Deep‑Penetration Multilayer Femtosecond Laser Cutting of Cf/SiC Composites

A femtosecond laser multilayer cutting method is proposed and implemented, achieving grooves with a depth‑to‑width ratio > 8. The cutting process is influenced by both thermal and non‑thermal ablation mechanisms, and the contour formation mechanism of femtosecond laser cutting of CMCs is clarified. As a preferred material for future aerospace high‑temperature components, Cf/SiC composites are in growing demand for cutting. However, conventional mechanical methods suffer from severe damage and tool wear. This study proposes a femtosecond laser multilayer cutting method and systematically investigates the femtosecond laser cutting process of Cf/SiC composites. Results show: scan line spacing significantly affects bottom flatness, with minimal impact beyond the surface layer; groove taper decreases with step depth, side edges are overly smooth, and fiber fractures occur neatly without pull‑out residue. Using a 12 µm scan line spacing and 20 µm step depth in the multilayer cutting parameters, a 400 µm‑wide groove with depth‑to‑width ratio > 8 and taper angle of 4.65° was achieved. The upper cross‑section is flat; the lower layer is covered by a thick recondensed layer with large undulation, mainly consisting of SiO₂ and fused silica. Groove ablation transitions from non‑thermal‑dominated at the top to thermal‑dominated at the bottom. The femtosecond laser multilayer cutting technique offers high quality and low material loss, making it one of the preferred methods for machining edges, grooves, and other features in Cf/SiC composite components.

Figure 1. Morphology and elemental composition of Cf/SiC composites

Figure 2. Schematic diagram of processing platform

Figure 3. Schematic diagram of scanning trajectory: (a) single-layer cutting; (b) multi-layer cutting

Figure 4. Laser distribution: (a) single-channel scanning spot distribution; (b) single-channel scanning energy distribution; (c) multi-channel scanning laser distribution diagram

Figure 5. Three-dimensional and two-dimensional distribution of multi-channel scanning laser energy density

Figure 6. Cutting results with different line spacing dl: (a1)~(a4) scanning plane diagram; (b1)~(b4) three-dimensional depth of field diagram; (c1)~(c4) cross-sectional profile

Figure 7. Cutting results with different step depths dp: (a-1)~(a-4) scanning plane diagram; (b-1)~(b-4) three-dimensional depth of field diagram; (c-1)~(c-4) cross-sectional profile

Figure 8. (a) Laser reflection diagram; (b) Relationship between the number of multi-layer scans and depth: 17 times = 30μm, 20 times = 25μm, 25 times = 20μm, 33 times = 15μm, 50 times

Figure 9. Microscopic morphology of the bottom of the groove cut with different line spacing

Figure 10. Microscopic morphology of the edge of the groove cut at different line spacings

Figure 11. Schematic diagram of laser stepping on groove edges

Figure 12. Microscopic morphology of the groove bottom cut with different stepping depths

Figure 13. Cutting sections with different line spacing: (a) 7μm; (b) 12μm; (c) 18μm; (d) 25μm

Figure 14. Microscopic images of the cutting sections with different step depths: (a) 10μm; (b) 20μm; (c) 30μm

Figure 15. Cross-sectional morphology of different parts after cutting: (a) Cross-sectional macroscopic morphology; (b) upper layer; (c) middle layer; (d) lower layer

Figure 16. Surface morphology of different parts after cutting: (a) Cross-sectional macroscopic morphology; (b) upper layer; (c) middle layer; (d) lower layer

Figure 17. Surface roughness of each part of the cutting surface: (a) upper layer; (b) middle layer; (c) lower layer

Figure 18. Schematic diagram of multi-layer cutting and forming process

Study findings:
Femtosecond laser multilayer cutting experiments clarify macro‑ and micro‑morphology evolution of grooves during single and multilayer passes, elucidate mechanism shifts and product distributions, achieving high‑quality cutting of Cf/SiC composites. Main conclusions:

1. Simulation of single‑layer laser energy distribution reveals that as scan line spacing (dₗ) increases, the top morphology transitions from smooth to serrated, while sides remain quasi‑Gaussian. This energy distribution explains bottom formation and supports parameter selection.

2. When dₗ < 1.5× laser spot radius, groove bottoms remain relatively flat with minimal subsurface impact. At dₗ = 18 µm, cutting efficiency is highest. As step depth (dₚ) decreases, groove width remains constant while depth increases, taper reduces and efficiency improves. Edges are overly smooth; fiber‑matrix side surfaces are flat with no pull‑outs. Deposits are mainly SiO₂; no oxidation in the cross section. Cutting efficiency increases with dₚ. Optimal parameters: dₗ = 12 µm, dₚ = 20 µm.

3. Using optimal parameters, high‑quality deep grooves (400 µm width, > 8 depth‑to‑width, 4.65° taper) are achieved. Top surface is flat with clear fiber–matrix interfaces; middle surface has a thin recondensed layer of SiO₂/fused silica, scoured along beam direction; bottom surface shows large undulations with ≥ 30 µm thick recondensed layer of SiO₂, fused silica, and carbon monomers—debris blockage during cutting leads to poor ablation quality. The recondensed layer thickens from top to bottom, indicating a transition from non‑thermal to thermal ablation downwards.

4. Groove formation mechanism comprises three stages: single‑pass ablation, single‑layer cutting, and multilayer cutting. Starting with V‑groove formation via single‑pass ablation to ensure flat bottoms and increased width, multilayer progressive cutting steps downward uniformly to increase depth while optimizing surface quality and reducing recondensed layer thickness.

Paper link：https://doi.org/10.1016/j.optlaseng.2025.108869

**--Cite the article published by 长三角G60激光联盟 on June 17, 2025, in the WeChat public account "Yangtze River Delta G60 Laser Alliance"