Role of powder microstructure in aerosol-deposited alumina coatings

The selection of feedstock powder particles is critical for coating formation in aerosol deposition (AD). AD favors sub-micrometer-sized powders, which are susceptible to aggregation and agglomeration. This study investigates the effect of powder characteristics on coating formation using two types of alumina powders: one consisting of dense, single particles, and the other of agglomerated particles. The morphology, microstructure, and mechanical properties of the coatings deposited on Si substrates were strongly influenced by the powder morphology and pretreatment. Without pretreatment, dense particle powders caused substrate erosion, whereas agglomerated powders produced thick coatings. Ball milling was employed to modify both types of powders, introducing surface defects with reduction in particle and crystallite size. Thick coatings were deposited using ball-milled dense or agglomerated powders. Although deposition with agglomerated powders was easier, coatings from dense powders exhibited higher density and hardness. Differences in particle behavior upon impact were discussed and related to coating microstructure.