Factors influencing the formation of a diffusion zone and the adherence of the top coat of high temperature coatings from micro-sized spherical aluminium particles

Abstract

Micro-sized spherical aluminium particles deposited as slurry by brushing or spraying on a substrate alloy oxidise at high temperatures to form a top coat from sintered hollow alumina spheres whilst forming an aluminised diffusion zone. The top coat has the potential to be effective as a thermal barrier by gas phase insulation. The formation of the diffusion zone and the adherence of the top coat are influenced not only by the parameters of the heat treatment, but by factors such as particle size and surface finishing. Coated samples of Alloy 321 were cured and heat treated at 650°C for 5 h and were analysed by field emission scanning electron microscopy. Coating samples of Alloy 321 containing spherical aluminium particles with the sizes 0.3 – 0.7 μm, 2–3 μm, 5 μm, and 30 – 50 μm and with a surface finishing of 120#, 500# and 1200# revealed that the size range of 2 – 5 μm and a surface finishing of 500# are more suitable for forming the coating structure of the diffusion zone with an adherent top coat. Using aluminium particles with a size of 30 – 50 μm, the temperature range where diffusion predominates at the expenses of the alumina sphere formation is wider and top coat spallation is observed.