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.
Introduction
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 () and mullite () 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.
The important commercially available CMCs are C/C, C/SiC, SiC/SiC and . They differ from conventional ceramics in the following properties, presented in more detail below:
Elongation to rupture up to 1%
Strongly increased fracture toughness
Extreme thermal shock resistance
Improved dynamical load capability
Anisotropic properties following the orientation of fibers
Manufacture
The manufacturing processes usually consist of the following three steps: Lay-up and fixation of the fibers, shaped like the desired component Infiltration of the matrix material Final machining and, if required, further treatments like coating or impregnation of the intrinsic porosity.
The first and the last step are almost the same for all CMCs: In step one, the fibers, often named rovings, are arranged and fixed using techniques used in fiber-reinforced plastic materials, such as lay-up of fabrics, filament winding, braiding and knotting. The result of this procedure is called fiber-preform or simply preform.