Ceramic matrix composite
In materials science ceramic matrix composites 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 zirconiathey 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 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 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 to increase the CMC stress for the progress of micro-cracks through the matrix, thereby increasing the energy expended during crack propagation; and then when thru-thickness cracks begin to form across the CMC at higher stress, to bridge these cracks without fracturing, thereby providing the CMC with a high ultimate tensile strength. 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 and metal matrix composites, where the fibers typically fracture before the matrix due to the higher failure strain capabilities of these matrices.
Carbon, special silicon carbide, 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 were investigated as ceramic matrix in a new class of CMC so-called ultra-high-temperature ceramic matrix composites or ultra-high-temperature ceramic composites.
Generally, CMC names include a combination of type of fiber/type of matrix. For example, C/C stands for carbon-fiber-reinforced 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 process 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
- 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.
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.
For the second step, five different procedures are used to fill the ceramic matrix in between the fibers of the preform:
- Deposition out of a gas mixture
- Pyrolysis of a pre-ceramic polymer
- Chemical reaction of elements
- Sintering at a relatively low temperature in the range
- Electrophoretic deposition of a ceramic powder
The third and final step of machininggrinding, drilling, lapping or milling has to be done with diamond tools. CMCs can also be processed with a water jet, laser, or ultrasonic machining.
Ceramic fibers
Ceramic fibers in CMCs can have a polycrystalline structure, as in conventional ceramics. They can also be amorphous or have inhomogeneous chemical composition, which develops upon pyrolysis of organic precursors. The high process temperatures required for making CMCs preclude the use of organic, metallic or glass fibers. Only fibers stable at temperatures above can be used, such as fibers of alumina, mullite, SiC, zirconia or carbon. Amorphous SiC fibers have an elongation capability above 2%much larger than in conventional ceramic materials. The reason for this property of SiC fibers is that most of them contain additional elements like oxygen, titanium and/or aluminum yielding a tensile strength above 3 GPa. These enhanced elastic properties are required for various three-dimensional fiber arrangements in textile fabrication, where a small bending radius is essential.Manufacturing procedures
Matrix deposition from a gas phase
is well suited for this purpose. In the presence of a fiber preform, CVD takes place in between the fibers and their individual filaments and therefore is called chemical vapor infiltration. One example is the manufacture of C/C composites: a C-fiber preform is exposed to a mixture of argon and a hydrocarbon gas at a pressure of around or below 100 kPa and a temperature above 1000 °C. The gas decomposes depositing carbon on and between the fibers. Another example is the deposition of silicon carbide, which is usually conducted from a mixture of hydrogen and methyl-trichlorosilane. Under defined condition this gas mixture deposits fine and crystalline silicon carbide on the hot surface within the preform.This CVI procedure leaves a body with a porosity of about 10–15%, as access of reactants to the interior of the preform is increasingly blocked by deposition on the exterior.
Matrix forming via pyrolysis of C- and Si-containing polymers
polymers shrink during pyrolysis, and upon outgassing form carbon with an amorphous, glass-like structure, which by additional heat treatment can be changed to a more graphite-like structure. Other special polymers, known as preceramic polymers where some carbon atoms are replaced by silicon atoms, the so-called polycarbosilanes, yield amorphous silicon carbide of more or less stoichiometric composition. A large variety of such silicon carbide, silicon oxycarbide, silicon carbonitride and silicon oxynitride precursors already exist and more preceramic polymers for the fabrication of polymer derived ceramics are being developed. To manufacture a CMC material, the fiber preform is infiltrated with the chosen polymer. Subsequent curing and pyrolysis yield a highly porous matrix, which is undesirable for most applications. Further cycles of polymer infiltration and pyrolysis are performed until the final and desired quality is achieved. Usually, five to eight cycles are necessary.The process is called liquid polymer infiltration, or polymer infiltration and pyrolysis. Here also a porosity of about 15% is common due to the shrinking of the polymer. The porosity is reduced after every cycle.