Microscopic study of failure mechanisms in infiltrated carbon fiber felts

Failure mechanisms in infiltrated carbon fiber felts have been studied by optical light microscopy, transmission electron microscopy and scanning electron microscopy combined with mechanical testing experiments. A model is presented which describes crack generation and propagation at layer-layer and fiber-matrix interfaces as well as within matrix carbon layers with different textures. Intensive cracking occurs within high- and less frequently in medium- and low-textured pyrolytic carbon layers. In particular, fracture does not occur directly at the fiber-matrix interface but within the low-textured matrix layer deposited on the fiber. Crack deflection in interface regions between layers with different textures, crack deflection along boundaries of columnar grains in high -textured layers and at interfaces between polyhedral nanoparticles, and finally crack bridging within high -textured lamellae are cooperative failure mechanisms contributing to the toughness enhancement.