Disclosing residual thermal stresses in UHT fibre-reinforced ceramic composites and their effect on mechanical behaviour and damage evolution

The reusability of CMCs with inherently high-temperature capabilities and ablation resistance goes through understanding of the effects of residual thermal stresses (RTS) caused by thermal expansion mismatch between fibre and matrix. In this work, the RTS of ultra-high temperature ZrB2-SiC based ceramic reinforced with carbon fibres was dependent on fibre anisotropy and fibre coating. A detailed microstructural analysis about the state of matrix (i.e. residual porosity and cracks) and fibre (i.e. porosity within the fibre bundles, volumetric content of the fibres and their degree of dispersion) was performed. The different level of RTS, among the composites, was studied by comparing stiffness-displacement curves of the bending test and the cyclic dilatometric curves. The correlation between the level of RTS and thermomechanical behaviour was also evaluated by flexural tests at 1500 C and after thermal shock. The results showed that RTS not only reduced flexural strength, but also influenced damage evolution and stiffness linearity. As a result, the release of RTS can alter not only the pristine geometry, obtained after sintering or machining, due to the formation of inner freed fibres (IFFs) but also the Young's modulus of the matrix and the stress and strain at failure onset of the investigated CMCs.