Microstructure and microhardness characterization of a Fe-based coating deposited by high-velocity oxy-fuel thermal spraying

Abstract

Based on the merits of high-velocity oxy-fuel (HVOF) thermal spraying, hardness and cavitation erosion resistance of steel were improved with a Fe–Cr–Si–B (5.8 wt.% B) alloy coating applied to stainless steel (1Cr18Ni9Ti) substrate by HVOF thermal spraying. The microstructures and microhardness of the coating were investigated. It was found that the morphologies of the coating were of well-flattened splats (containing some un-melted borides), semi-molten particles, featureless structures and a small amount of pores. The average Vickers microhardness of the coating reached 9.9 GPa, about four times that of the substrate (2.8 GPa). From the substrate to coating, the indentation marks or the corresponding microhardness values changed gradually. The higher microhardness values of the substrate near the coating can be attributed to work hardening resulting from the impact of the melted or semi-molten particles and grit sand on the substrate. The microhardness of the well-flattened splats (10.8 GPa) is higher than that of the semi-molten particles (about 6.2 GPa) and the featureless regions (6.4 GPa), and the microhardness of the featureless regions is higher than that of the semi-melted particles. This suggests that the fully molten state of the feedstock can result in the formation of featureless structures and well-flattened splats (containing some un-melted borides) present high microhardness.