Thermal barrier coating system for gas turbine application - A review

Abstract

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 oil a superalloy substrate. Atomised deposition or splat deposition of fine semi-molten particle technique deposits thin coatings of brittle ceramic. Thermal and mechanical strains arising from service exposure require structural compliance tolerances. This is facilitated by brittle constituent deposition over a ductile substrate. Electron beam, physical vapour deposition and plasma spray technique lead to a tortuous intergranular network of coating Porous deposition technique is applied in all cases instead of cementation or continuous section thickness. Thermal barrier coating is inevitable in aerospace engine sections operating at limiting conditions of strains. Thermal barrier coatings help in protection of high temperature components for maximum utilisation of component lives, and maximum utilisation of energy by operating at optimum allowable temperature limits. Thermo mechanical behaviour of TBC is optimised by in-silu formation and transformation mechanisms of alumina from aluminium of substrate/bond coat and metastable tetragonal zirconia to stable tetragonal zirconia respectively at temperature of service. While the former produces a volumetric contraction, the latter produces volumetric expansion. In service the composite system provides auto-toughening effects in due course. An intergranular tortuous network of coating forms cracks on exposure of strain and the crack tip blunting forms cubic allotropy from metastable tetragonal phase, resulting in an increase in toughness due to elimination of c/a ratio. However, a prolonged exposure forms, localised spallation zones, which are initiated by volumetric expansion stresses associated with nickel enrichment of thermally grown oxides (TGO) at bond coat/ceramic coat interface, and auto-sintering. Bond coat is applied to produce mechanical adherence and stress relaxation effects. Generally M-CrAlY families of bond coating alloys are used for this purpose. Exposure to operating/test temperature produces thermally grown oxides (TGO) at interface. This occupies an intermediate zone in response to property interactions. TGO mainly consists of alumina being catalyzed by chromia and adhered by yttria. Active research is going oil to study the mechanisms of auto sintering and auto-toughening of TBC. Work is in progress to explore how to decrease thermal expansion mismatch stresses by application of composite. coatings made from functionally graded materials, microlaminated, and multilayered ceramic/ceramic or metallic/ceramic or metallic/metallic coatings. The application of laser scaling or remelting to 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.