Published: Jan 1979
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The influence of materials and construction variables on the rain erosion behavior of layered infrared window materials has been investigated at 210 m/s in the simulated rain environment (1.8 mm drop diameter, 2.5 cm/h rainfall rate) of the Air Force Materials Laboratory rotating-arm apparatus.
Outer-layer materials included chemical vapor deposited (CVD) zinc sulfide, hotpressed zinc sulfide, and CVD gallium arsenide. These materials were either deposited directly onto CVD zinc selenide substrates or adhesively bonded to the zinc selenide.
In general, damage mechanisms and relative rates of degradation were predictable from studies of the erosion behavior of homogeneous specimens. The effect of layer thickness on erosion behavior was determined for outer layers of CVD zinc sulfide. For outer layers thicker than approximately 1.0 mm, damage in the layered specimens was comparable to that in homogeneous CVD zinc sulfide specimens. The amount of multiple impact damage in layers thinner than 1.0 mm compared with that in thicker layers depended upon the technique of fabrication. Damage was more severe for thinner layers bonded to zinc selenide but was somewhat reduced for thinner layers deposited directly on zinc selenide.
The effect of grain size on erosion resistance was demonstrated by comparison of the behavior of layers of CVD gallium arsenide bonded to zinc selenide with that of gradient-freeze grown gallium arsenide. The CVD material was more resistant to impact damage because of its much smaller grain size.
Thin films deposited on zinc sulfide and gallium arsenide as antireflection coatings were susceptible to removal by drop impacts and the subsequent radial outward flow of water away from the impact site. However, more durable antireflection coatings on zinc sulfide have been developed.
rain erosion, infrared window materials, zinc sulfide, drop impact, erosion mechanisms, gallium arsenide, erosion
Materials engineer, Air Force Materials Laboratory, Air Force Wright Aeronautical Laboratories, Wright-Patterson AFB, Ohio