Effects of low temperature on laser ablation of Si ₃ N ₄ : Multi‐physics thermal–mechanical simulations and experiments

Femtosecond laser ablation is widely used as a high‐efficiency, high‐precision, and non‐contact machining technology for fabricating surface microstructure of silicon nitride (Si 3 N 4 ) ceramic. Nevertheless, the thermal damage caused by laser spot overlap unavoidably leads to crack generation, impaired surface quality, and reduced service life of Si 3 N 4 workpieces. Thermal damage reduction is the key to improving the performance of laser‐machined Si 3 N 4 workpieces. In our previous research, we found that low temperature could improve machining quality, but its interaction mechanism was complicated, causing difficulty in selecting the appropriate temperature to obtain high surface quality. To determine the optimal processing temperature, a series of time‐consuming and labor‐intensive experiments must be conducted at various temperatures to identify the appropriate low‐temperature range. In this study, a 3D multi‐physics thermal–mechanical model was established to elaborate the ablation mechanism and predict ablation characteristics. Then, the formation of surface morphology was simulated, the temperature evolution was computed, and the thermal stress was quantified. The results were analyzed to assess the phenomena occurring during the ablation process. Variations in surface integrity with different temperatures were revealed utilizing this numerical model. Besides that, control experiments were performed. The finite element analysis results are in agreement with experimental data so that the precision of simulation has been verified. This work could provide a theoretical reference for cryogenic laser machining, a valuable machining technology that improves micromachining quality.