Transmission laser micro-welding of preheated glass substrates

Abstract

Glass is a widely used material for multiple applications such as building windows, medical implants, solar cell modules, and sensor packaging. In these applications, glass joining is a key process for producing the final functional design, which is traditionally performed by applying or inserting a second material like adhesives or polymer films. Cured adhesives and polymer films can join glass to glass and attain acceptable strength and transparency, however durability of the structure is limited by the material-environment interaction. Cured polymers absorb moisture and change color during service leading to a degradation in the sealed joints that affects the functionality of the respective application. Laser glass welding is a technique for joining surfaces without using an intermediate layer. Generally, the technology can be used to weld a transparent material to a transparent or non-transparent material such as glass-to-glass, glass-to-ceramic and glass-to-metal. The technique requires focusing and scanning a laser beam at the interface between pre-contacted surfaces to produce a seam weld. Hermetic sealing with a strength comparable to the glass shear strength can be achieved by optimizing laser and scanning process parameters. Laser process parameters are pulse energy, pulse duration, repetition rate, hatch distance and scanning speed. Generally, process parameter combinations fall into two categories: filamentation and accumulation. However, concurrent filamentation and accumulation can also be utilized for laser glass welding but effects of the filamentation may affect the quality of the welded parts. The current research investigates mitigation of the filamentation-related defects utilizing preheating that can also enhance modification topology and welding strength. This research provides fundamental knowledge and insights into laser glass-to-glass welding using preheated glass substrates by investigating pre-heating welding requirements compared to room temperature conditions. Since determination of the focal position before laser scanning is a critical requirement, especially when preheating is adopted, a fundamental analytical model is derived from basics of thermo-optical phenomenon for focal position prediction. The model is validated using focal position measurements of accumulation-induced tear-drop experiments under a range of pulse energy, scanning speed, distance between glass and lens, and temperature using femtosecond and picosecond laser pulses. The applicability of the model is investigated using welding experiments under picosecond conditions by introducing a compensation parameter to the analytical model. Temperature effects on the tear-drop modification are investigated under filamentation or self-focusing and accumulation conditions to elucidate temperature enhancement mechanisms by defining teardrop’s height (width) gain ratio. At last, effects of preheating on absorptivity rate, shear strength and defects of welded glass slides are explored to optimize the process parameter window and make the technique available for industrial applications. Predictions of the derived analytical model match the measurements of focal position in the distance of 9.56–9.96 mm at 200 °C where self-focusing is not significant while underestimating the measurements for the distance of 10.76 mm and 10.36 mm by 30–50 µm. Tear-drop modifications at 200 °C show a relaxed discoloration compared to 150 °C and room temperature. It is also found that in the focal range of 1.6 to 3.2 mm and using a 14 [mu]J pulse energy, the width and height gain are more than 18% and 13%, respectively. At 150 °C, tear-drop’s aspect ratio is enhanced by around 25% at a focal position of 1 mm for the pulse energy of 4.5, 6, 10 [mu]J and speed of 5 mm/s. Welding using optimum conditions of 5 scans and 200 [mu]m hatch produces a strength of around 50 MPa at RT and 40 MPa at 150 °C for 184 fs compared to 35 MPa at RT and 32 MPa at 150 °C for 10 ps. While preheating mitigates self-focusing discoloration, a lower fracture toughness is expected to be the major reason behind higher strengths for RT welding that needs independent future determination.