Interface Characterization and Bonding Mechanisms of Cold Gas-Sprayed Al Coatings on Ceramic Substrates

The aim of the present work is to contribute to the understanding of the adhesion mechanisms, which take effect at the interface of cold gas-sprayed metallic coatings on ceramic substrates. Former investigations revealed that it is possible to deposit well-adhering metallic coatings on atomically smooth ceramics. This led to the conclusion that mechanical interlocking is not always a necessary precondition for bonding. A combination of recrystallization processes induced by adiabatic shear processes and heteroepitaxial growth might be an explanation for the high observed adhesion strengths. The present work focuses on the examination of the interface area of cold gas-sprayed aluminum on various ceramic substrates by means of SEM and HRTEM. Beside sintered corundum plates, single-crystalline sapphire substrates with defined lattice orientations were used as substrates for the deposition of the coatings. In addition to Al2O3 substrates, aluminum coatings were also deposited on AlN, Si3N4, and SiC in order to investigate, whether the different amounts of ionic bonds in these substrate materials have an influence on the substrate/coating interface formation. Additional information about the local heteroepitaxy and its effect on the adhesion of Al coatings was obtained from experiments performed on coatings produced by means of physical vapor deposition.