Ceramic Fibers: The Backbone of Ceramic Matrix Composites

Both industrial and aerospace composites are manufactured by combining fibers with resins and matrices. The fibers act as a reinforcing material that gives a composite its strength and rigidity. As such, fibers are the backbone of a composite. And when it comes to ceramic matrix composites (CMCs) it is all about ceramic fibers.

CMCs make up a significant portion of what we do here at Axiom Materials. We are big proponents of CMCs because they are among the toughest composites on the market. Ceramic fibers are what make them so special.

Ceramic fibers improve material toughness through a number of key mechanisms that prevent the catastrophic brittle failure monolithic ceramics are known for. If you are not familiar with CMCs, consider the following:

1. Crack Deflection and Bridging

A crack in a monolithic ceramic would result in catastrophic failure. But when a crack develops in a ceramic matrix supported by a ceramic fiber, the crack is deflected. We call that ‘bridging’. In other words, the ceramic fibers hold the two faces of the crack in place. The crack is unable to open further or spread through the material.

Bridging makes it possible for CMCs to handle higher loads even in the early stages of crack formation. It is just the opposite of monolithic ceramics. Once a crack begins to form, it’s game over.

2. Fiber Pullout

The fibers and matrix of a CMC are purposely engineered with a somewhat weaker bond. While this might seem counterproductive, the weaker bond is actually that which enables CMCs to continue performing even with minor cracks.

The weaker bond allows fibers to pull out gradually as cracks form and spread. Slower fiber pullout absorbs more energy and increases the overall toughness of the material. As a bonus, fiber pullout also generates friction that dissipates energy and further boosts toughness.

3. Micro-Cracking Tendencies

One of the big things we worry about in the composites industry is a material developing a single, catastrophic crack. A single crack in a typical composite could be the end of that material. CMCs are different because they are engineered to discourage catastrophic cracks. Instead, CMCs are engineered with micro-cracking tendencies.

In the earliest stages of a crack, the ceramic fibers encourage multiple micro-cracks capable of dissipating stress across a wider surface. More energy is consumed by creating new crack surfaces, which ultimately delays failure.

4. Load Carrying

Micro-cracking has a huge advantage in that ceramic fibers continue to carry the load. The result is that CMC failure is gradual rather than abrupt. This behavior dictates that CMCs have a significantly higher fracture toughness compared to monolithic ceramics.

Note that the choice of ceramic fibers does play a role in this behavior. But as with most composites, fiber choices and orientation can be modified to meet unique needs. Across the board, the resulting CMCs are less prone to catastrophic failure.

5. Increased Energy Absorption

Composite design must pay attention to what is known as the stress-strain curve. This curve represents a material’s fracture energy. The higher the energy, the stronger the material. CMCs shine in this regard.

Thanks to ceramic fibers, CMCs are significantly tougher than monolith ceramics because the fibers absorb tremendous amounts of energy. All the previously mentioned properties owe their existence to ceramic fibers that behave in ways other fibers do not.

Ceramic matrix composites are among the most advanced composites on the market. They are as tough as they come. They are also an important part of our business. If you would like to know more about CMCs, just let us know.