Preparation and oxidation behavior of B4C–Ni and B4C–TiB2–TiC–Ni composite coatings produced by an HVOF process

Abstract In this study, B4C–Ni and B4C–TiB2–TiC–Ni composite coatings were thermally sprayed on the surface of 4130 steel substrate by a high velocity oxy-fuel (HVOF) torch. Before the HVOF process, the B4C–Ni and B4C–TiB2–TiC–Ni composite powders were prepared by mechanical alloying (MA) and spray drying processes. In order to study the oxidation behavior of coatings, thermo-gravimetric analysis (TGA) was done at three temperatures of 500, 700 and 900 °C for 35 h. Structural evolution of powder particles and coatings was investigated by X-ray diffractometry (XRD). The morphology and microstructure of powder particles and the coatings were studied by scanning electron microscopy (SEM). It was found that during TGA experiments of B4C–Ni coating, spallation was occurred at all temperatures, indicating the formation of non-protective oxide scale on the surface of this coating. First oxidation and then with increasing time, spallation were the predominant processes at 700 and 900 °C. No spallation was seen in TGA experiments of B4C–TiB2–TiC–Ni coating. Formation of the protective oxide scale on the surface of this coating decreased the oxidation rate with increasing time.