The stability of the composition during coating manufacture from blended Al2O3-Cr2O3-TiO2 ternary feedstocks is of high importance for its properties and, consequently, for its performance. Binary blends of single oxides, such as Al2O3-TiO2, have been widely used for decades for coating manufacturing by APS. For implementation of the expected improvements in ternary compositions changes in the composition during the spray process have to be avoided. When spraying a ternary powder blend containing equal contents by mass of Al2O3, C r2O3 and TiOx by APS using an Ar/H2 plasma gas mixture, tremendous changes of the composition were observed due to an increase in the argon flow rate. This change of the process conditions caused not only an increase in the total plasma gas flow rate but simultaneously increased the Ar/H2 ratio. Through the change of the average particle velocity to 140% and the average particle temperature to 90% of the initial values, respectively, alumina is nearly completely lost from the composition while the chromia content is decreased as well. This change in the coating composition indicates that the individual deposition rate of each oxide does not depend on the melting temperature alone but can be better described by the ‘ ‘difficulty of melting factor’’ (DMF) and the thermal diffusivity of the three oxides. With the selected analytical tools in this study, it is difficult to establish clearly the reason for the increased a-Al2O3 content in the coating, when sprayed with lower argon flow rates. In another study spraying of plain a-Al2O3 with the same parameter set, an expected high content of c-Al2O3 was found. Thus, poor melting of Al2O3 during the spray process when sprayed under the same conditions in a ternary blend is surprising but can be considered as a reason for the less intensive phase transformation from a-Al2O3 to c-Al2O3. However, Cr2O3 splats can also contribute to an additional content of a-Al2O3. At the same time, formation of rutile (TiO2) from TiOx was found, and EDS analysis showed a small amount of Ti in Cr2O3 lamellas.
Due to the very different thermal properties of the three oxides Al2O3, C r2O3, and TiO2, strict control of the thermal spray process conditions is a prerequisite for the minimization of any compositional changes. However, this is an indication that composite powders can be more effective in this case. At the same time, an accurate quantitative analysis of the coating components and more analytical efforts are necessary to investigate the complex interactions between different oxide particles, e.g., to distinguish nonmolten Al2O3 particles and their cores from possible stabilized molten areas, the oxidation of TiOx and the interaction of titania and chromia.
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