On the resistance of rare earth oxide-doped YSZ to high temperature volcanic ash attack

Abstract

Thermal barrier coatings (TBCs), such as 7-8 wt.% yttria stabilized zirconia (YSZ), are widely used to reduce the temperature of coated materials. However, when a turbine operates in a harsh environment, for example a volcanic ash attack, sand and ash particles ingested by the engine could be deposited on the TBC surfaces as molten calcium-magnesium-alumino-silicate (CMAS). CMAS melts and penetrates into TBCs at high temperatures, which causes a loss of strain tolerance and results in the premature failure of the top coat. It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash; therefore, CMAS mitigation solutions are among the top challenges for materials scientists. To some extent, CMAS is the biggest weakness of the traditional 7-8YSZ TBC material. The thermochemical interactions between YSZ and real volcanic ash are investigated in this paper as a means to alleviate the volcanic ash attack by understanding the underlying mechanisms. 7YSZ, 0.5Gd-8YSZ and 3Er-7YSZ powders and free-standing plates were prepared, respectively. The effect of the volcanic ash on the three different samples was investigated using an X-ray diffraction, scanning electron microscopy, a scanning transmission electron microscopy, and a differential scanning calorimetry. The phase transformations and reaction of TBCs materials subjected to a volcanic ash attack were examined and the volcanic ash degradation and mitigation mechanisms were discussed. It was found that rare earth-doped YSZ samples suffered less damage from the volcanic ash attack than the standard 7YSZ. The results demonstrate that rare earth-doped YSZ may be effectively utilized to mitigate a volcanic ash attack.

Thermodynamics of reaction between gas‐turbine ceramic coatings and ingested CMAS corrodents

“…Cubic, tetragonal and monoclinic zirconia and quartz were detected up to 1161 K. Tetragonal and cubic zirconia, walstromite and quartz were detected at 1263 K. Tetragonal and cubic zirconia, and quartz were detected between 1364 and 1465 K. Tetragonal and cubic zirconia, and zirconium silicate were detected between 1566 and 1666 K. The phase transformations in 7YSZ are related to dissolution and reprecipitation of material during reaction with CMAS . Zircon detected in this work also has been detected in the reaction between 7 and 8 wt% YSZ and volcanic ash at 1250°C for 5 h . The NASA CMAS is mainly amorphous and contains small amounts of akermanite, wollastonite, and quartz, and its amorphous phase crystallizes into diopside, akermanite, and wollastonite around 1114‐1229 K …”

Thermal Barrier Coatings—A State of the Art Review

“…The current work mainly deals with the development of the coating by nano-size ceramics powders, cermet powders, and powders doped with rare earth oxides and these modification are mainly performed to alter the microstructure of the coating [54,[60][61][62][63]. Addition of the Alumina in YSZ powder is done to produce a coating with increased resistance to diffusion of detrimental species.…”

Pore filling behavior of YSZ under CMAS attack: Implications for designing corrosion‐resistant thermal barrier coatings

“…Many CMAS mitigation strategies have been proposed in the literature . Most approaches aim to manipulate the chemical reaction between TBCs and CMAS by changing the chemical composition of TBCs.…”

“…15,16 Many CMAS mitigation strategies have been proposed in the literature. [17][18][19][20][21][22][23][24][25][26][27][28][29] Most approaches aim to manipulate the chemical reaction between TBCs and CMAS by changing the chemical composition of TBCs. This kind of approach causes the crystallization of CMAS and the attendant arrest of the infiltrating CMAS front.…”