Effect of Material Anisotropy on Thermal-Mechanical Instabilities in Metal-Free Friction Materials

An anisotropic ceramic matrix composite (CMC), which consists of a silicon carbide (SiC) based ceramic matrix reinforced with carbon (C) fibers, is considered as a metal-free friction material replacement in brake and clutch applications. The fibers are assumed to have a circular cross-section, arranged unidirectionally and packed in a rectangular array without the presence of voids. The rule of mixture showed the C-SiC composite to be transversely isotropic with the circumferential plane as the plane of isotropy. A set of parametric studies have been performed to computationally investigate the dominant parameters that affect thermal-mechanical instabilities. It is found that the chance of thermal buckling in the friction disc can be minimized by reducing the elastic moduli in the radial and circumferential directions, or by reducing the coefficient of thermal expansion in the same directions. Meanwhile, the material properties in the axial direction do not have a significant effect on the critical buckling temperature. On the other hand, a reduction of the onset of thermoelastic instability can be accomplished by decreasing the axial coefficient of thermal expansion or increasing the radial elastic modulus of the nonmetallic friction materials. Finally, there exists an optimal fiber volume fraction to minimize the occurrence of both types of thermal-mechanical instabilities, which can be determined by a numerical optimization method.