Dependence of the stabilization of alpha-alumina on the spray process

A phase change from alpha-alumina (corundum) in the feedstock powder to predominantly other alumina phases, such as gamma-alumina in the coating normally takes place, as a result of the spray process. It is expected that the prevention of this phase transformation will significantly improve the mechanical, electrical, and other properties of thermally sprayed alumina coatings. The results regarding the possibility of stabilization of a-alumina through addition of chromia published in the literature are ambiguous. In this work, stabilization using different spray processes (water-stabilized plasma (WSP), gas-stabilized plasma (APS), and high-velocity oxy-fuel spray (HVOF)) was studied. Mechanical mixtures of alumina and chromia were used, as were prealloyed powders consisting of solid solutions. The investigations focused on mechanical mixtures with both APS and WSP and on prealloyed powders with WSP. The coatings were studied by x-ray diffraction, including Rietveld analysis, and analysis of the lattice parameters. Microstructures were investigated by optical microscopy using metallographic cross-sections. It was shown that in the case of the mechanically mixed powders, the stabilization predominantly depends on the applied spray process. The stabilization of the a phase by use of the WSP process starting from mechanical mixtures was confirmed. It appears that stabilization exhibits a complex dependence on the spray process, the process parameters (in particular the thermal history), the nature of the powder (mechanically mixed or prealloyed), and the chromia content.