Sputtered zinc oxide coatings: structural study and application to the photoprotection of the polycarbonate

Zinc oxide (ZnO) was deposited by sputtering on silicon substrates. The structure and the composition of these deposits were studied by X-ray diffraction and Rutherford backscattering spectrometry, respectively. It was found that the thin films are polycrystalline with an hexagonal würtzite structure and have a crystallographic c-axis orientation perpendicular to the substrate surface. When the thickness of the coatings increases from 50 to 600 nm, the grain size and the density increase whereas the O/Zn atomic ratio decreases. Then ZnO coatings were deposited on bisphenol-A polycarbonate (PC) films in order to protect this polymer against the solar ultraviolet (UV) radiations. Samples were irradiated in artificial accelerated conditions with a polychromatic light (λ>300 nm). The rate of photodegradation was evaluated by infrared and UV–visible spectroscopies. ZnO coatings decrease the rate of oxidation and the rate of photo-yellowing of PC. The efficiency of the ceramic coating depends on its thickness and its structural properties. As a conclusion, ZnO coatings can improve the long-term behaviour of PC for outdoor applications.