A study of laser-remelted flame-sprayed NiCrBSi/W composite coatings: the influence of thermal diffusivity

The article presents research results on the microstructure of laser-remelted NiCrBSi coatings deposited with flame spraying. Two types of coating powders were used. Commercial Metco 15E powder is strengthened with chromium-based carbide and boride phases based on a eutectic form of Ni(Cr) solid solution matrix with borides and Ni silicides. The second powder was also Metco 15E but with the addition of 50 mass% metallic tungsten granules. In both cases, identical parameters of the flame spraying and laser melting processes were used. It was found that introducing tungsten significantly changes the morphology of the coating. This applies to the size of the heat-affected zone, its microstructural nature (grain growth), and the dilution effect. These elements influenced the level of hardness obtained, which was lower in the case of the modified coating. At the same time, however, the modified coating was characterised by a stable microhardness distribution in the cross section, especially in connection with the substrate material. This indicates a strong tendency of tungsten to annihilate the dilution effect and inhibits the decrease in the hardness of the carbide and boride phases associated with the increase in iron content. The factor responsible for the described effects is a significant difference in the thermal diffusivity of the materials used to produce the coatings. This size significantly impacts the scale of the created heat-affected zone and directly affects the mixing effect.