Micro-Welding of Sapphire and Copper and Interface Regulation Via Femtosecond Laser Micromachining

Femtosecond laser-induced micro-welding tests of sapphire and copper (Cu) were conducted under non-optical contact conditions to study influences of two measures on the bonding strength: one is to prefabricate micro-textures on the Cu surface using femtosecond laser and the other is to prefabricate titanium (Ti) coating on the sapphire surface via femtosecond laser-induced backward transfer (LIBT). When no pretreatment measure is adopted, Cu is found to enter the damage zone in the sapphire side on the sapphire/Cu interface prepared by femtosecond laser-induced micro-welding; metallurgical bonding is formed on the sapphire/Cu interface, where the shear strength is 46.4 MPa. When grooves are prefabricated on the Cu surface using femtosecond laser, it is observed on the sapphire/Cu interface prepared via femtosecond laser-induced micro-welding that the sapphire/Cu mixture is embedded in grooves on the Cu surface. The shear strength of the bonding interface ramps up to 64.6 MPa. In the case that the Ti coating is prepared on the sapphire surface through femtosecond LIBT, a Ti transition layer between sapphire and cooper is observed on the bonding interface of the joint. The mixture of sapphire, Cu, Ti, and Ti oxide is filled in the gap at the interface, which further improves the shear strength of the joint to 115.5 MPa. Analysis reveals that the fusion and damage of sapphire are probably a result of the nonlinear absorption of femtosecond laser and the ablation and back-flushing of high-temperature plasmas. When grooves are prepared on the Cu surface using femtosecond laser, mechanical interlocking is formed on the bonding interface of the joint, so the shear strength of the joint can be improved. The coefficient of linear expansion of Ti is somewhat between those of sapphire and Cu, so the presence of Ti coating prepared on the sapphire surface through femtosecond LIBT can to some extent reduce the stress gradient on the interface and therefore improve the shear strength of the joint. The research methods and results provide certain reference for the ultrafast laser micro-welding of transparent brittle materials and metals.