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 degrees 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 mu m, 2-3 mu m, 5 mu m, and 30-50 mu m and with a surface finishing of 120#, 500# and 1200# revealed that the size range of 2-5 mu m 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 mu m, the temperature range where diffusion predominates at the expenses of the alumina sphere formation is wider and top coat spallation is observed
A novel coating concept for high temperature protection based on spherical micro sized aluminum particles in combination with boron micro particles has been developed and investigated. Fractions of 10% to 20% boron micro particles were added to slurries of spherical micro-sized Al, and deposited by brushing on the surface of the austenitic steel alloy 321. A single-sized 3-5 µm Al and a multi sized 1-20 µm Al powder were used. After a heat treatment of 5 hours at 650°C, a homogeneous diffusion layer was formed in the substrate as well as an adherent topcoat. The latter consisted of sintered hollow alumina spheres, which has the potential to provide thermal barrier effect by gas phase insulation. The results from exposure experiments up to 3000 h show that for the 1-20 µm Al particles, the addition of boron yields topcoats with improved adherence, higher stability and resistance to high temperatures, as well as an outstanding sintering degree. Nano-sized whiskers of Al 4 B 2 O 9 were observed on the surface of the Al 2 O 3 particles when boron was added. The formation mechanism of the borate was studied in situ by high temperature X-ray diffraction.
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