Laser Induced Modification and Integration of Glasses

Abstract

Glasses have been widely used as substrates in new technologies especially flat panel displays (FPD), organic light-emitting diode (OLED) lighting, and lab-on-a-chip (LOC) applications. They are inexpensive, chemically inert with excellent optical, mechanical and thermal properties. In addition, they are biocompatible, and some compositions possess bioactive properties which are highly desirable in biomedical applications. This dissertation seeks to develop fundamental understanding of feature formation mechanisms and changes in morphology, structural, and mechanical properties of glasses induced by lasers in both high (a femtosecond laser) and low (a continuous wave laser) intensity regimes, and to investigate novel processes for modification and integration of glasses. Due to its nonlinear absorption capability in glasses, a femtosecond laser is used to generate internal features inside glass. Their morphology, structure, and mechanical properties such as modulus, hardness, ductility, and fracture toughness are experimentally characterized. Fundamental understanding of the feature formation and these property changes is developed through differential interference contrast (DIC) microscopy, spatially resolved Raman spectroscopy, spatially resolved nanoindentation, and predictive numerical simulations. The improved understanding lays ground work to investigate novel processes of transmission welding (TW) and single step channeling (SSC) in glasses. Joining or sealing of glasses in FPD, OLED, and LOC applications are currently based on adhesives. They are susceptible to moisture permeability and require high curing temperatures of entire parts for a long period of time. TW is investigated and the mechanism of joint formation is analyzed. A numerical model, developed to predict the welding widths, demonstrates the inverse teardrop-shaped absorption volume like the experimental weld seam geometry. Using indentation fracture analysis, the joint is determined to have better mechanical properties than the base material. Fabrication of microfluidic networks in LOC using traditional lithographic processes, or other hybrid processes is cumbersome because they involve multiple steps. SSC is investigated, and numerical models are also developed and experimentally validated to predict the channel lengths resulting from different laser and focusing parameters. The understanding of the channel formation mechanism, and the channel length variation corresponding to the working parameters is developed. TW has the potential to achieve a reliable, highly localized sealing process while SSC has the potential to simplify LOC designs requiring no adhesive for FPD and biomedical industry, respectively. Reduction of the risk of early failure for load-bearing biomedical implants could be achieved by coating bioactive glass onto bioinert metallic substrates. High bioactivity of bioactive glass accelerates the bone-bonding time. Coatings of 45S5 Bioglass, which has the highest rate of bioactivity by plasma spraying and enameling usually fail due to its significant crystallization and weak adhesion to the substrates. Double layer coating by a continuous wave (CW) laser is investigated to produce a dense bond coat having a strong adhesion and a porous top coat having high bioactivity. The morphology and microstructure of the resultant laser coatings are experimentally characterized. A mixed interfacial layer is found at the glass-titanium interface indicating a relatively strong chemical bonding. The top coat is examined revealing a porous structure with low crystallinity. A numerical model is developed to aid in understanding laser sintering mechanisms and is validated experimentally to predict the overall porosity and crystallinity of laser coating