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Comparative Sand and Rain Erosion Studies of Spinel, Aluminum Oxynitride (ALON), Magnesium Fluoride, and Germanate Glass

Abstract : Comparative studies of erosion resistance were performed with Alpha Optical Company spinel, Raytheon aluminum oxynitride (ALON), Bausch and Lomb polycrystalline magnesium fluoride, and Corning 9754 germanate glass. Materials were tested on their bare surfaces, or with two different midwave infrared (3-5 microns wavelength) antireflection coatings. Magnesium fluoride was only used as the bare material. In sand erosion experiments, spinel and ALON performed best, with little impact damage and no loss of infrared transmission. Coatings on spinel and ALON were readily removed by sand erosion, and magnesium fluoride was readily eroded. (Germanate glass was not tested.) In rain erosion, ALON was nearly undamaged. Magnesium fluoride and spinel both suffered very slight impact damage, but differences in the level of damage could not be distinguished with the limited exposure in this test. Antireflection coatings were readily eroded by rain. The germanate glass, with or without coatings, was seriously damaged by rain drops. Magnesium fluoride has a midwave infrared optical scatter near l%. The infrared optical scatter of spinel, ALON and germanate glass are 0.5%, 1-3%, and 0.20%, respectively. ALON will be of limited use at elevated temperature because of midwave infrared emission.