New ablation evolution behaviors in micro-hole drilling of 2.5D Cf/SiC composites with millisecond laser

2.5D Cf/SiC composite has been a key heat-resistant ceramic matrix composite (CMC) in aerospace field due to the special structure characteristics. Against the existing research almost focus on 'ablation behavior' in laser processing of CMC, this paper put forward the 'ablation evolution behavior' for the first time, and reveals the 'ablation evolution behavior' in micro-hole machining of 2.5D Cf/SiC composites with millisecond laser. The results show that 0 degrees fiber experiences the ablation evolution from filiform connection, flat ellipse, needle-like to convex structure. And the 90 degrees fiber suffers the ablation evolution from cylindrical structure, lotus lead shape, bud-shape to closed shape. The core of the 90 degrees fibers undergoes the topography structures from cylindrical protrusion, spherical protrusion to needle-like. Except for the honeycomb structure studied in existing research, four new recast layer structures, named transverse strip, longitudinal strip, shell structure, and multi-layer structure are found and analyzed. Further, the mechanism analysis reveals that the recast layer contains both oxidized characteristics and highly carbonized characteristics. In addition, the microstructural analysis shows that three types of particle topography adhere to the recast layer, that are spherical micro-protrusion (20-48 mu m), bubble particles (5-15 mu m), and sub-micron particles (<1 mu m).