03-08 resistivity in CMCs

[Image above] As airplanes begin to include more ceramic matrix composite (CMC) materials in their structure, it is necessary that effective methods for checking damage to CMCs are developed. Credit: Dushan Hanuska, Flickr (CC BY-SA 2.0)

Ceramic composites are everywhere, from sports equipment to aerospace engineering. They offer high strength with lower weight and at lower costs than other materials. Additionally, ceramic matrix composites (CMCs) are expanding the operating range to higher temperatures, allowing the use of them in applications such as gas turbine engines.

However, ceramic composites can be brittle and sustain damage when too much force is applied in the wrong direction. And while you may be out some money when your skis or tennis racket breaks, failure of CMCs in aerospace applications can be catastrophic. Therefore, airlines and manufacturers are scrambling to solve technical problems with advanced jet engines that are disrupting their operations.

In a recent article published in International Journal of Applied Ceramic Technology, researchers from The University of Akron in Ohio and the Institute of Structures and Design at the German Aerospace Center in Germany demonstrated a nondestructive method for measuring resistivity that has great potential for in-situ monitoring of the health of composites.

Resistivity has been shown to act as a sensitive probe for identifying damage in composites. Because some types of damage affect in-plane structure while others disrupt stacking of the layers, monitoring resistivity both in-plane and out-of-plane is required for a complete picture of the composite. But due to sampling constraints and architectural complexity, the out-of-plane measurement is rarely reported.

Is there an easier way to take out-of-plane resistivity measurements?

Using simple electrical measurements and sample geometry along with the concept of length constant from the field of neurology, the U.S. and German researchers were able to determine both in-plane and out-of-plane resistivity simultaneously for three composite samples: melt‐infiltrated SiC/SiC composite (MI SiC/SiC), polymer impregnation and pyrolysis SiC/SiNC composite (PIP SiC/SiNC), and melt‐infiltrated C/C‐SiC (MI C/C‐SiC). To verify the validity of their technique, the researchers also measured out-of-plane resistivity values using a standard 4-point method. The result? The values calculated from both techniques were comparable for the MI SiC/SiC and PIP SiC/SiNC composites, but their third composite, MI C/C‐SiC, was too thin to use in the standard method.

Though only early results were presented, it is easy to imagine refining the technique and applying it to real-time monitoring of damage resulting from mechanical and thermal stresses on composite materials, both in the lab and in end-use products.

Interested in learning more about the use of ceramic matrix composites in aerospace applications? Keep your eye out for the April issue of the ACerS Bulletin, which is all about ceramics for aerospace!

The paper, published in International Journal of Applied Ceramic Technology, is “Determination of out-of-plane electrical resistivity for nonoxide ceramic matrix composites” (DOI: 10.1111/ijac.12865)