PREPARATION AND STRUCTURES OF PLASMA-SPRAYED GAMMA-AL2O3 COATINGS AND ALPHA-AL2O3 COATINGS

As-sprayed Al2O3 coatings containing more than 99.5% gamma-Al2O3 were prepared by optimizing torch parameters. Lamellae were 1-4-mu-m thick with grains ranging from 0.1 to 1-mu-m in diameter. Using the same torch conditions, but slowly traversing the torch over the substrate, coatings containing alpha-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 gamma-Al2O3 and transformed to alpha-Al2O3 from the heat of the torch and coating; type II, above type I, which nucleated as alpha-Al2O3 and had separate lamellae because solidification was completed before the next droplet arrived; type III, in the upper region, which nucleated as alpha-Al2O3 and contained grains over 200-mu-m 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.