Low-temperature synthesis of silica coating on a poly(ethylene terephthalate) film from perhydropolysilazane using vacuum ultraviolet light irradiation

A silica coating was formed on a poly(ethylene terephthalate) (PET) film at a low temperature by using the reactive oxygen species (ROS) generated by vacuum ultraviolet (VUV) radiation of 172 nm wavelength. The ROS were produced using a xenon excimer lamp in the presence of oxygen. A xylene solution of perhydropolysilazane (PHPS) consisting of repeating units of (SiH2NH) was used as the precursor. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR)-based analyses confirmed that the VUV irradiation of the spin-coated PHPS film resulted in the removal of hydrogen and nitrogen atoms from the film and the incorporation of oxygen atoms in it, resulting in the formation of a silica network. Surface profilometric measurements showed no appreciable difference in the thicknesses of the untreated and irradiation-treated PHPS coatings. The results of X-ray photoelectron spectroscopy (XPS) showed a shift of the Si2p binding energy to higher energies after irradiation, indicating that the nitrogen atoms in PHPS that formed bonds with silicon atoms were replaced by oxygen atoms. The XPS depth profile of the film indicated that carbon and nitrogen were nearly absent in the irradiated film and that the composition of the film was uniform across its thickness. The silica coating formed comprised silicon-rich silica SiOx (x < 2), and was colorless and transparent. The pencil hardness of the silica-coated film was greater than 6H. Nanoindentation tests confirmed that the SiOx coating adhered strongly to the PET film and that the SiOx-coated PET film had a higher surface hardness and elastic modulus than did the uncoated PET film.