Material removal mechanisms during micro-hole drilling of UD-Cf/SiC composites: A study via nano-scratch and drilling tests

Carbon fiber-reinforced ceramic matrix composites (C_f/SiCs) are widely used in aerospace due to their exceptional strength-to-weight ratio. However, their high hardness and anisotropy often lead to rapid tool wear and poor drilling performance. To investigate their fracture behavior, nano scratch was conducted on unidirectional carbon fiber-reinforced SiC composites (UD-C_f/SiCs). The ductile-brittle transition depths of carbon fibers in the radial, axial, and end-face directions were 907.2 nm, 961.3 nm, and 455.6 nm, respectively. Meanwhile, the micro-macro brittle transition depths were 3013.4 nm, 2759.4 nm, and 5101.2 nm, respectively. Furthermore, drilling tests were conducted on UD-C_f/SiCs to produce 600 μm diameter holes using parallel drilling (PD) and vertical drilling (VD) processes. The thrust force in the VD process was higher than in the PD process at a feed rate (f) of 0.3 μm/r, nearly equal at f of 0.9 μm/r, and lower at f of 1.8 μm/r. The PD process caused less exit damage than the VD process at 0.3–1.5 μm/r but more at 1.8 μm/r. With increasing f, the carbon fibers gradually transitioned from ductile fracture to micro- and macro-brittle fracture. The primary removal mechanisms of the carbon fibers involved compression, shear, and bending fracture. Hole-wall with longitudinal fibers exhibited the best integrity (R_a increasing from 0.3940 μm to 1.1110 μm), whereas perpendicular and transverse bundles deteriorated more severely (R_a reaching up to 1.6991 μm–3.7058 μm). Notably, the most severe subsurface damage occurred in holes with perpendicularly oriented fibers, reaching a depth of approximately 6.93 μm.