A Comparative Microstructural Investigation of Nanostructured and Conventional Al2O3 Coatings Deposited by Plasma Spraying

Nanostructured and conventional Al2O3 powders have been plasma sprayed to produce coatings. The parameters for retaining a fraction of the nanostructure were investigated. Dissimilarities were observed between the two types of coating, regarding properties and phase proportions, which are related with the different percentages of semimolten particles in the coatings. The nanocoatings retained a higher percentage of semimolten particles than the conventional coatings owing to the higher porosity of the nanoparticle agglomerates. The molten part of both conventional and nanostructured coatings consisted of γ-Al2O3 of columnar morphology. In order to investigate the mechanism of the melting front advance into the particle interior, the particles were sprayed directly into deionized water. The nanoparticles mainly formed hollow spheres, whereas the conventional particles mainly formed compact spheres. The internal porosity of the solidified nanoparticle agglomerates, which affected the overall coating porosity and, consequently, coating properties such as hardness, adhesion, and surface roughness, was linked to the hollow sphere phenomenon.