**1. Introduction**

SiC ceramics can adapt to extreme working environments due to their excellent thermal shock resistance, high strength at high temperature, low thermal expansion coefficient, excellent corrosion resistance and low density, which have huge application prospects in the fields of aerospace, space optics, semiconductors and high-temperature components [1–3]. With the gradual development of SiC ceramic–related products in the direction of high-precision and high-quality, higher requirements for the surface quality of SiC ceramics have been put forward. Due to the poor surface quality of SiC ceramics prepared by sintering technology and SiC ceramics being hard and brittle materials with high hardness and brittleness, it is very difficult to obtain high-precision SiC ceramic surfaces by traditional processing methods, which limits the application of SiC ceramics in the precision manufacturing field.

The traditional surface precision machining technologies of SiC ceramics include mechanical polishing [4], ELID grinding [5], plasma polishing [6], chemical mechanical

**Citation:** Zhang, X.; Chen, X.; Chen, T.; Ma, G.; Zhang, W.; Huang, L. Influence of Pulse Energy and Defocus Amount on the Mechanism and Surface Characteristics of Femtosecond Laser Polishing of SiC Ceramics. *Micromachines* **2022**, *13*, 1118. https://doi.org/10.3390/ mi13071118

Academic Editors: Youqiang Xing, Xiuqing Hao and Duanzhi Duan

Received: 10 June 2022 Accepted: 11 July 2022 Published: 15 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

polishing [7], magnetorheological polishing [8], etc. These processing methods have defects such as low processing quality, low processing efficiency, high cost and environmental pollution, which make it difficult to meet actual needs [9]. Laser polishing technology is noncontact polishing, which can not only effectively avoid the above defects but also has the advantages of high flexibility, easy combination with CNC technology to realize automation, a wide processing range and suitability for the surface polishing of complex parts such as planar, spherical and free-form surfaces, making it a surface precision machining method for hard and brittle materials with application prospects and development potential [10]. Considering the complex interaction mechanism between laser and materials involving complex physical and chemical processes, the laser polishing mechanism can be divided into thermal effect and photochemical decomposition (such as thermal polishing and cold polishing) [11]. Thermal polishing generally uses continuous wave lasers or mediumand long-pulse lasers. When the laser beam is radiated to the surface of material, the material absorbs laser energy, causing the local area temperature to rise rapidly. When the energy density of the laser spot reaches a certain level, the material in the irradiated area melts or evaporates, and the surface roughness is reduced by material remelting distribution or removal of the material. Domestic and foreign scientific researchers have conducted more research on laser thermal polishing, which is more suitable for polishing metal materials. Miller et al. [12] used the continuous wave laser to polish H13 die steel and found when the optimal transient combination of laser power and scanning speed was used, surface quality improvement of 83% was obtained. Ma et al. [13] used the fiber laser to polish additive-manufactured Ti-based alloy surfaces. The results showed that the surface roughness, wear resistance and microhardness after laser polishing were better than those of the original surface. Xu et al. [14] used a continuous wave laser and a nanosecond pulsed laser to polish the surface of TiAl alloy fabricated by laser deposition and studied the differences in surface morphology, microstructure, microhardness, corrosion resistance and wear resistance between the two polishing processes. Gao et al. [15] used a nanosecond laser to ablate SiC ceramics and found that when the laser energy is low, the material is removed by evaporation. When the incident laser energy is high, the material removal mechanism is liquid phase explosion, producing a splash of liquid around the ablation area. Some studies have shown that when a continuous wave laser or a medium- and long-pulse laser was used to polish hard and brittle materials, due to the large heat-affected zone, defects such as debris deposition, microcracks and oxidation were prone to occur [16]. Therefore, it is difficult for laser thermal polishing to meet the surface precision machining requirements of hard and brittle materials.

Compared with continuous wave lasers and medium- and long-pulse lasers, ultrashortpulse lasers have the advantages of small heat-affected zone, fewer thermal defects and high machining accuracy and can more easily meet the requirements of surface polishing accuracy of SiC ceramics. Ultrashort-pulse lasers mainly include picosecond, femtosecond and attosecond lasers. Zhang et al. [17] used the overlapping parallel line scanning mode to achieve high-efficiency, large-area and high-precision polishing of alumina ceramics by picosecond laser and determined the ablation law and ablation threshold during the polishing process. Ihleman et al. [18] conducted polishing experiments on different oxide ceramics using nanosecond and femtosecond lasers. It was found that when using nanosecond pulsed laser polishing, the material removal mechanism was mainly plasma-induced ablation. When using femtosecond laser polishing, photochemical decomposition dominated. Since the pulse width of the femtosecond laser is extremely narrow, the interaction time of the femtosecond laser with the material is very short, and it hardly brings thermal effect to the surrounding materials. Therefore, femtosecond laser polishing is also called cold polishing. The action mechanism of cold polishing is that a single photon or multiple photons interact with the lattice or chemical bond of the material, and as a result, some components in the material are directly peeled off, that is, photochemically decomposed [19]. Kurita et al. [20] found that the number and size of deposited debris on the surface of SiC ceramics processed by femtosecond laser were much smaller than nanosecond laser

processing. Taylor et al. [21] used femtosecond laser to polish SiC ceramics, by optimizing the polishing process parameters, and the problem of thermal oxidation on the surface of the material due to the high laser frequency was avoided. Chen et al. [22] reported a femtosecond laser polishing method for SiC ceramics and studied the influence of laser wavelength and pulse number on the surface morphology and composition formation mechanism. By fine-tuning the processing parameters, the subsurface defects were eliminated. After polishing, the subsurface structure was uniform and the friction coefficient was stable, and a high-quality polished surface was obtained. On the basis of single-laser polishing technology, researchers have carried out research on the composite polishing technology of laser and other energy fields. Wang et al. [23] used femtosecond laser-assisted chemical mechanical polishing of SiC crystals, and the corresponding composite polishing mechanism was explored. It was found that the surface quality and polishing efficiency of chemical mechanical polishing SiC crystals could be significantly improved when the laser process parameters were properly selected. Zheng et al. [24] proposed a new method of underwater femtosecond laser polishing for SiC ceramics, studied the influence of scanning trajectory and laser pulse energy on the surface morphology and polishing depth during underwater polishing and, finally, obtained a smooth polished surface.

In summary, researchers from different countries have carried out some investigations on laser polishing for die steel, alloy, SiC ceramics and other materials. At present, the related research on the ultrashort-pulse laser polishing mechanism and process for hard and brittle materials is still limited and needs to be further carried out. Femtosecond laser polishing has certain technical advantages for the surface precision machining of hard and brittle materials. However, there are many factors that affect the polished surface quality, among which the regulation of laser energy density has a significant impact on the polishing effect. This work studies the influence of laser energy density on femtosecond laser polishing of SiC ceramics under different working conditions in order to further promote the process improvement and technological progress of the laser polishing of hard and brittle materials. In this paper, the ablation and polishing experiments with SiC ceramics were carried out by infrared femtosecond laser, and the laser ablation threshold of SiC ceramics was calculated, and the influence of pulse energy and defocus amount on the surface morphology, surface roughness, polishing depth and oxidation degree of femtosecond laser polishing SiC ceramics were studied. The research results can guide the selection and optimization of process parameters for femtosecond laser polishing of SiC ceramics.
