Study on the ablation evolution and surface morphology of fused silica as function of CO2 laser processing parameters

In this work, CO2 laser pulses with a wavelength of 10.6 μm, a beam diameter of about 400 μm, and an intensity of the order of 190 kW/cm2 are used to produce microablations on fused silica samples with pulse fluences ranging from the silica ablation threshold up to 38 J/cm2. We obtain pits with average depths ranging from a few nanometers to tens of micrometers and study their morphology evolution as a function of pulse width and pulse-repetition frequency. Our experimental results are used to improve a 2D axisymmetric finite-element model that analyzes the heat transfer process of CO2 laser ablation in fused silica. We propose an empirical function added to the reflection coefficient parameter, which is triggered when the material temperature exceeds its boiling temperature. This empirical function helps to control the energy deposited on the surface material and, as a result, ensures more precise calculations of pit depths for a large range of pulse widths and pulse-repetition frequencies. The improvements made to the 2D axisymmetric model are tested on a 3D model whose results are compared to microchannels ablated with different processing parameters (e.g., scanning speed, pulse-repetition frequency, and pulse width). Good agreement between experimental and numerical results was obtained. The numerical results provide a comprehensive understanding of the evolution of the residual surface temperature and heat-affected zones of the material as a function of pulse width and pulse-repetition frequency.