Thermochemical Interaction of Thermal Barrier Coatings with Molten CaO–MgO–Al₂O₃–SiO₂ (CMAS) Deposits

Abstract

Thermal barrier coatings (TBCs) are increasingly susceptible to degradation by molten calcium-magnesium alumino silicate (CMAS) deposits in advanced engines that operate at higher temperatures and in environments laden with siliceous debris. This paper investigates the thermochemical aspects of the degradation phenomena using a model CMAS composition and ZrO 2 -7.6%YO 1.5 (7YSZ) grown by vapor deposition on alumina substrates. The changes in microstructure and chemistry are characterized after isothermal treatments of 4 h at 12001-14001C. It is found that CMAS rapidly penetrates the open structure of the coating as soon as melting occurs, whereupon the original 7YSZ dissolves in the CMAS and reprecipitates with a different morphology and composition that depends on the local melt chemistry. The attack is minimal in the bulk of the coating but severe near the surface and the interface with the substrate, which is also partially dissolved by the melt. The phase evolution is discussed in terms of available thermodynamic information.

Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties

“…A similar degradation mechanism at high operation temperatures is caused by the environmentally ingested airborne sand/ash particles melt on the hot TBC surfaces resulting in the deposition of the CMAS glass deposits (Ref [46][47][48]. At high surface temperatures, the CMAS rapidly penetrates the porosity of the coating and lead to premature failure of it as a consequence of mechanical and chemical interactions.…”

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

“…But it undergoes significant sintering above 1200 º C, which is not desirable [4]. Furthermore, YSZ is susceptible to CMAS infiltration, which results in loss of strain tolerance and change in near surface mechanical properties leading to early coating spallation [5][6][7][8][9][10][11][12][13].…”

Degradation of Thermal Barrier Coatings from Deposits and Its Mitigation

“…This type of damage to 7YSZ TBCs, both APS and EB-PVD, is similar to what has been observed in aircraft gas-turbine engines but from deposits of molten calciummagnesium-alumino-silicate (CMAS) sand from the environment [15][16][17][18][19][20][21][22][23][24]. The molten CMAS glass penetrates the TBC microstructure and forms a relatively stiff glaze upon cooling [15][16][17][18][19]24].…”

“…The molten CMAS glass penetrates the TBC microstructure and forms a relatively stiff glaze upon cooling [15][16][17][18][19]24]. This results in the reduction of TBC strain-tolerance during engine heating-cooling cycles, causing the TBC to detach from the substrate much more quickly than if the CMAS was not present [20,21].…”