The rapid advancement of ultrafast laser technology has opened new avenues for precision manufacturing, particularly in fabricating microstructures on hard and brittle materials, such as glass. However, achieving high-quality microstructure fabrication on such materials remains challenging owing to their inherent properties, including high hardness and brittleness. Therefore, this study aims to investigate and optimize the methods for ultrafast laser microstructuring of hard and brittle materials, focusing on the influence of laser power on crack formation, propagation, and processing efficiency. We propose a gradient power processing strategy to enhance machining quality while maintaining processing efficiency. The importance of this study lies in its potential to mitigate issues related to crack generation and propagation during ultrafast laser processing of hard and brittle materials, thereby enhancing quality and efficiency. Furthermore, the findings can expand the application of microstructures on hard and brittle materials in advanced technological fields.

A femtosecond laser with a wavelength of 1030 nm, pulse duration of 900 fs, and Gaussian beam profile was employed herein. Microholes arrays were processed in quartz glass (0.15 mm thick) at a constant pulse frequency of 10 kHz and an average scanning speed of 0.1 m/s. The experimental procedure involved initially investigating the generation and propagation mechanisms of cracks during machining. Subsequently, the processing effects of varying laser power on quartz glass were compared to analyze the relation between processing power, crack formation, and processing efficiency. Based on these analyses, a gradient power processing strategy was proposed. In-situ temperature measurements were taken during processing to validate the effectiveness of the proposed strategy in reducing the temperature gradient and minimizing thermal stress generation.

1. During ultrafast laser processing of quartz glass, crack propagation can be divided into four stages: a) solidification of the melt at the bottom of the hole creates microcracks, b) these cracks propagate outward through the hole wall, c) the outward-propagating cracks form annular cracks around the hole, and d) the connection of multiple annular cracks within the microhole array leads to processing failure (Fig. 2).

2. Comparative analysis of the impact of different laser powers on machining effects reveals that low-power processing effectively reduces recast layer formation and thermal stress generation but decreases processing efficiency (Fig. 3). Therefore, a gradient power processing strategy is proposed to optimize processing quality while maintaining efficiency. The effects of different gradient power strategies on machining surface quality and efficiency are explored. The results show that a lower and gradual power gradient yields better processing outcomes. This is attributed to the smooth transition of power from low to high, leading to a gradual temperature increase, thereby avoiding notable temperature gradients. This effectively suppresses crack formation and propagation, thus achieving an optimized balance between efficiency and quality (Fig. 4).

3. In-situ temperature measurements are used to compare temperature variations during microhole array processing of quartz glass under constant and gradient power conditions. The results indicate that gradient power processing reduces the maximum temperature in the initial processing stage and lowers the temperature at each power level, allowing the processing temperature to stabilize quickly (Fig. 5).

4. The crack suppression mechanism of gradient power processing includes the following: in the initial stage, low-power processing increases surface roughness, enhancing laser absorption and promoting heat dissipation through phase transition in subsequent stages, thereby reducing thermal stress. With increasing power, the hole depth has already developed to a certain extent, resulting in a reduced stress concentration at the hole wall. This reduces crack propagation along the hole wall and blocks the crack path (Fig. 6).

This study investigates the mechanisms of crack initiation and propagation during microhole array fabrication on quartz glass using 900 fs laser pulse and demonstrates the effective suppression of crack formation and expansion through a gradient power dynamic regulation strategy. The findings reveal that crack formation and propagation occur in four stages and that adopting a “low-power-to-high-power” gradient power processing strategy can suppress crack propagation without compromising processing efficiency. In-situ temperature measurements indicate that gradient power processing considerably reduces the maximum temperature difference, enhancing fabrication process stability. Furthermore, the crack suppression mechanism of gradient power processing involves the initial low-power phase that increases surface roughness to enhance laser absorption, reduce internal thermal stress, and allow the hole depth to reach a critical level. Transitioning to high-power processing subsequently prevents crack propagation from the bottom of the hole to the sidewalls. The proposed gradient power processing strategy is broadly applicable to laser micromachining of hard and brittle materials and holds considerable potential for various applications.