Analysis of TEA-CO₂-laser-induced plasma to establish a new measurement technique

There are needs to establish a new hardness measurement method to meet the demands of high through-put production. The new method is based on laser-induced shockwaves. To analyze this process, it is important to characterize the plasma position and size, which influence the laser-induced shockwave and finally the hardness indentation. First, plasma is created with a nanosecond pulsed TEA-CO₂-laser by optical breakdown on top of a target surface. The plasma becomes unstable and results in a shockwave. The pressure of the shockwave can be used to push a spherical shaped test specimen inside a material surface. So far, laser-induced hardness indentations showed high standard deviations. To understand the process and decrease the deviation of the hardness indentation process, the laser-induced plasma size and its reproducibility are analyzed. The focal position of the laser is changed and correlated with the forming of plasma. The experiments are performed in ambient air and have shown that the size of the plasma increases up to a focal position of −20 mm compared to processing in focus. In the conducted experiments, the focal position has a positive influence on the indentation depth. Moreover, plasma shows lowest standard deviations in pressure and plasma at a positive focal position of −9 mm.