Thermal fracture phenomenon in laser-assisted direct glass imprinting (LADGI)

This study proposes a laser-assisted direct glass imprinting (LADGI) process. In LADGI, laser irradiation is used to directly heat the microstructures on a mold, and a glass surface is locally heated and softened through thermal conduction, which enables rapid imprinting. However, this process involves rapid heating and cooling, which can induce local tensile stress and increases the risk of thermal fractures in the glass. Therefore, the ability to replicate microstructures while preventing thermal fractures is required. Model experiments with laser spot irradiation and coupled thermal stress analysis were conducted using the finite element method to investigate the temperature conditions required for imprinting and the thermal fracture phenomenon. During LAGDI, imprinting occurred at temperatures above the yield point of the glass. After laser irradiation, thermal fractures initiated in areas of the glass surface near the annealing point because the coefficient of linear expansion of the glass changed rapidly around the annealing point; hence, the volumetric expansion rate at the boundary between the high-temperature region and surrounding low-temperature regions was in a non-equilibrium state. This generated local tensile stress during cooling. This study showed that the relative risk of thermal fractures can be evaluated based on the rate at which the glass volume increased when it was heated above the annealing point. Based on these findings, new indicators E i and I spot were defined to evaluate the LADGI process conditions. These indicators can be used to quantify the efficiency of the process conditions and guide the optimization of the LADGI process. • A rapid laser-assisted direct glass imprinting (LADGI) process is proposed. • The fractures were caused by rapid changes in the coefficient of linear expansion. • Indicators E i and I spot can be used to evaluate LADGI process conditions. • E i can be used to assess LADGI for different materials and laser parameters. • I spot can be used to optimize LADGI including temporal process margin.