Microstructure characteristics of Al2O3–13 wt.% TiO2 coating plasma spray deposited with nanocrystalline powders

Nanostructured Al2O3–13 wt.% TiO2 coating was fabricated by plasma spray with nanocrystalline powders and the microstructures of the feedstock and the coating were characterized by means of XRD, SEM and TEM. It was found that three forms of substructure existed in the coating: one evolving from the unmelted part of the feedstock and showing a round-shaped morphology; one resembling the liquid-phase-sintered structure consisting of the flattened partially melted region and fully melted region; another being of the particulate-reinforced-solid solution type with fine particles distributed in the matrix. The TEM analysis revealed that partially melted α-Al2O3 particles were in the size range of 20–70 nm and were embedded in the TiO2-rich matrix. The mechanism of the substructure formation was also explained in terms of the melting and flattening behavior of the powders during plasma spray processing.