Enhancement of the photoprotection and nanomechanical properties of polycarbonate by deposition of thin ceramic coatings

摘要

The chemical reactions resulting from ultraviolet radiation produce discoloration and significant changes in the surface properties of polycarbonate (PC). To prevent photon absorption from irradiation and oxygen diffusion and to enhance the surface nanomechanical properties of PC, thin ceramic coatings of ZnO and Al_2O_3 (both single- and multi-layer) were deposited on bulk PC by radio-frequency magnetron sputtering. The samples were irradiated at wavelengths greater than 300 nm, representative of outdoor conditions. Despite the effectiveness of ZnO to protect PC from irradiation damage, photocatalytic oxidation at the PC/ZnO interface was the limiting factor. To overcome this deficiency, a thin Al_2O_3 coating was used both as intermediate and top layer because of its higher hardness and wear resistance than ZnO. Therefore, PC/Al_2O_3/ZnO, PC/ZnO/Al_2O_3, and PC/Al_2O_3/ZnO/Al_2O_3 layered media were fabricated and their photodegradation properties were examined by infrared and ultraviolet-visible spectroscopy. It was found that the photocatalytic activity at the PC/ZnO interface was reduced in the presence of the intermediate Al_2O_3 layer that limited the oxygen permeability. Nanomechanical experiments performed with a surface force apparatus revealed that the previous coating systems enhanced both the surface nanohardness and the elastic modulus and reduced the coefficient of friction in the order of ZnO, Al_2O_3, and Al_2O_3/ZnO/Al_2O_3. Although irradiation increased the nanohardness and the elastic modulus of PC, the irradiation effect on the surface mechanical properties of ceramic-coated PC was secondary.