Acoustic process monitoring during the structuring of the diffusion media for fuel cells with Ultrashort Laser Pulses

Inline monitoring is already an essential part of laser material processing. Due to the ongoing technological improvements as well as the devel-opment of new sensor technology, new fields of application emerge. In this regard, optical microphones are a promising alternative to conven-Tional microphones as the membrane-free measuring principle allows them to cover a significantly wider frequency spectrum and, thus, enable new possibilities for process observation. Simultaneously to the development in sensor technology, ultrashort-pulse laser beam sources are being further commercialized, which results in a gradual enlargement of their fields of application due to the improved economic efficiency. In order to increase the productivity of ultrashort-pulse laser beam sources, high pulse repetition rates in the upper kilohertz or even in the megahertz region are often used. Together with the emitted pulses exhibiting durations in the picosecond or femtosecond range, the inline process observation is challenging. In this work, the diffusion media for polymer electrolyte membrane fuel cells, consisting of a carbon black particle-based mi-croporous layer and a carbon fiber-based substrate layer, were structured with a laser beam source emitting pulses in the picosecond range. An optical microphone was used for process monitoring in combination with dedicated data processing methods. It was shown that acoustic data can be used to determine the focus position as well as the material transitions during ultrashort-pulsed laser material processing. Additionally, infor-mation about the ablation volume and the presence of surface defects can be drawn from the data.