Organic/Inorganic Hybrid Sol-Gel Derived Hard Coatings on Plastics
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ABSTRACT We investigated sol-gel derived silica based hard coatings on modified polyester substrates. The silica network was modified by incorporating an organic component and adding transition metal oxides. These modifications resulted in tailored thermal, optical and mechanical properties of the coatings. Various low temperature densification techniques were studied including appropriate sol-preparation procedure, enhanced solvent evaporation, ultraviolet (UV) irradiation, and low-temperature (below 150 'C) heating. Oxygen plasma etching was applied to improve the adhesion of the sol-gel coatings on the plastic surface. Nanoindentation analysis revealed that the sol-gel coatings have a surface hardness up to 2.5±0.27 GPa, approximately an order of magnitude higher than that of the plastic surface. INTRODUCTION A number of film deposition techniques have been employed to form inorganic coatings on polymers to improve their surface mechanical properties such as abrasion resistance. Vapor phase techniques, such as evaporation, sputtering and chemical vapor deposition (CVD) [e.g., 1-3], produce high quality films (high density and high mechanical properties), but are expensive, require sophisticated equipment, and often require substrates to be maintained at high temperatures. In addition, uniform coatings of complex shaped substrates are difficult with line-of-sight vacuum techniques. Solution deposition is an alternative approach for applying oxide films to temperature sensitive substrates. For example, the biomimetic synthesis [e.g., 4-8] has two important characteristics: (1) control of solution conditions, including ion concentration (supersaturation levels), pH, and temperature and (2) the use of functionalized interfaces to promote mineralization at the substrate surface. By controlling the surface energy of the substrate along with the solution supersaturation, deposition conditions can be established that favor heterogeneous growth of the thin film on the substrate without bulk particle precipitation (homogeneous growth). The big drawback of this approach is the need of functionalized surface. Langmuir films and self-assembled monolayers (SAMs) are often applied to introduce functionality to the polymer surfaces. Oxide or oxide-based coatings have been deposited on various substrates using sol-gel technique [9-19]. In this approach, the substrate withdrawal rate can be controlled to obtain desired film thickness [19]. Sol-gel processing also offers easy manipulation of chemical composition and 100% recycle of precursor materials. However, post deposition heat treatments at elevated temperatures are commonly required to convert porous structure to a dense film. Organically modified oxide (also known as ormocer or ormosil) coatings have been studied extensively [9-13] and have been successfully commercialized as coatings for optical glasses [20]. Organic components are introduced through hydrolysis and co-polymerization of organic and inorganic precursors together during sol preparation. The unhydrolyzable org
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