Laser ablation mechanism of silicon nitride with nanosecond and picosecond lasers

Silicon nitride (Si3N4) has exceptional mechanical, thermal and chemical properties and is therefore, advantageous for many applications. However, the employment of Si3N4 is impeded by the high finishing cost. Short and ultrashort pulse laser ablation are considered as a non-contact machining processes and are utilised in recent years to overcome constrains of conventional machining of Si3N4. However, there is still a lack of research on the effects of different laser parameters such as pulse duration on the mechanism of laser ablation. In this study, laser-material interaction of two different types of solid-state lasers with 10 ps and 100 ns pulse durations was investigated at the constant power of 50 W on a Si3N4 workpiece. The width of thermal damages and heataffected zone (HAZ) is extremely reduced within laser input energy density (EL-input) less than 40 J/mm(2) for both types of laser. It is found that in case of the nanosecond laser ablation, molten droplets inside the laser cut prevent diffusion of the laser beam at EL-input higher than 62.5 J/mm(2). This effect could not be observed in ablation with the picosecond laser. Compared to the nanosecond laser, the picosecond laser is more efficient for ablating the workpiece material at laser input energy densities higher than 120 J/mm(2).