Unraveling the CMAS corrosion mechanism of CrTaO4: A promising dual function oxidation and thermal protection material for RHEAs

Abstract

CrTaO₄ has been found to play a pivotal role in the protection of refractory high-entropy alloys (RHEAs) from high-temperature oxidation and thermal attack due to its high melting point, low thermal conductivity, close thermal expansion coefficient (TEC) to RHEAs. These appealing properties enable CrTaO₄ as a new type of protective scale material for high-temperature applications such as in air breathing jet engines. For such engine applications, CaO-MgO-Al₂O₃-SiO₂ (CMAS) corrosion is a critical issue. However, the corrosion behavior of CrTaO₄ under CMAS attack remains unknown so far. Here, the corrosion resistance of CrTaO₄ to molten CMAS is comprehensively studied. It is demonstrated that the CMAS corrosion resistance is significantly superior over commercial yttrium-stabilized zirconia and the commonly investigated thermal barrier coating materials. Element and phase compositional analyses indicate dense and CMAS corrosion-resistant layers are established between CMAS and the CrTaO₄ substrate. The interface reaction between the CrTaO₄ substrate and CMAS at 1250 and 1300°C gives rise to a dense layer composed of CaTa₂O₆ and Mg(Cr,Al)₂O₄ spinel just beneath the molten CMAS. At 1350°C, a phase composition gradient layer, composed of crystalline phases CaTa₂O₆/CaTa₂O₆ + Mg(Cr,Al)₂O₄/CaTa₂O₆ + Cr₂O₃, is formed. With increased calcium consumption due to more Ca-containing crystalline phase formation upon elevating temperature, the Ca/Si ratio in CMAS melt declines, thereby increasing the viscosity of the melt and mitigating the penetration of CMAS into the CrTaO₄ substrate.