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Damage due to thermal shock in cross-ply Nicalon/calcium aluminosilicate ceramic matrix composites has been investigated. Heated specimens of two simple [(0º/90º) s and (90º/0º) s ] and two multi-layer [(0º/90º) 3s and (90º/0º) 3s ] materials were quenched into water at room temperature. Crack morphologies were assessed by reflected light microscopy and scanning electron microscopy.The use of image assembling software allowed the generation of reflected light microscopy images of all of the thermally-shocked surfaces onto which the crack patterns were then superimposed. This allowed clear identification of damage mechanisms and accurate quantification of damage accumulation with increasing severity of thermal shock. Damage was first detected in the central plies of each composite. Composites with 0º central plies exhibited slightly higher resistance to thermal shock than their counterparts with 90º central plies.Although damage extended to the outer plies as the severity of the shock increased, crack density was found to vary with position at every shock: it was highest in the central plies and gradually reduced towards the outer plies.Multiple matrix cracking perpendicular to the fibre direction was the damage mode identified in 0º plies, while 90º plies contained cracks that ran along the ply length. At more severe shocks the morphology of these crack patterns was affected in significantly different ways. In addition, the thinner, simple cross-ply * Author to whom all correspondence should be addressed composites exhibited much higher resistance to thermal shock than their multilayer counterparts.

The behaviour of two Nicalon/calcium aluminosilicate ceramic composite laminates (a (±45°) 3s and a plain-weave woven) under conditions of thermal shock has been studied. Test specimens heated at various temperatures were quenched into room-temperature water. This was followed by detailed damage characterisation. In addition, post-shock mechanical properties were assessed by tensile tests (for the woven laminate) and flexural tests (for both laminates). Both materials were found to have comparable thermal shock resistance. Crack morphologies comprised matrix cracks of various orientations that exhibited similar characteristics to those described for thermally shocked cross-ply laminates with the same constituents, but cracking was found to be less widespread in the woven laminate. Fibre breaks were also detected on the woven material when high-temperature degradation of the fibre-matrix interface was present. A gradual reduction in properties (stiffness, proportional limit stress, fracture strength) of thermally shocked specimens was identified, which began at larger shocks than those at which thermal shock damage initiated. This was attributed to the extension of some matrix cracks into the bulk of material.

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