Optically transparent functional coatings for polymeric glazings [electronic resource]
Keywords: Materials Science and Engineering.

Atmospheric plasma deposition is an emerging technology for depositing functional coatings in ambient air without a vacuum chamber at low cost on large substrates for energy, display, and aerospace applications. Among these functional coatings, carbon-chain included organic−inorganic hybrid silicates is the most popular candidates used as protective glazings for polymer substrates. The incorporation of organic components into molecular network of coatings is necessary to control network conductivity and fracture properties. The incorporation process can be controlled by tuning the condition of plasma deposition to deposit either high toughness or high stiffness coatings. One possible way to incorporate organic components to the coating is to use organosilane precursors containing the Si-CxHy-Si structure. However, the abundant oxygen in air poses a significant oxidation challenge for incorporating specific oxygen-sensitive organic components in the coating. The key factor to control the oxidation process was found to be maintaining a high ratio of precursor delivery rate vs plasma power. Other deposition parameters, such as the deposition distance, the plasma gas flow rate, and the deposition rate to achieve good substrate surface properties for high adhesion were also studied. By using proper deposition condition, we deposited adhesive coatings with ultra-high adhesion properties using atmospheric plasma with epoxy ring organosilicate precursors such as GPTMS and trimethoxy[2-(7-oxabicyclo[4.1.0]hept-3-yl)ethyl] silane (TOHES). The coating was demonstrated to have a ~70J/m2 adhesion with PMMA. The debond interface was cohesive in the PMMA substrate. The epoxy ring structure in precursors was demonstrated to increase the adhesion by increasing bonding density at the coating/plastic interface and increasing plasticity of the coating from the organic network forming. The other way to incorporate organic components into molecular network of coatings is using dual precursors for atmospheric plasma deposition. Two different types of organic secondary precursors were found and revealed to have different functions. 1, 5-cyclooctadiene (CYC) is an organic ring structure that allowed incorporation of carbon chain into molecular network of coatings when co-deposit with other functional silane precursor including tetraethoxysiline (TEOS), 1,2-bis(triethoxysilyl)-ethane (BTESE) and bis(trimethoxysilyl) hexane (BTMSH), etc. The deposited coating using the dual precursor method showed increased carbon content and improved adhesion with polymer substrates. The other type of organic precursor, toluene, worked as a buffering precursor during deposition and its fragmented product was not retained in the deposited coating. When toluene worked as a second precursor and co-deposit with 1, 4-bis(triethoxysilyl)benzene (BTESB) silane precursor, it can protect the benzene structure in BTESB from fragmentation of oxidative atmospheric plasma environment. The deposited bottom layer from dual precursors showed better UV-absorbing properties and adhesion than a single BTESB precursor on plastic substrate because more benzene groups were kept in the coating molecular structure. After that, I demonstrated the successful deposition of bilayer protective coatings on plastics using a combined spray and atmospheric plasma deposition method. The bottom layer was a sprayed adhesive coating using (3-glycidyloxypropyl) trimethoxysilane (GPTMS) and tetrapropyl zirconate (TPOZ) precursors. The top layer was a dense coating deposited by atmospheric plasma deposition with a tetraethyl orthosilicate (TEOS) precursor. The coating deposition rate, chemical composition, elastic modulus, hardness and adhesion to poly methyl methacrylate (PMMA) substrates were investigated. The adhesive layer adhesion to the polymer substrate was found to decrease with increasing TPOZ ratio in the precursor solution. A silane surface pretreatment of the PMMA substrate before coating deposition was shown to significantly increase the bilayer coating adhesion. The debond interface changed from adhesive failure at the coating/PMMA interface to cohesive failure in the PMMA substrate. The combined bilayer structure showed > 90% transparency in the visible light wavelength region, ~eight times the adhesion energy and five times the Young's modulus of commercial poly-siloxane sol-gel coatings. The approach provides a strategy for unprecedented combination of adhesion and mechanical properties. Finally, the study included deposition of transparent TiNx/TiO2 hybrid conducting thin films on Si wafer and polycarbonate (PC) from titanium ethoxide using atmospheric plasma with a high-temperature precursor delivery system. The hybrid film chemical composition, deposition rate, optical and electrical properties as well as adhesion energy with the polycarbonate substrate were investigated as a function of plasma power and nitrogen flow rate. The film was a hybrid of amorphous TiNx and rutile phase TiO2, and the TiNx content increased with higher plasma power and nitrogen flow rate. The visible transmittance increased from 71% to 83% with decreasing plasma power and nitrogen flow rate. The film resistivity was in the range of 8.5×101 ohm cm to 2.4×105 ohm cm. The adhesion energy on the polycarbonate ranged from 1.2 J/m2 to 8.5 J/m2 with increasing plasma power and decreasing nitrogen flow rate. Finally, it was found that annealing the film or adding H2 to the primary plasma gas significantly affected the composition and decreased thin film resistivity.