Emergence and impact of Al2TiO5 in Al2O3-TiO2 APS coatings

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.