Finite element modelling of thermal stresses in ceramic-matrix composites

The combination of computational prediction and microstructural control in ceramic-matrix composites is a powerful method for the better understanding of CMC behaviour In the present work sialon and zirconia matrices have been separately reinforced with carbon microbeads and fibres using standard fabrication techniques based on cold isostatic pressing and controlled atmosphere firing. The result of such processing is that residual stresses arise during cooling from sintering temperature as a result of the difference in coefficient of thermal expansion between the matrix and reinforcement and these stresses have a major effect on the mechanical behaviour of the material. This study compares the behaviour of sialon and zirconia composites reinforced with carbon microbeads and short fibres and shows that in the case of sialon the hardness decreases and toughness increases, but for zirconia there is a decrease in both strength and toughness compared to the monolithic material. The behaviour of these materials has also been assessed by finite element calculation of the thermal stresses. Assuming that chemical incompatibility between the two phases does not result in undue degradation of the fibre or matrix (that is, there is a discontinuous interface) then the stress state of the composite at room temperature can be modelled. The results are consistent with the experimental observations and give some insight into the behaviour of the interface between the matrix and reinforcement.