Coating recession effects in ceramic composite strength

Abstract

BN fiber coatings in SiC-SiC composites are vulnerable to oxidation and volatilization at elevated temperature in the presence of water vapor. These processes lead to coating recession in the composite interior with recession fronts starting from matrix cracks and proceeding axially along the fibers. In some operational domains, the main effect of recession is to de-couple the fibers from the matrix, precluding load transfer that would otherwise occur through frictional sliding. Here we present a modelling framework to address effects of coating recession on composite strength. The framework is built upon the characteristic strengths and transfer lengths that govern fiber and matrix fragmentation during tensile loading. Four dominant behavioral domains are identified and analyzed. Recession causes shifts in the domain boundaries and reductions in composite strength. Variations in degraded strength with the extent of recession are interpreted in terms of stress rupture plots. In the regime where recession leads to time-dependent fracture, the composite response is defined by two strengths: (i) the pristine composite strength (governed by the in situ fiber bundle properties) and (ii) a threshold stress below which failure is not predicted to occur (governed by the strength of an equivalent fiber bundle in the absence of a matrix). The time scales are set by the square of the ratio of recession length and the matrix crack spacing, the latter governed by a combination of the matrix strength distribution, interface sliding stress, fiber content and applied stress.