Reaction-bonded silicon carbide (RB-SiC) is considered one of the most excellent and feasible materials for lightweight large telescope optics owing to its high specific stiffness and strength. It has broad application prospects in ultra-high-resolution astronomical observation, deep space exploration, and space situational awareness. However, the material has high hardness and a complex two-phase structure, which leads to a complex surface polishing and modification treatment process, low efficiency, and high cost. This has become a bottleneck limiting its further development. Although this problem can be solved using a femtosecond laser, which is a highly promising method for high-precision processing, few applications involving femtosecond lasers in RB-SiC have been reported, and the interaction processes and ablation patterns with RB-SiC remain unclear. Therefore, this study elucidates the interaction process between the femtosecond laser and the RB-SiC surface based on the one-dimensional two-temperature model. On this basis, research on the femtosecond laser ablation law and polishing process on the RB-SiC surface is conducted.
RB-SiC is used in this study. First, the interaction process between the femtosecond laser and RB-SiC is clarified based on the two-temperature model. Second, the RB-SiC ablation law under the action of a femtosecond laser is studied experimentally, and the influence of various process parameters on the RB-SiC ablation during femtosecond laser scanning is analyzed. The process parameters for femtosecond laser polishing of the RB-SiC surface are preliminarily explored, providing a theoretical and experimental basis for its application as a polishing tool for RB-SiC surfaces.
The process of RB-SiC femtosecond laser ablation is elaborated based on the dual-temperature model (Fig.2), and the results show that during the interaction of the femtosecond laser with RB-SiC, the femtosecond laser pulse causes the electron temperature to rise sharply and transfers heat to the lattice through electron-phonon coupling, resulting in a continuous increase in lattice temperature, while the electron temperature gradually decreases. Finally, the electron and lattice reach the same temperature, resulting in material removal when the pulse energy density reaches the ablation threshold.
In addition, using experiments, the influence law of the ablation process parameters on the ablation effect is analyzed (Figs.5?7), and the results show that process parameters such as pulse energy, pulse frequency, scanning speed, and scanning spacing have a significant influence on the ablation morphology. The increase in laser energy and pulse frequency and the decrease in scanning speed cause an increase in the ablation surface roughness and ablation depth. However, smaller scanning spacing benefits from the ablated surface flattening and surface roughness reduction. It is confirmed that it is difficult to obtain high surface quality on the RB-SiC cutting surface using femtosecond laser ablation.
On this basis, the femtosecond laser polishing process of the RB-SiC surface is explored (Figs.8 and 9). The RB-SiC cutting surface is pre-polished with CeO2 slurry for approximately 32 h, and this pre-polished surface is used as the basis for studying the femtosecond laser polishing process. The surface roughness of the RB-SiC substrate before femtosecond laser polishing is approximately 30 nm, which decreases significantly after femtosecond laser polishing with a pulse energy of 0.09 μJ. After polishing twice, the surface roughness shows a decreasing trend and subsequently increases with a decrease in the scanning spacing. At a scanning spacing of 0.0012 mm, a lower roughness surface can be obtained with a surface roughness of 11.56 nm, which achieves an improved polishing result. This indicates that the RB-SiC surface roughness after pre-polishing treatment can be effectively reduced by regulating the femtosecond laser ablation amount, and the feasibility of RB-SiC surface femtosecond polishing is initially verified.
In this study, the femtosecond laser ablation process of RB-SiC is elaborated based on a one-dimensional two-temperature model, and the influence law of the femtosecond laser parameters on the surface roughness and ablation depth is analyzed using an area ablation study. Femtosecond laser polishing of the RB-SiC surface is subsequently performed. The results show that during the interaction of the femtosecond laser with RB-SiC, the femtosecond laser pulse causes the electron temperature to rise sharply and transfers heat to the lattice through electron-phonon coupling, resulting in a continuous increase in the lattice temperature. The electron temperature gradually decreases and finally reaches the same temperature as the lattice achieves temperature equilibrium, which produces material removal when the pulse energy density reaches its ablation threshold. In addition, this study indicates that it is difficult to obtain a higher-quality surface directly using femtosecond laser polishing on the RB-SiC cutting surface. With an increase in laser energy and spot lap rate, the ablation surface roughness increases and the ablation depth increases, however, the smaller scan spacing facilitates the ablation surface flattening and assists in reducing surface roughness. However, on the pre-polished RB-SiC surface, the surface roughness can be polished from 36.90 nm to 11.56 nm by regulating the femtosecond laser ablation amount, which verifies the feasibility of polishing the RB-SiC surface using the femtosecond laser. Femtosecond laser polishing efficiency can be improved by the subsequent optimization of process parameters, and this is expected to address the challenge of low efficiency RB-SiC polishing.
