Applications of thermal barrier coating system in gas turbines - A review

Abstract

Ceramic coatings are refractory metal oxides deposited on substrate to reduce thermal loss and to protect components, being operuted at high temperature. Thermal barrier coatings (TBC) are composite overlay of bond coat and ceramic coat on a superallor substrate implemented in combustion engines. Thermal and mechanical strains of service exposure requires structural compliance tolerances. This is facilitated by brittle constituent deposition technique over a ductile substrate. Electron beamphysical vapour deposition and plasma spray technique provides this from tortuous intergrunular network of coating. Porous deposition technique is applied in most cases instead of cementation or continuous section thickness. Thermal barrier coating is inevitable in aerospace engine sections operating at limiting conditions of strains. This is attributed to reduction of temperature levels and grat1iets.Subsequent advantages are 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 ill-situ formation and transformation mechanisms of alumina from aluminium of substrate/bond coat and metastable tetragonal zirconia to stable tetmgonal zirconia respectively at temperature of service. The former produces a volumetric contraction whereas later produces volumetric expansion. In service the composite system provides an auto-toughening effects in due courses. Mechanical behaviour of TBC is attributed to the intergranular tortuous network. This on strain exposure form cracks and at the second stage of crack tip blunting form cubic allotropy from metastable tetragonal phase. Reduction of cia ratio thus increases toughness. However a prolonged exposure form localised spallation zones being initiated by volumetric expansion stresses associated with nickel enrichment of thermally grown oxides (TBC) at bond coat/ceramic coat interface and auto-sintering. TBC interaction directly with substrate is a wide variation of ductility. So bond coat is applied to produce mechanical adherence and stress relaxation effects. Generally M-CrAIY family of bond coating alloys are used. Exposure to operatingltest temperature produces thermally grown oxides (TGO) at interface. This occupies an intermediate zone in response to property interactions. TGO mainly consists of alumina being catalysed by chromia and adhered by yttria. Current scenario is to study contmdiction between auto-sintering and auto-toughening mechanisms and to decrease effects of thermal expansion mismatch stresses by application of composite coatings made from functionally graded materials microlaminated,and multilayeredceramic/ceramicor metallic/ceramicor metallic/metallic coatings. Application of laser scaling or remelting to reduce porosity of free surface and to increase glaze is another avenues to reduce diffusion of reactive gases and to increase internal heat transfer respectively. The former state increases life of bond coat/substrate, where as later increase energy efficiency by maximu mutilisation of heat.The main unsolved problems are spallation of ceramic coating being cohesively induced in either side of interface and spread out to interfacesof adhesion. TBC increase life more than twice for cases of aerospace engines however localised spallation by high temperature corrosion of bond coat/substrate, TGO stresses, gaseous/liquid contaminant diffusion/impregnation through tolerance networking of voids. erosion and their addItive effects increase pressure over preventive measure.