Abstract

The advancement of gas turbine technology depends on the development of durable thermal barrier coating (TBC) systems capable of withstanding high temperatures and oxidative environments. At the core of these systems, the NiCoCrAlY bond coat provides both oxidation protection and adhesion between the ceramic topcoat and metallic substrate. The microstructure and oxidation resistance of these coatings are strongly influenced by the deposition process. This dissertation investigates the High Velocity Air Fuel (HVAF) thermal spray process for NiCoCrAlY bond coats, comparing it with the conventional High Velocity Oxy-Fuel (HVOF) method. Lower flame temperature and higher particle velocity in HVAF produced dense coatings with minimal in-flight oxidation. Using AccuraSpray 4.0, real-time diagnostics of particle velocity and temperature were correlated with microstructural and compositional analysis via SEM, EDS, and FIB. HVAF based splat analysis revealed ability to generate partially molten and fully deformed splats with preserved aluminum reservoirs, promoting the formation of continuous α-Al₂O₃ scales critical for oxidation resistance. Longer nozzles improved coating density through enhanced dwell time, while partial melting introduced localized splat-boundary porosity. Oxidation testing at 1000 °C confirmed that HVAF coatings exhibited superior alumina formation, reduced β-phase depletion, and lower weight gain than HVOF coatings. The results establish HVAF as a superior route for producing oxidation-resistant NiCoCrAlY bond coats for advanced gas turbine systems.