Surface treatment effects on ceramic matrix composites: Case of a thermal sprayed alumina coating on SiC composites

Abstract High-temperature-resistant ceramics are already used for many industrial applications. In response to the growing demand, the need for further research considering the final application and the global behaviour of the material is becoming increasingly apparent. In general, it is possible to comply with many specifications, just by treating the surface of the ceramics. For instance, it is possible to achieve an adequate mechanical strength by depositing a protective layer with different structure and/or chemical composition. Regardless to the specification, the adhesion coating/substrate is the most crucial property to be considered. Conventionally, surface degreasing (applying solvents to remove organic impurities) and grit blasting (corundum) are carried out as a two steps pretreatment prior to the thermal-spraying operation to guarantee a mechanical anchorage of the molten particles to the substrate. However, some substrates are grit-blasting sensitive and therefore, alternative treatments should be considered. In these cases, the adherence of the coating must be attained by others means, either from a chemical point of view by modifying the surface wettability, or from a photonic one. Within this context, this study aims to explore the surface modifications induced on SiC composites (Ceramic Matrix Composites, CMC) before plasma spraying of alumina coatings. The effect of two different pre-treatments, deposition of a silicon bonding layer and short-pulse laser treatment (Nd:YAG), on the coating–substrate interface is investigated. A better chemical affinity between the alumina coating and the silicon carbide CMC has been observed with the silicon bond coating. On the other hand, a mixture of chemical and mechanical interaction induced by a cone-like structure occurs when treating the SiC CMC surface by laser prior thermal-spraying.