Music-mania!: March 2019

Ceramic Matrix Composite (CMC), materials are considered a subgroup of both ceramics and composite materials.

Composite materials are comprised of at least two parts: the reinforcement, which provides special mechanical properties such as stiffness or strength, and the matrix material, which holds everything together. In the case of a ceramic matrix composite, both the reinforcing material (sometimes referred to as refractory fibers) and the matrix material are ceramics. In some instances, the same ceramic material may be used for both the matrix and the fibers, and additional secondary fibers may also be included.

CMC's are normally identified by fiber/matrix. For example, C/SiC would be a carbon fiber in a silicon carbide matrix. The most commonly used CMCs include:

    •    C/C
    •    C/SiC
    •    SiC/SiC
    •    Al2O3/Al2O3

You may see CMC naming conventions that include the manufacturing process, such as LPI-C/SiC.

    •   LPI = liquid polymer infiltration (or polymer infiltration and pyrolysis);
    •   LSI = liquid silicon infiltration;
    •   CVI = chemical vapor infiltration;
    •   CVD = chemical vapor deposition.

The main processes for fabricating CMCs include chemical vapor or liquid phase infiltration, polymer infiltration and pyrolysis (PIP), and hot press sintering techniques. However, the most common method is PIP. In PIP, the ceramic matrix is formed from a fluid that is infiltrated into the fiber reinforcement.

Pyrolysis is defined as a thermal decomposition of an organic (carbon-containing) substance. This decomposition is brought about at high temperatures in the presence of an inert atmosphere and usually takes place in the absence of oxygen. In the context of CMC, pyrolysis causes the substance to decompose into a ceramic and takes place in an argon, nitrogen, or ammonia atmosphere (depending in part on the type of ceramic matrix being used).

Various products are released during pyrolysis, some of which are volatile. These products may include CO, H2, CH2, H20, and CO2. Pyrolysis is preferred over sintering and hot processing. Both sintering and hot pressing can result in high porosity, mechanical damage to the fibers, and fiber degradation as chemical reactions occur at the interface between the fibers and the matrix.