Degradation and Failure Analysis of Environmental Barrier Coatings under Adverse Operating Environment: Multi-Physics Modelling

Abstract

Environmental barrier coatings (EBCs) are essential for protecting ceramic matrix composite (CMC) substrates in advanced gas turbines and aero-engines. However, their durability is limited by thermo-mechanical stresses, oxidation, and water-vapor-induced degradation under high-temperature operating conditions. The premature failure of EBCs, particularly delamination along the interfaces of thermally grown oxide (TGO) poses major challenges to coating reliability. This research presents a systematic study of the degradation of bi-layer Yb₂Si₂O₇/Si EBCs using COMSOL Multiphysics methodologies. Thermal cycle-induced temperature fields were implemented into the EBC model, aiming to simulate the system's in-service conditions. Coupled thermo-mechanical processes were simulated, incorporating high-temperature creep behavior, phase transformation from Yb₂Si₂O₇ to Yb₂SiO₅, and TGO growth kinetics. The model also accounted for the β-α phase transformation of cristobalite TGO between 220°C and 270°C, which introduces significant volumetric changes during cooling. Based on the evaluated local stress evolution and distribution in the EBC system during thermal cycles in water-vapor environments, the experimentally observed EBC degradation mode was explained in terms of the simulated results. Fracture behavior was investigated using J-integral, Virtual Crack Extension, and a phase-field damage model to capture crack initiation and propagation within the coating system. The results show that large tensile stresses in the TGO promote spontaneous crack formation, which can bifurcate and propagate along Yb₂Si₂O₇/TGO and Si/TGO interfaces. Coalescence of these bifurcation-induced delamination cracks could be the primary mechanism leading to the EBC's spallation and failure. The simulated TGO crack growth pattern was compared with that experimentally observed in the literature. Additionally, the effect of interface morphology on crack evolution was systematically investigated. Increasing interface undulation was found to intensify local stress concentrations, enhance mixed-mode fracture driving forces, and accelerate interfacial delamination. The findings of this study enhance the understanding of EBC degradation and provides an integrated multi-physics framework for predicting failure.