Hybrid Aerosol Deposition of Dense Al₂O₃ Coatings and Evaluation of the Through-Thickness Effect

Hybrid aerosol deposition (HAD) has been proposed recently as a new coating regime to deposit homogeneous ceramic coatings via the utilization of mesoplasma and solid particle deposition. This study will discuss the implementation of HAD for the deposition of alumina (Al₂O₃) coatings on 304 stainless steel and aluminum substrates, and evaluation of the hardness and Young’s modulus using a nanoindentation method to clarify the through-thickness properties. Dense and uniform coatings with a nanocrystalline structure were fabricated on both substrate materials. The fabricated HAD coatings consisted of α-Al₂O₃ phase. The hardness and Young’s modulus distributions along the through-thickness direction showed a significant difference across the coating-substrate interface and tended to show a slight decrease by 10-15% as the measured position went close the surface. Increasing the hardness and Young’s modulus on the substrate side near the interface is presumably related to the peeing effect of the substrate as well as the increase of interface roughness during the room temperature impact consolidation (RTIC) and deformation of the hard ceramic particles on the substrate. The decrease in the coating’s mechanical properties along the through-thickness direction is considered to be related to the particle deformation tendency during the coating build-up. At the beginning stage of the deposition, initial particles are impacting on a metallic substrate which is ductile enough to facile plastic deformation and the deposited layer can have an enough hammering effect by the subsequent impacting particles. The hardness and Young’s modulus in this location are 15.6 GPa and 246 GPa, respectively, and the highest through the thickness in case of the stainless steel substrate. However, the later particles are impacting on a hard ceramic surface (initially formed HAD Al₂O₃ layers), which hardly undergo plastic deformation or led to less particle deformation. In addition, through-thickness measurements revealed that the deposited coatings on the stainless steel substrate showed higher hardness than deposited coatings on aluminum substrates. Thus, the stainless steel enhances the degree of deformation of the deposited particles, and the resulted smaller crystallite size and strain lead to increased hardness and modulus.