Fabrication of TiO2/SiO2 multilayer coatings for near-infrared thermal radiation control

Sol-gel coatings of TiO 2 /SiO 2 multilayers potentially control thermal radiation spectra from electric heaters, providing an energy-efficient solution.This study investigated sol-gel techniques to develop crack-free thick coatings that are crucial for infrared applications.Similar to many reports, the application of multiple thin coatings significantly enhanced fracture resistance.Residual stress measurements using Stoney's method showed that the stresses in multiple coatings were comparable to those in single coatings despite their different critical cracking thicknesses.Based on the results, a specific fracture mechanism was proposed for the single coatings.Further coatings beyond the critical thickness resulted in delamination from the substrate.Pull-off tests showed that the adhesion between TiO 2 and glass was weakest among the TiO 2 , SiO 2 , and glass interfaces.To prevent delamination, a SiO 2 buffer layer was added to the TiO 2 /glass interface, facilitating the formation of TiO 2 films with thicknesses of several micrometers.At high temperatures, surface roughening occurred owing to the grain growth of rutile TiO 2 , which was prevented by applying a SiO 2 overcoat.A long-pass optical filter was fabricated using TiO 2 /SiO 2 stacks with the buffer layer and overcoat, which effectively blocked the thermal radiation from a red-hot heater at wavelengths near 1 μm.