Transformation of fracture mechanism and damage behavior of ceramic-matrix composites during nano-scratching

The material removal mechanism of C-f/SiC composites in brittle regime is more complex than that of single-phase materials. Variable loading nano-scratch tests of unidirectional C-f/SiC composites along different fiber orientations were carried out. According to different fracture mechanism of fibers, the brittle regime of C-f/SiC composites can be subdivided into micro brittle fracture regime and macro brittle fracture regime. In micro brittle fracture regime, cracks were initiated under the extrusion of the indenter, and propagated between the graphite crystallites, resulting in material removal. The scratched surface of fibers was rough. The thickness of fiber subsurface damage layer was the largest when scratching along transverse fibers, and was the smallest when scratching along across fibers. In macro brittle fracture regime, fibers occurred debonding and flexural deformation under the extrusion of the indenter, which caused bending-induced fracture. The scratched surface fluctuated greatly, but the fractures or the surface of fibers were smooth.