Relation between interphase porosity and crack deflection tendency in all-oxide Ceramic Matrix Composites

Abstract

The concept of Ceramic Matrix Composites (CMC) is aimed at overcoming the brittleness of conventional monolithic ceramics while retaining their favourable properties, such as high-temperature stability and thermal shock resistance. This concept is based on a sufficiently weak bonding between the ceramic fibres and the ceramic matrices, which is realized either by applying fibre coatings or by using low-strength, highly porous matrix interphases. Both approaches assure fibre-matrix de-bonding under mechanical loading, which is required to use the comparatively high failure strain of the fibres and to enable an overall damage tolerant behaviour of CMCs. During the service life of CMC components, however, the fibre-matrix interface or interphase is subject to changes: Due to exposure to high application temperatures, sintering processes re-initiate, leading to morphological alterations in the CMC microstructure, which ultimately affect the CMCs damage tolerance. For an all-oxide CMC with porous fibre-matrix interphase, the effect of varying interphase porosity on the expectable damage tolerance has been studied. The relation between interphase porosity and the tendency of a matrix crack to deviate in the vicinity of an embedded fibre was described by a simple analytical equation. The interphase properties required for solving this equation were predicted using microstructure-based numerical models. Experimental investigations were carried out to evaluate the analytical approach.