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

Abstract

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-Fe_2-3(N, C) and γ ′-Fe₄(N, C), with a surface hardness of 1250 HV_0.1. 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.