A study of localised fracture events in continuous fibre reinforced ceramic matrix composites.

Various aspects of localised fracture events in glass-ceramic matrices (calcium alumino-silicate (CAS) and barium magnesium alumino-silicate (BMAS)) reinforced with continuous SiC fibres (Nicalon(TM) and Tyranno(TM)) have been examined. An analytical model has been developed which enables the residual thermo-elastic stresses that are present in the composites at room temperature (as a result of the difference in coefficient of thermal expansion (CTE) between the fibres and the surrounding matrix) to be calculated. A simple consideration of the effect of introducing a free surface coupled with experimental results has shown that a portion of the fibre/matrix interface is likely to be debonded in the vicinity of the free surface. Quasi-static indentation loading of the composites has shown that lateral cracks are the predominant fracture event. The technique of confocal scanning laser microscopy has been used to provide quantitative data relating to the size of these lateral cracks as well as their sensitivity to the local fibre volume fraction. In a CAS/Nicalon composite, where the CTE of the matrix is greater than that of the fibres, lateral cracks have been found to form preferentially in regions of relatively high local fibre volume fraction. This behaviour is consistent with residual stress calculations, in that matrix in regions of high fibre volume are in a state of axial tension, which promotes lateral cracking. The converse has been found to be true in a BMAS/Tyranno system with a CTE mismatch in the opposite sense. Experiments where contact is by single particle impact have shown similar fracture patterns, suggesting that dynamic fracture follows mechanisms observed during quasi-static indentation. Utilisation of data acquired from quasi-static indentations has facilitated an upper bound prediction of the erosion wear rate. The mechanism of material removal and the estimate of wear rate have been shown to be consistent with results acquired from a limited study of erosive wear in one of the composite systems