Ultrafast Laser Micromachining Studies of Borosilicate Substrates Using Various Fluid Immersion Media to Characterize Cavitation Damage

The discovery of fluid cavitation and the invention of the laser changed their respective fields of study permanently. Over the past 70 years, cavitation has become more widely understood, and laser technology has advanced drastically. The combination of these two is a narrow field of study, and an even more-focused concept is the tailoring of specific laser-machining immersion fluids to enhance or suppress cavitation damage on a substrate. In this work, a study involving low vapor pressure fluids (LVPFs) such as ionic liquids (ILs) for ultrafast laser immersion micromachining was performed. ILs are conductive, molten salts. These chemicals exhibit exceptionally low vapor pressure, which is theorized to make them good candidates for immersion machining. The results of the experiments conducted in this Thesis suggest that LVPFs help to mitigate damage from fluid cavitation during ultrafast laser machining. More specifically, it was determined that LVPFs in general appear to mitigate the damage to a substrate surface. Other physical properties of the immersion fluids tested in these experiments were also considered, including density and viscosities (both kinematic and dynamic), and how these properties also affect cavitation damage to a substrate surface. A total of eight fluids were tested in this paper, whereby the surface roughness of a borosilicate glass substrate was examined, as well as the cross section of the features made by laser ablation. Additionally, the ability of each fluid to modify the effective focal range relative to the substrate surface, where material may still be removed from the substrate, was studied. It was determined from these experiments that ILs may offer specialized, functional immersion alternatives to the most commonly used fluids (water or air) due to their ability to reduce surface damage from cavitation and, compared to air, reduce the amount of ejecta that resolidifies onto the substrate surface during laser machining.