Study On Y₂O₃Interlayers For Protective Coatings Of Sapphire Infrared Windows

Sapphire is considered to be one of the most promising detection windowsmaterials at mid-wave infrared because of its excellent combination of mechanicalproperties and optical performances. With the development of high-speed aircraftand infrared image detecting system, higher performances of the infrared detectionwindows are required. However, there are some problems with sapphire inhigh-speed flying, such as hot crack, thermal distortion and protective filmsdesquamated. SiO₂film is one of the important protective films of sapphire.However, there is a thermal mismatch issue between sapphire and SiO₂film. So aninterlayer is needed to be introduced to solve the thermal mismatch problem. Y₂O₃film has interesting properties, including high refractive index, and appropriatethermal expansion coefficient. These make it suitable to be used as high refractiveindex film for the antireflective and protective coatings of sapphire. Y₂O₃film issuitable to form a double layer coating system with SiO₂film. This makes itpossible to improve the performance of the sapphire. This study mainly focuses onthe influence of the Y₂O₃film, which is used as the interlayer to form a double layercoating system with SiO₂film, on the performance of the sapphire at infrared.In this study, the Y₂O₃films and SiO₂films are deposited on the sapphire by RFreactive magnetron sputtering. The influence of the deposition parameters on thedeposition rate and the refractive index is studied through orthogonal design. Themain factor affecting the deposition rate and the refractive index of Y₂O₃films isproved to be sputtering pressure, and the deposition parameters of Y₂O₃films areoptimized. A series of tests, such as SEM, XRD, ellipsometer, nano-indenter and soon, are used to analyze the films.The analysis results indicate that the Y₂O₃films are polycrystalline and the SiO₂films are amorphous. The grain size of Y₂O₃films increases after annealing, whilethe SiO₂films remain amorphous after annealing. After in situ annealing at500℃for one hour in the chamber, the radio of O/Y of the Y₂O₃films is increased and thestoichiometric ratio of the SiO₂films is close to theoretic value. The Y₂O₃and SiO₂films have good combination of mechanical properties and optical performances.The effect of Y₂O₃interlayer on the performances of windows material isstudied. The results show that the introduction of Y₂O₃film can led to a betteradhesion of the films. The average transmittance of the sapphire coated by Y₂O₃/SiO₂films is improved significantly. The average transmittance of sapphire coatedby Y₂O₃/SiO₂films is much higher than the un-coated one. The averagetransmittance decreased with increasing temperature as the same as that of the un-coated one. Moreover, the sapphire coated by Y₂O₃/SiO₂films has good thermalshock resistance. This shows that the performance of the sapphire is improved byusing Y₂O₃film as the interlayer to form a double layers coating system with SiO₂film.