Aluminum oxide is a relatively cheap, abundant material that is widely used for plasma-spray applications. This material, however, exists in many crystallographic modifications with different properties. In addition, most of these modifications are metastable and cannot he used in applications employed at elevated temperatures. Usually )', ~, or other phases form after spraying, while ~ phase (corundum) is often the most desirable phase due to high corrosion resistance and hardness. This paper first reviews the method of ~ stabilization in the as-sprayed materials offered in literature. Then, as an example, it summarizes the results of an extensive study of chromia additions to alumina. Chromia was chosen because of its complete solid solubility in alumina and its crystal lattice type, which is similar to that of alumina. It was demonstrated that the addition of approximately 20 wt % chromia results in the formation of one solid solution of (AI-Cr)203 in the s-modification.Finally, this paper discusses the thermal stability of various alumina phases. Phase change routes of heating for different starting alumina modifications are discussed, and a case study of alumina-chromia is presented. Both types of as-sprayed structures, a mixture of cq ~, and y phases, and 100% (AI-Cr)203 were annealed up to 1300 *C and the phase composition checked. At lower te~aperatures and shorter holding times, the amount of c~ phase decreases while another metastable 0 phase appears, and the fraction of)' + fi, if present, increases. At temperature above 1100 ~ the amount of c~ phase increases again.

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A phase change from a-alumina (corundum) in the feedstock powder to predominantly other alumina phases, such as c-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.

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