Modeling of oxidation behavior of SiC-reinforced ceramic matrix composites

Internal oxidation of SiC reinforcement is a major factor affecting the environmental stability of SiC reinforced ceramic matrix composites (CMCs) for high temperature applications. A simple phenomenological model describing the unidirectional oxidation of SiC reinforced oxide CMCs is presented. The model allows to calculate the thickness of the silica layer formed on a SiC reinforcement as a function of its location (depth beneath the surface) and time, if the oxygen permeabilities of silica and the matrix are known. The oxidation mode can thereby be predicted. Alternatively, the model allows to evaluate the oxygen permeabilities of silica and the matrix from the experimental oxidation data. Moreover, the expected mode of oxidation, I or II, can be predicted depending on oxygen permeabilities and volume fraction of the reinforcement phase. Application of the model to the results of the microscopic study of the oxidation of SiC reinforced mullite–zirconia matrix composites allowed to evaluate oxygen permeabilities of the matrix and of the growing silica layer on the SiC particles. It was found that while oxygen permeability of the silica layer on the SiC particles may depend significantly on the type of SiC reinforcement, it is reasonably close to the values obtained from the experiments on direct oxidation of SiC and permeation through vitreous silica. Oxygen permeability of the mullite–ZrO2 matrix showed a dependence on the microstructure and composition of the matrix.