Examination of Crystalline Structure of Plasma-Sprayed Al2O3 Coatings At Different Surface Temperatures

Plasma-sprayed alumina coatings, prepared by the conventional route, consist mainly of the metastable γ-Al₂O₃ phase and a certain amount of α-Al₂O₃, resulting from nonmelted fraction of partially melted alumina particles remaining into coatings. How to control the crystalline structure of plasma-sprayed deposit is still a challenge. In this study, Al₂O₃ coatings were plasma-sprayed on the sintered Al₂O₃ substrates preheated to different temperatures. The crystalline structure of the coatings was examined by x-ray diffraction analysis. The crystalline structure of typical splats was characterized by high resolution transmission electron microscopy. It was shown that the phase composition of plasma-sprayed alumina coatings is influenced by the deposition temperature. The coatings consisted of a fraction of amorphous phase besides γ-Al₂O₃ and α-Al₂O₃ phases when the deposition temperature was increased from room temperature up to 800°C. When the deposition temperature was increased to above 800°C, δ, α, θ phase appear in the coatings. The mass fraction of α-Al₂O₃ increased with increasing deposition temperature while the coatings are thinner than 100 μm. The fraction of α-Al₂O₃ was increased to higher than 50% and 80% as the deposition temperature reached to 800°C and 1000°C, respectively.