Preparation and structures of plasma-sprayed γ- and α-Al₂O₃ coatings

As-sprayed Al₂O₃ coatings containing more than 99.5% γ-Al₂O₃ were prepared by optimizing torch parameters. Lamellae were 1-4 μm thick with grains ranging from 0.1 to 1 μm in diameter. Using the same torch conditions, but slowly traversing the torch over the substrate, coatings containing α-Al₂O₃ were produced. Three types of microstructures were observed in each pass of the torch: type I, in the lower region, believed to have nucleated as γ-Al₂O₃ and transformed to α-Al₂O₃ from the heat of the torch and coating; type II, above type I, which nucleated as α-Al₂O₃ and had separate lamellae because solidification was completed before the next droplet arrived; type III, in the upper region, which nucleated as α-Al₂O₃ and contained grains over 200 μm in length because the previous droplet had not finished solidifying before the next droplet impacted. Thermal expansion and pore size distributions were measured, as also were temperatures within the coatings during their deposition.