Adhesion of ceramic coating on thin and smooth metal substrate: A novel approach with a nano-structured ceramic interlayer

The adhesion of plasma-sprayed coating is, to a large extent, controlled by the cleanness and roughness of the surface on which the coating is deposited. So, most of the plasma spray procedures involve surface pretreatment by grit-blasting to adapt the roughness of the surface to the size of the impacting particles. This preparation process brings about compressive stresses that make it inappropriate for thin substrates. The present works aim to elaborate a thick ceramic coating (about 0.5 mm thick) on a thin metal substrate (1 mm thick) with a smooth surface (Ra of about 0.4 μm). The coating system is intended for use in a Generation-IV nuclear energy system. It must exhibit a good adhesion between the ceramic topcoat and the smooth metal substrate to meet the specifications of the application. Our approach consisted of depositing the ceramic topcoat by air plasma spraying on a few micrometers thick ceramic layer made by suspension plasma spraying. This nano-structured layer played the role of a bond coat for the topcoat and made it possible to deposit it on the as-received substrate. The adhesion of the nano-structured layer was measured by the Vickers indentation cracking technique and that of the ceramic duplex coating system by tensile test. (authors)