Effect of boron addition on micro-aluminum-particle-based multifunctional high-temperature coatings

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 10-20 % 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 h 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 3,000 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 Al4B2O9 were observed on the surface of the Al2O3 particles when boron was added. The formation mechanism of the borate was studied in situ by high temperature X-ray diffraction.