Influence of porosity and fibre coating on engineering elastic moduli of fibre-reinforced ceramics (SiC/SiC)

Porosity generally has a large influence on the modulus of monolithic ceramics. Little, however, is known about its influence in fibre-reinforced ceramics. In the present investigation the influence of porosity on the elastic moduli, i.e. the longitudinal modulus E_L, the transverse modulus E_T and the longitudinal shear modulus G_LT, of unidirectional SiC/SiC ceramic matrix composites is studied. SiC fibres (Nicalon NLM 202) with pyrocarbon coatings of different thicknesses were chemically vapour impregnated by SiC. The axial tensile modulus proved to be little influenced by a porosity content of 13–14%. In fact, the measured modulus was 4–7% smaller than the value predicted with the classical linear rule of mixtures making use of elastic constants of each component and assuming porosity does not reduce the moduli of the different components. On the other hand, the shear and transverse moduli are excessively reduced by the presence of porosity. For a porosity content of 14%, the reduction is roughly 50 and 60% compared with theoretical values for pore insensitive and dense material computed with the model of composite cylinder assemblage and the polarization extremum principle. Finally, the influence of pore morphology on the elastic moduli is discussed.