Calcia–magnesia–alumina–silicate (CMAS) attack mechanisms and roadmap towards Sandphobic thermal and environmental barrier coatings

ABSTRACT This review critically examines the current understanding of calcia–magnesia–alumina–silicate (CMAS) degradation mechanisms and mitigation approaches in thermal and environmental barrier coatings. First, the review introduces case studies of field returned engine components exposed to CMAS attack, followed by fundamental aspects of CMAS-induced degradation. Understanding CMAS adhesion, infiltration, spallation mechanics, and thermochemical attack mechanisms is crucial to designing materials approaches to mitigate CMAS attack. CMAS mitigation strategies have focused on reactive approaches aimed at crystallising molten CMAS at the earliest stage possible to inhibit infiltration. Promising approaches are presented, starting with fundamental reaction kinetics studies, followed by the effects of microstructure in actual coatings systems. Salient results on coating systems tested in various burner rigs and a full engine test are presented to benchmark the success of various mitigation strategies. Lastly, several key future research areas are presented in order to provide a roadmap towards ‘sandphobic’ thermal and environmental barrier systems.