2.12 - Mechanical Behavior of SiC Fiber-Reinforced Ceramic Matrix Composites

Abstract

Ceramic–matrix composites (CMCs) are interesting materials for aeronautic applications because of their excellent mechanical properties at elevated temperatures even in oxidizing environments like air. Unlike bulk ceramics, CMCs demonstrate nonbrittle mechanical behavior due to the high strength of the fibers, and optimized fiber/matrix interactions after multicracking of the matrix. Interactions between fibers and matrix occur mainly at the interface. When fibers and matrix debonded in a zone close to a matrix crack, sliding of the bridging fibers in the surrounding matrix results in friction. This friction is characterized by the interfacial frictional shear stress τ and is the main phenomenon leading to energy dissipation during cyclic fatigue of CMCs. Under monotonic tension, the mechanical behavior is nonlinear due to the multicracking of matrix and fiber bridging, while under cyclic or static fatigue at high temperature, interfacial effects change with the number of cycles or during time leading to a change of the energy dissipation and the maximum strain. At higher temperatures, creep of fibers may occur leading to a larger deformation and to a limited lifetime. For nonoxide composites with SiC fibers, lifetime under fatigue may be affected by oxidation. Therefore protection of interphases and of fibers leads to longer lifetimes at high temperatures in air as for example observed on self-healing SiC matrices.