Air-stable low-emissive AlN-Ag based coatings for energy-efficient retrofitting of existing windows

Low-emissive (low-E) coatings, based on alternating thin layers of aluminium nitride (AlN) and ultra-thin layers of silver (Ag), have been deposited by sputtering for use on glass surface outside the inert gas filled gap of insulating multiple-pane windows. Such low-E coatings being optical filters containing thin metallic layers, undergo degradation phenomena when exposed to air with the appearance of a bluish colour and delamination. An in-depth study has been conducted to identify the degradation mechanism in moist atmosphere by means of several characterization techniques before and after accelerated aging tests. Photoluminescence measurements have evidenced formation of the complex defect VAl-3ON at the AlN/Ag interfaces as responsible for the hydrolytic degradation process. Relationship between degradation process and chemical-physical nature of AlN lattice defects has been established. Modifications in the sputtering process of AlN films were implemented and the antireflective layer was replaced with less hydrophilic material. The coating deposited with modified structure has been found to be highly stable and with excellent thermo-optical performances (SHGC 0.54, visible transmittance 80%, emissivity 4.8%). The modified low-E coatings have been also deposited on polycarbonate substrate with similar results of high stability in the air. This stabilized coating on polymer can be considered an excellent candidate for low-cost energy-saving retrofitting.