Physical and chemical characterization of the surface and removal process of silicon carbide ceramics by femtosecond laser processing

Objectives : Silicon carbide ceramics, as a typical hard and brittle material, are difficult to process, with challenges such as low efficiency, significant tool wear, and poor surface quality during processing. Ultrafast laser processing can effectively inhibit the processing damage, which is an important method for the precision processing of silicon carbide ceramics. However, existing research on femtosecond laser processing of silicon carbide primarily focuses on the laser ablation characteristics of the material. The specific removal process of the material, in relation to the composition distribution and laser ablation mechanism, is still lacking in the relevant research. This paper analyzes the microstructure of the surface of silicon carbide ceramics processed by femtosecond laser pulses and its evolution law. It reveals the material removal process by examining the changes in chemical components in the ablation area, further improving the femtosecond laser processing mechanism of silicon carbide ceramics from the perspective of changes in material's physicochemical properties. Methods : Under the premise of a fixed laser repetition frequency, the silicon carbide ceramics were processed by varying the laser energy density and the number of pulses. The changes in the physical and chemical properties of silicon carbide were observed and analyzed. A field emission scanning electron microscope was used to observe the microscopic morphology of the processed surface, while a matching energy spectrum analyzer was used to analyze the composition of the processed area. Additionally, X-ray photoelectron spectroscopy was employed to detect the chemical components of the surface before and after processing. A field emission transmission electron microscope was used to detect the cross-sectional morphology of the microstructure. An OLS4100 confocal laser microscope was used to scan the three-dimensional morphology of the micropores formed by the pulsed processing, measuring the diameter and depth of the micropores. The diameter and depth of the micropores were measured. Results : It was found that when femtosecond laser processing silicon carbide ceramics with a single pulse at low energy density, the surface of the processed ceramic was slightly ablated, forming a melting characteristic zone. At high energy density, localized high temperatures were generated in the central processed area, reaching the boiling point of the material, leading to the boiling of the material and the formation of a boiling characteristic zone. Simultaneously, due to the relatively lower energy in the edge area, the material melted, forming a melting characteristic zone. The surface morphology of the boiling region mainly consisted of gasification pits and micro-projectile structures formed by gasification, while the morphology of the melting region was not obvious, with the local appearance of unclear contours of a periodic stripe structure. Using the radius of the feature area and the Gaussian laser intensity function, the evaporation and melting thresholds for the formation of the boiling and melting zones were calculated to be 3.779 J/cm² and 0.860 J/cm², respectively. Under the influence of multi-pulse laser processing, the morphology of the processed area was primarily striped when the laser energy was between the melting and evaporation thresholds, with coarse stripe structures in the central area and fine stripe structures in the edge area. The center region produced coarse streaks, and fine streaks were produced in the edge region. When the laser energy exceeded the evaporation threshold, the central region's structural appeared as a hole structure formed by vaporization, followed by the formation of concentric coarse and fine stripe structures extending to the edge region. As the laser energy density and the number of pulses increased, the micropore diameter and ablation depth exhibited an increasing trend, with the micropore diameter leveling off after an energy density of 9.46 J/cm² and 50 pulses. Phase explosion performed a shielding effect on oxygen and material oxidation reactions; due to the decreasing distribution of laser energy and temperature from the center to the edge of the laser beam, the extent of the phase explosion decreased. In the central region, oxygen in the air did not center the material, while in the middle to the edge regions, exposure to oxygen increased, resulting in an increasing trend in oxygen content from the center to the edge. Meanwhile, through transmission and compositional analysis, it was found that the material subjected to laser action produced a metamorphic layer in the depth direction, showing a distribution pattern of an oxide layer-C-rich layer-silicon carbide matrix. Conclusions : This article investigates the ablation mechanism of silicon carbide ceramics processed by femtosecond laser. In terms of the removal process, laser removal of silicon carbide ceramics is a process in which photothermal and photochemical effects act sequentially. The laser beam irradiates the surface of the material, and the absorbed energy causes an internal temperature rise, accelerating atomic movement. As the energy continues to increase, it leads to a plasma phase explosion of the material, which jets outward, allowing oxygen from the air to react chemically within the material, completing the removal process. In terms of pulse processing, two characteristic regions, boiling and melting, are formed in the ablation region under the action of a Gaussian beam when processing with a single pulse. The evaporation threshold and the melting threshold of the characteristic region are 3.779 J /cm² and 0.860 J/cm², respectively. During multi-pulse processing, when the laser energy is between the melting and the boiling thresholds, the generation of structural defects in the hole structure can be avoided. The temperature of the ablation region decreases from the center of the laser beam to the edge region. In the high-temperature area, the material removal mechanism is primarily direct evaporation of the matrix, while in the low-temperature region, it is the thermal decomposition of the material and the oxidation reaction. This results in the formation of microstructures in the ablation region that are consistent from the center to the edge during multi-pulse processing.