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Two of the materials most often used as optical windows due to their high transmittance at infrared wavelengths are zinc selenide (ZnSe) and zinc sulfide (ZnS). However, these materials are soft and often degrade when subjected to a particle-impacting environment. Diamondlike carbon (DLC) films have the potential to protect optical windows, such as ZnSe and ZnS, from rain and particle erosion as well as chemical attack. Diamondlike carbon films were deposited on ZnSe and ZnS, and have been evaluated as protective coatings for the optical windows exposed to particle and rain erosion. The DLC films were deposited on the windows using three different ion beam methods. One method was sputter deposition, while the other two methods used a 30 cm hollow cathode ion source with hydrocarbon/argon gas to directly deposit the DLC films. In an attempt to improve the adherence of the DLC films on ZnSe and ZnS, techniques such as ion beam cleaning, ion implantation using helium and neon ions, and thin ion beam sputter deposited intermediate coatings were employed prior to deposition of the film and were also evaluated. The protection the DLC films afforded the windows was quantitatively determined by exposing the surfaces to 27-μm-diam A1203 particles in a microsandblaster. A Perkin-Elmer IR spectrophotometer was then utilized to indicated the change in specular transmittance between 2.5 and 50 μm as a result of the erosion. The DLC coated windows were also subjected to water droplets at 400 mph for exposure times up to 15 minutes. These samples were qualitatively evaluated by optically viewing the surfaces. The DLC films were also evaluated for adherence, intrinsic stress, and infrared transmittance.
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