In materials science ceramic matrix composites (CMCs) are a subgroup of composite materials and a subgroup of ceramics. They consist of ceramic fibers embedded in a ceramic matrix. The fibers and the matrix both can consist of any ceramic material, including carbon and carbon fibers.

The motivation to develop CMCs was to overcome the problems associated with the conventional technical ceramics like alumina, silicon carbide, aluminum nitride, silicon nitride or zirconia – they fracture easily under mechanical or thermo-mechanical loads because of cracks initiated by small defects or scratches. The crack resistance is very low, as in glass.

To increase the crack resistance or fracture toughness, particles (so-called monocrystalline whiskers or platelets) were embedded into the matrix. However, the improvement was limited, and those products found application only in some ceramic cutting tools.

The integration of long multi-strand fibers has drastically increased the crack resistance, elongation and thermal shock resistance, and resulted in several new applications. The reinforcements used in ceramic matrix composites (CMC) serve to enhance the fracture toughness of the combined material system while still taking advantage of the inherent high strength and Young's modulus of the ceramic matrix.

The most common reinforcement embodiment is a continuous-length ceramic fiber, with an elastic modulus that is typically somewhat lower than the matrix. The functional role of this fiber is (1) to increase the CMC stress for the progress of micro-cracks through the matrix, thereby increasing the energy expended during crack propagation; and then (2) when thru-thickness cracks begin to form across the CMC at higher stress (proportional limit stress, PLS), to bridge these cracks without fracturing, thereby providing the CMC with a high ultimate tensile strength (UTS). In this way, ceramic fiber reinforcements not only increase the composite structure's initial resistance to crack propagation but also allow the CMC to avoid abrupt brittle failure that is characteristic of monolithic ceramics.

This behavior is distinct from the behavior of ceramic fibers in polymer matrix composites (PMC) and metal matrix composites (MMC), where the fibers typically fracture before the matrix due to the higher failure strain capabilities of these matrices.

Carbon (C), special silicon carbide (SiC), alumina (Al₂O₃) and mullite (Al₂O₃−SiO₂) fibers are most commonly used for CMCs. The matrix materials are usually the same, that is C, SiC, alumina and mullite. In certain ceramic systems, including SiC and silicon nitride, processes of abnormal grain growth may result in a microstructure exhibiting elongated large grains in a matrix of finer rounded grains. AGG derived microstructures exhibit toughening due to crack bridging and crack deflection by the elongated grains, which can be considered as an in-situ produced fibre reinforcement. Recently ultra-high-temperature ceramics (UHTCs) were investigated as ceramic matrix in a new class of CMC so-called ultra-high-temperature ceramic matrix composites (UHTCMC) or ultra-high-temperature ceramic composites (UHTCC).

Generally, CMC names include a combination of type of fiber/type of matrix. For example, C/C stands for carbon-fiber-reinforced carbon (carbon/carbon), or C/SiC for carbon-fiber-reinforced silicon carbide. Sometimes the manufacturing process is included, and a C/SiC composite manufactured with the liquid polymer infiltration (LPI) process (see below) is abbreviated as LPI-C/SiC.