Suppression of crystallization during high velocity impact quenching of alumina droplets: Observations and characterization

Droplets of alumina were deposited and quenched onto various smooth substrates using the high-velocity oxy-fuel (HVOF) thermal spray process. The microstructure of single splats was examined using optical microscopy, atomic force microscopy (AFM), focused ion beam assisted scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM). It was found that the high impact velocity inherent in HVOF formed splats that were in intimate contact with the substrate, leading to a high solidification rate up to 10¹⁰ K/s. The solidified alumina splats exhibited a smooth surface devoid of "mud-crack" networks of surface micro-cracking, commonly observed in ceramic plasma sprayed systems. Microstructural examinations of the smooth areas of the splat revealed an amorphous microstructure as confirmed by selected area electron diffraction. In local areas of poor contact between the splat and substrate, a polycrystalline microstructure was discernible and a metastable γ-Al₂O₃ phase was detected. Second layer splats, that landed on top of a first layer splat and experienced slower cooling rate, were found to have surface cracking, and revealed principally a γ-Al₂O₃ nano-crystalline structure. The observation of amorphous alumina is explained by the suppression of a first order liquid-to-solid phase transformation, due to a high solidification rate that we estimate.