Transition from penetration cracking to spallation in environmental barrier coatings on ceramic composites

Abstract

The article addresses the competition between driving forces for penetration cracking and for spallation in bilayer environmental barrier coatings on SiC-based composites. Coatings of interest consist of an outer rare-earth monosilicate (MS) and an inner rare-earth disilicate (DS). Finite element analysis is used to compute energy release rates (ERR) for cracking as functions of misfit strains, layer thicknesses and size of putative cracks. The results indicate that, for certain property combinations, ERRs for spallation in the DS layer exceed those for continued penetration. The implication is that penetration cracks could branch and cause coating spallation. A case study comparing bilayers of Yb- and Y-silicates reveals domains in which (i) the tolerable thickness of MS is high, but eventual spallation could potentially remove the entire DS coating along with the adjoining MS, and (ii) the tolerable thickness of MS is low, but, if it were to occur, spallation would follow a path near the MS/DS interface, leaving most of the DS intact. The behaviors prove to be sensitive to the thermal expansion coefficient of the underlying composite, even over the rather narrow range of values typical of SiC-based composites.