Surface integrity analysis of Si3N4 under cryogenic assisted femtosecond laser ablation

Femtosecond laser is suitable to process Si3N4, but heat effect forming during machining process could degrade the performance of specimen. To reduce heat damage, in this study, the effects of cryogenic assisted laser machining on the surface quality and micro-structures of Si3N4 was investigated. The femtosecond laser was employed to ablate Si3N4 at various low temperatures, accompanied with comprehensive analysis of the surface roughness, material removal rate and SEM topography. To further illustrate the formation mechanism of surface structure, finite element analysis was conducted. Results indicated that while low temperature exerted negligible effects in single-layer ablation, multi-layer laser machining benefits from improved surface quality. Low temperature mitigated thermal effects and reduced the recast layer area, resulting in decreased surface roughness. The lowest surface roughness was obtained during the machining of the specimen at -60 degrees C of 1.072 mu m, representing a 54.6 % reduction compared to ambient air machining. However, further decreasing the temperature below -60 degrees C led to an increase in surface roughness. This was attributed to the combined effects of Gaussian beam energy distribution and low temperature, resulting in incomplete ablation of the surface region between two scanning pitches, thereby decreasing surface quality. This research provides valuable insights into the cryogenic assisted femtosecond laser machining.