Thermal shock of fibre-reinforced CMCs

With optimum selection of fibres and matrices, favourable residual stress conditions can be established in the matrix, which lead to increased A Tc. Above A Tc, matrix cracks appear but the presence of crack-deflecting fibre-matrix interfaces ensures minimal effect on mechanical properties as the fibres remain largely unaffected. As damage is also confined mostly to the surface of the materials, changes in mechanical and thermal properties are more readily identified by means other than mechanical testing. 

In the following paragraphs an overview of damage due to thermal shock and its effect on the mechanical properties of CMCs with different fibre architectures is provided for a number of different reinforcement architectures. Subsequently, the effect of thermal shock on interfacial properties is discussed, followed by a description of attempts to analyse and model the thermal shock behaviour of these materials. 

2 Thermal shock damage and its effect on mechanical and thermal properties 

In the LAS-matrix system matrix cracks parallel to the fibre axis were observed at ATc = 800°C, accompanied by a reduction in Young s modulus, although flexure strength seemed to remain unaffected by thermal shock treatment. This was attributed to the difference in the direction of matrix crack propagation in the two composites due to the formation of a-spodumene- 

Multiple matrix cracking perpendicular to the fibre axis was also reported by Blissett et al. (1997) for a UD Nicalon /CAS (calcium aluminosilicate) (Fig. 15.6). The density of these cracks increased with increasing AT but showed a reduction for AT 800°C, which seemed to be consistent with the formation of strong silica bridging between the matrix and the fibres. 

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