Low-damage thermally assisted femtosecond laser processing of borosilicate glass: mechanism and in situ acoustic emission characterization

This study proposes a thermally assisted processing strategy combined with acoustic emission (AE) monitoring and time–frequency analysis to address crack damage and the limited capability for in situ damage characterization in femtosecond laser processing of borosilicate glass. The effects of preheating temperature and scanning speed on groove morphology, surface quality, crack behavior, and groove depth are systematically investigated. The results show that increasing the preheating temperature markedly suppresses crack damage, improves ablated surface quality, and enhances material removal. At a scanning speed of 40 mm/s, the groove depth increases by 10.0% and 17.5% at 250 °C and 350 °C, respectively, compared with that at room temperature. No visible surface cracks are detected along the evaluated 3-mm tracks at 60–100 mm/s under stage set temperatures of 250 °C and 350 °C. In contrast, only slight residual cracking remains at 40 mm/s under the highest preheating condition, indicating a pronounced suppression of crack damage. AE measurements are conducted from room temperature to 250 °C. Crack-related AE activity is mainly distributed within approximately 200 kHz–700 kHz, with components higher than approximately 500 kHz showing a stronger association with the rapid initial stage of crack formation. On the other hand, 200 kHz–500 kHz components are more closely related to subsequent crack evolution. In addition, the low-frequency AE energy near 101 kHz exhibits better empirical fitting and predictive performance for groove depth than the conventional root mean square (RMS) parameter. High-speed imaging and microscopic morphology characterization results indicate that thermal assistance suppresses crack damage through coupled reductions in thermal mismatch, the overall thermally induced stress retained after cooling, and the formation of subsurface flaws that provide preferential paths for deflected crack propagation. Moreover, the elevated initial thermal state promotes material modification and removal and prolongs the softened state of the laser-affected surface, facilitating material redistribution and surface reconstruction. This work provides experimental evidence and methodological support for low-damage femtosecond laser machining and AE-based in situ crack characterization of brittle transparent materials.