Al₂O₃, Cr₂O₃, and TiO₂ are most commonly used oxide materials for thermal spray coating solutions. Each oxide shows unique properties comprising behavior in the spray process, hardness, corrosion, and wear resistance. In order to exploit the different advantages, binary compositions are often used, while ternary compositions are not studied yet. Atmospheric plasma spraying (APS) of ternary compositions in the Al₂O₃-Cr₂O₃-TiO₂ system was studied using blends of plain powders with different ratios and identical spray parameters. Coatings from the plain oxides were studied for comparison. For these powder blends, different deposition rates were observed. The microstructure, roughness, porosity, hardness, and wear resistance were investigated. The formation of the splats from particles of each oxide occurs separately, without interaction between the particles. The exception are the chromium oxide splats, which contained some amounts of titanium. The predominant oxide present in each blend has a decisive influence on the properties of the coatings. While TiO_x causes a low coating porosity, the wear resistance can be increased by adding Cr₂O₃.

Despite numerous studies and decades of industrial application, there is still a lack of understanding about the formation and the impact of aluminum titanate (Al₂TiO₅) in Al₂O₃-TiO₂ thermal spray coatings. Especially the influence of the feedstock powder characteristics on the phase composition has only crudely been investigated so far. Therefore, in this work we have characterized commercial fused and crushed Al₂O₃-TiO₂ feedstock powders: three of them containing 13 wt.% TiO₂ and three containing 40 wt.% TiO₂. The effect of the varying phase compositions of the powders and their relevance on the deposition efficiency, the phase compositions, the porosity, and the hardness of the respective APS coatings is described in detail. While detrimental to the mechanical properties of 40 wt.% TiO₂ coatings, we have found an enhancement of the hardness for 13 wt.% TiO₂ coatings with a high Al₂TiO₅/Al₆Ti₂O₁₃ content in the feedstock powder. Furthermore, it was found that Al₂TiO₅ may reform during APS when sprayed from an Al₂TiO₅-free powder.

Thermally sprayed coatings from the single oxides and binary compositions of the Al₂O₃-Cr₂O₃-TiO₂system show multifunctional properties. Ternary compositions are promising for further improvement in their performance. The stability of the composition during coating formation is an important issue for blended feedstock powders in order to obtain the desired properties. This work focuses on the compositional changes of a ternary blend of Al₂O₃, Cr₂O₃and TiO_xpowders of equal content by mass in a conventional atmospheric plasma spraying (APS) process using an Ar/H₂plasma gas mixture. By increasing the argon flow rate at constant hydrogen flow rate, the total plasma gas flow rate and the Ar/H₂ratio were varied. For the highest argon flow rate, this resulted in an average particle velocity of 140% and an average particle temperature of 90% of the initial values, respectively. Coating composition and microstructure were studied by optical microscopy, SEM, including EDS analyses, and XRD. In addition, the coating hardness and electrical impedance were also measured. Differences in the “difficulty of melting factor” (DMF) and the thermal diffusivity of the three oxides appear to be responsible for the dramatic change of the coating composition with an increasing argon flow rate. For the highest argon flow rate applied, besides TiO₂, the coating contains only 8 wt.% Al₂O₃, while the Cr₂O₃content remained almost constant. At the same time, the change of the Ar/H₂ratio resulted in the formation of stoichiometric TiO₂in the coating by oxidation of TiO_xin the feedstock powder. Moreover, a small content of titanium was found in the Cr₂O₃splats, showing that there are only limited interactions between the large oxide powder particles. Thus, the study has shown that stability of the chemical composition during spraying of ternary powder blends is strongly influenced by the process conditions.