Development of Axial Injection Suspension Thermal Spraying

Suspension and solution thermal spraying are innovative processes that enable the deposition of layers with ultrafine microstructure. These processes have been investigated by many research teams over the recent years moving the technology from a laboratory-scale curiosity to emerging industrial applications. This presentation will focus on current research efforts at the National Research Council of Canada on the development of functional coatings and devices deposited using robust and fully automatic axial injection systems for plasma and HVOF technologies. Axial injection can generate important benefits in terms of reduced process sensitivity and improved heat and momentum transfer to the particles. During the development, close attention was paid to adapt on-line particle diagnostics to the constraints of suspension spraying. As compared to conventional thermal spraying, the small particle size in suspension spraying, which is not a priori known, poses unique challenges for optical diagnostics. The particle state was found to play a critical role in the impact behaviour of the droplets on the substrate, which in turn can have significant effect on the coating microstructure, porosity and grain-size as well as on the efficiency of the deposition process. The relationship between the operating conditions, particle velocity and temperature and coating microstructure was continuously assessed. Selected activities will be presented in the fields of suspension spraying of zirconia-based thermal barrier coatings for aerospace and land-based gas turbine systems, alumina-zirconia nanocomposites for TBCs and wear protective coatings, mullite-based environmental barrier coatings, and the fabrication of metal-supported solid oxide fuel cell anodes and electrolytes by laser-assisted suspension plasma spraying and HVOF suspension spraying.