Blistering of hafnia/hafnon composites exposed to molten silicate deposits

Abstract

Environmental barrier coatings (EBCs) are critical for protecting ceramic matrix composites in gas turbine engines. However, their effectiveness can be compromised by blistering when exposed to molten silicate deposits. This study investigates the mechanisms driving blistering in hafnia/hafnon EBCs through numerical modelling. Two potential mechanisms are explored: one driven by partial dissolution of the parent phases into the melt followed by crystallization into the other phase (hafnia to hafnon or vice-versa) and another driven by partial dissolution and reprecipitation into the same phase via Ostwald ripening. Models for both mechanisms make use of the observations that melt ingress and macroscopic blistering are decoupled in time, with melt ingress occurring relatively rapidly along grain triple junctions and blistering occurring later through a slower process of melt spreading, dissolution and crystallization or reprecipitation. A finite element model incorporating randomly shaped grains and cohesive grain boundaries is developed to simulate these two mechanisms. Phase transformation-driven cracking is found to produce minimal damage and negligible changes in pellet thickness, inconsistent with experimental observations. In contrast, the Ostwald ripening mechanism generates damage patterns and pellet thickness increases closely resembling experimental blistering observations, even though the ripening process produces only modest stresses. Combined effects of phase transformation and Ostwald ripening show no significant synergy. These findings suggest Ostwald ripening as the dominant mechanism responsible for blistering in hafnia/hafnon EBCs exposed to molten silicate deposits. The study provides valuable insights into EBC failure modes and offers a framework for investigations of similar phenomena in other coating systems.