Thermal Barrier Coating System for Gas Turbine Application - A Review

Ceramic coatings are refractory metal compounds deposited on substrates to reduce thermal loss and to protect components from high temperature. Thermal barrier coatings (TBC) are composite overlay of bond coat and ceramic coat on a superalloy substrate. Atomised deposition or splat deposition of fine semimolten particle technique deposits thin coatings of brittle ceramic. Thermal and mechanical strains arising from service Vol. 23, No. 2, 2004 Thermal Barrier Coating System for Gas Turbine Application-A Review reduce porosity of free surface and to increase glaze are other avenues to reduce diffusion of reactive gases and to increase internal heat transfer respectively. The former increases life of bond coat/substrate, whereas the latter increases energy efficiency by maximum utilisation of heat. The main unsolved problem is spallation of ceramic coating, which is cohesively induced in either side of interface and spread out to interfaces of adhesion. TBC increases life more than two-fold for cases of aerospace engines. However localised spallation may rise by high temperature corrosion of bond coat/substrate, TGO stresses, gaseous/liquid contaminant diffusion/ impregnation through tolerance networking of voids, and erosion.