Microstructure and formation mechanism of the Si-Cr dual-alloyed coating prepared by pack-cementation

In this study, two types of Si-Cr coatings (named as SiCr-1 and SiCr-2) were fabricated on AISI 5140 steels by pack-cementation via different Si feedstocks, aiming to strengthen the steel surface and avoid the carbon-poor layer caused by chromizing. Phases, microstructure, and composition were characterized by X-ray diffraction technique (XRD), secondary electron imaging (SEI) and backscattering electron imaging (BSEI), and energy dispersive spectroscopy (EDS), respectively. The results show that both the coatings mainly consist of alpha-Fe (with Si and Cr in concentration), Fe3Si, CrFe8Si, and CrC. However, there are some apparent differences between the two coatings including the morphology, element content, microhardness, and wear resistance, which is owing to the difference in the coating growth mechanism. The coating growth of SiCr-1 sample using Si powders as Si feedstock is controlled by the interdiffusion mechanism of the substrate and coating elements, while the coating growth of SiCr-2 sample using ferrosilicon powders as Si feedstock is mainly dominated by the reaction deposition mechanism. For both samples, the presence of micropores in the coatings is observed and their formation is mostly due to the non-equilibrium diffusion induced Kirkendall effect. A new gradient growth mode of micropores based on the Kirkendall effect was proposed.