Driving mechanism of the morphology evolution involved in the micro-defect healing process of fused silica optics under the non-evaporative CO₂ laser irradiation

Currently, the machining of hard and brittle fused silica material still relies on material removal by contact-based machining tools. Due to the inherent unevenness of grinding wheels and abrasives, surface defects inevitably persist on precision fused silica optics. Recently, the non-contact, non-destructive CO₂ laser processing of fused silica begins to attract research attentions. Through the spontaneous micro-flow of the molten materials, the fused silica surface can be smoothed and the defects can be healed without mass loss. However, the micro-flow and material rearrangement in the defect area under laser irradiation involves complex physical interactions such as laser energy deposition, heat transfer, fluid dynamics, and volumetric strain. The contribution mechanisms to the morphological evolution remain unclear, preventing precise control of defects and the achievement of high-quality surfaces. In this work, a new numerical model coupling volume strain with the traditional heat transfer and micro-flow models is established. The computational model shows a high degree of consistency with the experimental results, with a deviation of less than 5%. The contribution of various thermo-mechanical effects, including surface tension, Marangoni force and thermal modification, to the evolution of surface morphology during the defect healing process was revealed. It is found that, the surface tension and thermal strain are the primary factors dominating the surface deformation. Both calculated and experimental results demonstrated that, surface tension is the dominant contributor to the defect healing behavior, with the contribution of the other forces to the defect morphology being less than 1% of that of surface tension. This work can provide guidance for the regulation of the defect healing and other laser processing procedures of fused silica optics.