Adhesion of electron-beam evaporated SiO2/TiO2 anti-reflective coating on ion-treated polycarbonate substrate under xenon arc accelerated weathering test

Abstract

Anti-reflective (AR) coatings on polymer substrates enhance light transmittance and minimize reflectance loss. However, durability of multi-layered AR coatings on polymer substrates under various weathering conditions (temperature, radiation, and humidity) can be challenging and affecting adhesion of the coating to the substrate. Therefore, in the present work, Xenon arc accelerated weathering test followed ISO 16474 guidelines was used to investigate durability of electron-beam evaporated SiO2/TiO2 stacking layer as the AR coating on an ion-treated polycarbonate (PC) substrate by varying anode voltage of the ion treatment, oxygen flow rate during the treatment, and thickness of a binding layer (Chromium-based alloy) sandwiched between AR coating and PC substrate. Crosshatch tape test based on ASTM D3359 was used to evaluate the adhesion performance. Physical changes of ion-treated PC surface were characterized by atomic force microscopy and scanning electron microscopy, whereas Fourier transform infrared spectroscopy was adopted to identify chemical functional groups on the treated PC surface. Residual stress and optical reflection of AR coating on the PC were measured by laser interferometry and UV–Vis spectrophotometry, respectively. As anode voltage increased with no oxygen flow, average surface roughness of PC increased but the roughness decreased as oxygen flow rate increased when the highest anode voltage (100 V) was applied. A reversed trend of the roughness achieved with the lowest anode voltage (80 V). Amount of C–O functional group on treated PC increased when the average surface roughness decreased. The adhesion improved by incorporating a binding layer, depending on anode voltage and oxygen flow rate, that can alter residual compressive stress of the material system, reduce moisture and UV attack without sacrificing optical performance of the AR coating. Generally, a better AR coating adhesion can be achieved with smoother PC surface having higher amount of C–O bonding, and higher overall residual compressive stress.