Heat treatment of thermal barrier coatings

Thermal barrier coatings (TBCs) with CoNiCrAlY bond-coat and 0.3 mm thick zirconia top-coat were air plasma sprayed (APS) onto a nickel-base super alloy, Hastelloy-X and 316L stainless substrates. The samples were heat treated at different temperatures. The resulting microstructural features and fracture surfaces of the zirconia top-coat were studied by optical and electron microscopies. Formation of spinels was observed close to the thermal grown oxide (TGO)/top-coat interface, and it was noticed that this layer became thicker during long heat treatments. Top-coat phase stability was assessed by X-ray diffraction, and it was observed that for long heat treatments the top-coat transformed up to 26% into monoclinic phase. Residual stresses and relative interfacial toughness are also important factors, which affect the life of the TBC. They were determined by the hole drill method and by Vickers indentation. The relative interfacial toughness between the top-coat and bond-coat increased with increasing temperature for nickel-base super alloy substrates whereas, for stainless steel based TBCs it increased only up to 650 °C and then decreased with increasing temperature. For long heat treatments, the nickel-base TBCs showed that the interface between top-coat and bond-coat deteriorated with extended exposure time.