CMAS resistance of high-entropy (Dy0.2Ho0.2Er0.2Tm0.2Lu0.2)2Hf2O7 in-serviced as top TBC of Si-HfO2/Yb2Si2O7/Yb2SiO5 EBCs system: an experimental and atomistic modeling study

Abstract

The creation of a modified T/EBC system was driven by the absence of a proven strategy to enhance the intrinsic resistance of silicate EBCs to molten silicate deposits (CMAS). To address this, a high-entropy hafnate material with a defect-fluorite structure was designed using five rare earth (RE) elements: Dy, Ho, Er, Tm, and Lu. Quadruple-layer coatings consisting of (Dy_0.2Ho_0.2Er_0.2Tm_0.2Lu_0.2)₂Hf₂O₇/Yb₂SiO₅/Yb₂Si₂O₇/Si-15 mol% HfO₂ ((5RE)₂Hf₂O₇/YbMS/YbDS/15SH), prepared by plasma spray-physical vapor deposition (PS-PVD), were deposited to protect ceramic matrix composites (CMC). These coatings were evaluated for their interfacial thermochemical stability and resistance to CMAS attack. The study revealed no thermochemical reactions at the interface between the (5RE)₂Hf₂O₇ and YbMS layers following isothermal annealing at 1300 °C, 1400 °C, and 1500 °C. The (5RE)₂Hf₂O₇ layer acts as a reactive barrier, resisting CMAS infiltration and maintaining the integrity of the coating. At 1300 °C and 1400 °C, reactions between the melt and the coating result in the formation of RE-garnet phases on the surfaces. At 1500 °C, RE-garnet phases were observed at the interface between the (5RE)₂Hf₂O₇ and YbMS layers, along with the consistently observed apatite and fluorite reaction products. Most of the RE³⁺ ions dissolved from the (5RE)₂Hf₂O₇ TBCs were captured by apatite, maintaining the crystallization stability of the reaction products and acting as a permeability barrier. Furthermore, melt/coating interaction models were developed using the ab initio molecular dynamics method (CP2K package) to understand coating/deposit reactions at the atomic level, including diffusivity dynamics and diffusion energy barriers. During the cooling stage, vertical mud-cracks and bifurcated microcracks are induced by thermal stresses resulting from the mismatch in coefficients of thermal expansion (CTE) between the corroded layer and the underlying layer. As CMAS continues to infiltrate the coating, a Yb-bearing apatite layer forms and gradually thickens through the dissolution and reprecipitation reactions with the YbMS. This corrosion layer introduces additional compressive stress on the unreacted YbMS layer, promoting the formation of more bifurcated microcracks. With further infiltration of newly molten CMAS, these microstructural damages accelerate the overall degradation of the coating system.