<title>Rain Erosion-Resistant Antireflective Coatings For Infrared Windows</title>

Abstract

AbstractAR coatings for potential window materials (ZnS and GaAs) were developed. Emphasis wasplaced on developing AR coatings with rain erosion resistance.Rain erosion resistance of two -layer AR coatings (NdF3 /ZnSe, LaF /ZnSe, and ThF4 /ZnSe)on small ZnS substrates was demonstrated. When exposed to a simulated one inch per hourrainfall at 470 mph for 20 minutes, the coatings remained intact with about 10 percent lossin transmittance. The same coatings on GaAs were severely degraded by the rain erosion test. The surface finish of the substrate appeared to be an important factor for rainerosion resistance. Coatings on substrates that satisfied the scratch -dig specificationof 60 -40 exhibited superior rain erosion resitance.IntroductionTo be suitable as an aperture on the multisensor systems being developed for high -per-formance aircraft, a window material must possess the following properties: 1) high trans - mittance at the wavelength of interest, 2) rain erosion resistance, 3) high temperature(200 °C) stability, and 4) low -cost for sizes up to 12 x 18 x 3/4 . ZnS and GaAs, althoughnot having all of these properties, are being developed as IR windows for certain applica-