Thermostructural Ceramic Matrix Composites: An Overview

Ceramic matrix composites (CMCs) consist of a ceramic matrix reinforced with ceramic fibers. They have been designed to be used in severe environments. With respect to monolithic ceramics, CMCs are characterized by a non-linear stress/strain mechanical behavior, a high resistance to crack propagation, a non-catastrophic failure and thus an improved reliability. CMCs are processed according to either a gas phase route (the matrix being deposited chemically from a gaseous precursor in the pores of a fiber preform) or a liquid phase route (by hot-pressing fiber tapes impregnated with a liquid precursor of the matrix). The most important CMCs are those made from SiC-based fibers embedded in glassceramic or SiC matrices. CMCs are inverse composites with the result that under loading the matrix fails first. They exhibit a tough behavior only when the fibers are weakly bonded to the matrix: the fiber/matrix interfaces arresting or deflecting the matrix microcracks preventing thus the early failure of the fibers and a catastrophic propagation of a macrocrack. The control of the fiber/matrix bonding is achieved through the use of an interphase material (e.g. a layer of carbon or BN) and a proper choice of the respective thermal expansion coefficients. Since long exposures to oxidizing atmospheres at high temperatures may have a detrimental effect (e.g. a tough/brittle transition due to the oxidation of the interphase), most CMCs receive a protective surface treatment, however, some of them exhibit a self-healing character.