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Exploratory Development of Rain Erosion Resistant Infrared Window Materials.

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Final summary rept. 16 Feb 76-15 Feb 77,

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A combined experimental and analytical program was performed to investigate the behavior of selected infrared transmitting materials exposed to raindrop environments. The experiments consisted of single drop impacts 0.7, 2.0 and 2.5 mm drops and impact velocities of 730 and 1120 fps overlapping drop impacts 2.0 mm drops at 730 fps and multiple drop impacts at 730 fps in a standard rainfield 1.8 mm drops, 1 rainfall. Erosion damage mechanisms were defined by detailed optical and electron microscopic analyses performed on the materials before and after exposure to the selected environments. Single drop impact tests performed on zinc selenide, zinc sulfide, and gallium arsenide showed that each drop impact produced a ring fracture pattern characteristic of the material. Resistance to drop impact damage increased in the order zinc selenide, gallium arsenide, and zinc sulfide. The superior performance of zinc sulfide as compared to zinc selenide appeared to be related to the order of magnitude smaller grain size of zinc sulfide. Damage produced by overlapping drop impacts was essentially additive, at least for the first two or three drops impacting the same site. A zinc sulfide outer layer as thin as 0.25 mm protected zinc selenide from damage by impact of a 2.0 mm drop at 730 fps. Exposure of zinc sulfide at 730 fps to multiple drop impact in the rainfield showed transmittance loss at short wavelengths 0.5 to 2.1 micrometers was linear with exposure time indicating a dependence on the extent of subsurface damage. At longer wavelengths 2.5 to 25 micrometers, there was an incubation period during which transmittance did not decrease.

Subject Categories:

  • Lasers and Masers
  • Optics

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