Abstract
Nanostructured and conventional Al2O3 powders have been plasma sprayed, under the same parameters, to produce coatings on 304 stainless steel. The nanopowder was fabricated by mechanical attrition techniques and composed of agglomerates of nanoparticles, whereas the conventional powder consisted of solid granules. In order to maintain the beneficial effect of the nanostructure in the final coating, a fraction of the powder’s nanostructure should be retained in the coating after spraying. Dissimilarities were observed between the two types of coating, regarding porosity, roughness, deposition efficiency and α-Al2O3 to γ-Al2O3 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, as compared to the conventional solid granules. 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 during their presence in the plasma stream, the particles were sprayed directly into distilled 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, was linked to the hollow sphere phenomenon.
