Features

Faster, lighter, stronger, hotter – words anyone working on military product development is all too used to hearing. In a world where advanced computer modeling and simulation packages are helping engineers optimize new product designs to increase performance, oftentimes the limiting factors to meeting spec are the mechanical and thermal restrictions inherent to currently available engineering materials. Ceramic matrix composites, or CMCs, provide an entire world of new thermo-mechanical properties, allowing engineers the ability to unlock the potential of some of their most advanced high temperature and high speed designs.

Figure 1. CMC Fabrication via the PIP Process
For many people the first thoughts that come to mind when hearing the word “ceramic” are coffee mugs, delicate pottery, or grandma’s collection of Precious Moments® figurines. These items represent a class of ceramics known as “monolithics”, or homogeneous ceramics. Monolithic materials like porcelain, graphite and silicon carbide have excellent thermal performance in both oxidative and inert atmospheres, but mechanical performance suffers as these materials are prone to catastrophic shatter during thermal and/or mechanical shock events. Additionally, monolithic materials leave limited ability for tuning items such as surface, electrical, and mechanical properties for specific applications. Many of the negative features of monolithic ceramics can be addressed through the adoption of tailored, application-engineered CMCs.

What Are CMCs?

Ceramic matrix composites, as the name suggests, are composite materials consisting of a ceramic matrix and one or more additional property-modifying components. Unlike homogeneous materials, CMCs are commonly reinforced with fiber which adds mechanical strength to the ceramic matrix, allowing for successful utilization in applications where a monolithic ceramic would fail catastrophically due to either impact or thermal shock events.

Reinforcing fiber composition (carbon, quartz, alumina, etc.) can be selected and tuned based on thermal and electrical needs, and fiber architecture (chopped, woven, braided, etc.) can be tailored to address specific mechanical design criteria. To further refine and enhance performance, particulate fillers such as silicon carbide or zirconia can be added to modify both surface and bulk properties. The end result is a family of materials that can successfully withstand temperatures above that of the most advanced high temperature polymers and metals, while at the same time being resilient to the chipping and shattering associated with common monolithic ceramics.

How CMCs Work

Figure 2. CMC Radial Bearings for Industrial Pump Applications
Physically, CMCs derive their remarkable impact and thermal shock strength from their internal reinforcing fibers. Similar to polymer matrix composites (PMCs), mechanical stresses are transferred from the weaker matrix to the stronger internal fibers where the forces are dispersed and mitigated through the bulk of the composite. This results in consistent stress transfer and limited damage to the composite as a whole. It is important to note though that PMCs are much different than CMCs in that polymer composites rely on a very strong interface between the matrix and the reinforcing fibers for adequate stress transfer, while ceramics have optimal shock performance with only a “decent” interface.

The explanation for this is quite simple. If one were to think about a laminated automotive windshield as an example, a small crack will spread, or propagate, as more energy is put into the system due to the excellent interface between the glass and the reinforcing lamination layers. If there was a perfect interface between the ceramic matrix and reinforcing fibers in a CMC, cracks would similarly propagate along the length of the fibers, leading to brittle material failure and shatter.

To address this crack propagation phenomena, CMCs utilize fiber interfacing technology which allows for a good, but not great, interface between the matrix and the fibers. This allows for small-scale delocalization of the matrix from the fibers in areas affected by shock, reducing propagation and allowing the composites to continue functioning properly, though at the cost of reduced mechanical performance compared to common PMCs and engineering metals. The fiber/matrix interface is the single most important aspect in the design of a CMC, and largely dominates the methods by which the composites are processed.

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