Growth kinetics and microstructure of composite coatings on H13 by thermal reactive diffusion

The kinetics and microstructure of composite coatings on H13 steel in different chemical compositions of baths were studied in the present research. Three metal baths and three oxide baths with chromium to vanadium molar ratios of 0.5, 1 and 2 were used. The coating process was performed at seven different times at 1000 °C for all the baths. Field emission scanning electron microscopy (FE-SEM), electron probe micro analyzer (EPMA) and X-ray diffraction analysis (XRD) were applied to study the coatings. The coating produced in the metal bath had a greater thickness than that in the oxide bath in all the experiments. Before 1 h and between 1 h to 12 h of the coating process, the interface and the diffusion controlled the growth, respectively. Regarding this mechanism of growth, before 1 h, the relationship between coating thickness and time was fitted to a linear function (d = at + b) and from 1 h to 12 h, it was fitted to a parabolic function (d = ct0.5 + e). Chromium carbide and vanadium carbide coatings grew as columnar and equiaxed grains, respectively. Distribution of chromium carbide-rich regions in vanadium carbide-rich regions became more homogenous with time and a composite coating was formed.