Effect of residual stresses on air plasma sprayed thermal barrier coatings

Thermal barrier coating with a CoNiCrAlY bond coating and a 0.3 mm thick zirconia, (7% yttria stabilized) top coating were air plasma sprayed onto a Hastelloy-X nickel based super alloy substrate coupons. Substrate preparation is a key initial step in the production of quality thermal sprayed coatings. Grit blasting is usually used to roughen the substrate surface so that better adhesion of the coating to the substrate is achieved. The effects of different grit blasting variables on the substrate surface roughness were examined by using the Taguchi designed experimentation. The residual stress profiles were determined for a different set of conditions by hole drill method. The Anstis et al. model was modified for the determination of indent toughness of the substrate and bond coat interface. The results showed that the combination of grit size with distance and pressure during the grit blasting were the most influential parameters. The adhesion of the coatings increases with increase of substrate roughness up to certain limits and then decreases. It was observed that the residual stresses have an impact over the adhesion of the coatings. The indentation toughness results showed that with increase of substrate roughness there is an increase in interfacial toughness due to high compressive stresses associated with high rough surfaces. Further, the final deposition temperature and heat treatment effect the residual stress profile and subsequently the interfacial toughness of the coatings.