Ceramic Matrix Composites

August 26, 2016

Ceramic matrix composite materials (CMCs) are formed from ceramic fibers embedded in a ceramic matrix and have the potential to overcome many of the problems associated with conventional technical ceramics, such as their tendency to fracture easily because of cracks initiated by small defects.

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Ceramic matrix composites (above at 500x magnification) can overcome the problems usually associated with conventional ceramics.

Ceramic matrix composite materials (CMCs) are formed from ceramic fibers embedded in a ceramic matrix, known as ceramic fiber reinforced ceramic material. CMCs have the potential to overcome many of the problems associated with conventional technical ceramics, such as their tendency to fracture easily because of cracks initiated by small defects. Conventional ceramics have low crack resistances, so it is very easy for a crack to form and cause fracture. When particles are embedded in the matrix to increase crack resistance, there is a limited improvement in mechanical properties. However, when long, multi-strand fibers are integrated into the ceramic matrix, crack resistance, elongation, and thermal shock resistance are all increased dramatically.

CMC Manufacturing

CMCs are manufactured in much the same way as conventional composites. The manufacturing process begins with lay-up of the fibers, which are shaped to create the preform. Then, the matrix material is infiltrated into the fiber layers in a variety of ways including deposition out of a gas mixture, pyrolysis of a pre-ceramic polymer, chemical reaction of elements, sintering at a relatively low temperature (1000-1200 deg. C), and electrophoretic deposition of a ceramic powder which has not yet been established in industrial processes. Finally, the part is machined for post-production and further treatments (coating or porosity impregnation) are applied.

A Quasi-Plastic Material

Under a mechanical load, the ceramic matrix cracks at 0.05% elongation like conventional ceramics do. To increase the strength of the material, embedded fibers bridge the cracks. This process only works when the matrix can slide along the fibers, so the bond between the fibers and matrix must be weak. CMC tensile tests generally show nonlinear stress-strain curves, which might lead an observer to believe that the material is experiencing plastic deformation. However, CMCs are termed quasi-plastic materials because the nonlinearity is caused by microcracks which are formed and bridged as the load increases. The Young’s modulus of the load-carrying fibers is typically lower than the Young’s modulus of the matrix, so the slope of the curve naturally decreases as the load increases.

Delamination Issues

Finally, because the reinforcing fibers are arranged as 2D stacked layers, the material is anisotropic, meaning that its physical properties are not symmetric in all directions. Cracks that form between the layers are not bridged by fibers, causing the interlaminar shear strength and the strength perpendicular to the 2D fiber orientation to be significantly lower for these materials. Because of that, delamination can occur quite easily under mechanical loads in certain orientations.

In general, the use of ceramic matrix composites in applications that require increased tensile strength and resistance to both high temperatures and corrosive environments can provide weight reduction as well as increased stability and durability. However, they must never be used in situations that require significant interlaminar shear strength, as CMCs can delaminate very readily. To avoid this, an engineer must always analyze the stress pattern the material will be subjected to once placed in service.

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