On the relationship between particle melting degree and phase transformation of alumina and alumina-based solid solution powders during atmospheric plasma spraying

The polymorphism and the thermo-physical properties of alumina create significant challenges for deposition of thermally sprayed coatings. For a better understanding of the associated phenomena, in this study two feedstock powders (one commercial plain alumina powder and one experimental solid solution powder (initial composition 83.0 mol% Al2O3, 8.5 mol% Cr2O3 and 8.5 mol% TiO2) both consisting mainly of the corundum phase were deposited by atmospheric plasma spraying (APS) using three different spray parameter sets. For each parameter set, the plasma jet area was studied as well as the fluctuation using a high-speed camera. Particle temperature and velocity were measured using a particle diagnostic tool. In addition, powders were collected in flight to study the melting state of the particles to improve the understanding of the phase transformation. The results show that the particle melting state is closely related to the particle temperature and is significantly influenced by the electrical power and the particle velocity. The experimental Al2O3-based solid solution powder shows significantly improved melting behavior. Controlling the melting state of the particles in order to achieve a high amount of α-phase is more difficult for the plain Al2O3 powder than for the alumina-based solid solution powder. Under the conditions studied, for plain Al2O3, a deterioration of the particle melting state leads to a reduction in deposition efficiency without effect on the phase composition in the coating, while for the alumina-based solid solution, the phase composition can be influenced in favor of a higher α-phase content without affecting the deposition efficiency.