Microstructure and wear mechanisms in C/C composites

The micromechanisms of the failure and related wear processes in the two-dimensional (2D) randomly chopped and the three-dimensional (3D) non-woven carbon/carbon (C/C) composites with varying fiber type and matrix architecture have been studied by light microscopy, scanning electron microscopy and transmission electron microscopy after the subscale brake dynamometer tests. The instrumented indentation technique (nanoindentation) was used to characterize the localized mechanical properties. The presented wear model incorporates the formation and propagation of the microcracks within the C/C composites with different matrices and fiber types. Intensive cracking occurred within the highly anisotropic pyrolitic carbon and less frequent cracking was detected in the medium textured carbons on layer/layer levels. The varying mechanical properties (modulus of elasticity) of carbon fiber do not have a significant effect on the micromechanisms of the crack formation and wear. The fracture tip deviates at the fiber/matrix interface in the 2D C/C composite. However, in the 3D C/C composite, the deviation of the advancing crack occurred dominantly within CVI carbon. The crack deflection along the columnar grain boundaries and the crack bridging was observed. Since the frequency of the crack deviation within CVI is very high, an increase in the fracture toughness was detected for 3D samples. As a result of the different wear mechanisms, different friction layers were formed on the friction surface of 2D and 3D samples. It is possible to optimize the frictional performance of the C/C composites by controlling the microstructure.