CrMoAl steel. For the plasma nitrocarburized and post-sulfurized samples, a sulfurized layer mainly consisting of FeS and FeS2 was formed on the nitrocarburized layer. The wear mass loss and friction coefficient of the duplex samples are significantly reduced due to the thin FeS layer acting as a solid lubricant. A two-step method, plasma nitrocarburized and post-sulfurized duplex treatment, further improves tribological performance, which is crucial for engineering applications.