Optical coatings have thicknesses smaller than the thickness of the finest human body hair. Despite this miniscule size, there are applications that demand from them durability and resistance to harsh external forces. Coatings used in terrestrial & space, scientific, commercial, communication, NASA/NOAA, military, and medical applications can suffer potentially damaging or catastrophic failure in their operational environments. The following discusses the environmental challenges faced and how these micrometer-thin layers are designed to be able to survive and function over long-exposure lifetimes.
In terrestrial applications, thin-film coatings are required to operate and survive in environments that include high humidity, temperature swings, abrasive sand and high-velocity rain impacts, salt water exposure and organic solvent immersion. In previous issues of Coating Materials News (CMN), we have discussed the deposition processes and materials that produce coatings that exhibit the required resistance to these environmental stresses. Briefly, we learned that high-energy deposition processes are employed to produce thin-film layers with morphology that possesses high packing density, low stress, and correct chemical composition. Such deposition processes include ion-assist (IAD), magnetron sputtering (MS), ion-beam sputtering (IBS), atomic layer deposition (ALD), and variations on these techniques.
The PVD processes, IAD, MS, and IBS produce durable film layers by supplying high kinetic and reactive energy to the condensing vapor species, so they grow film layers that are dense enough to prevent the permeation of water and other liquids and vapors that can alter physical and optical properties. Simultaneously, chemical stoichiometry, such as oxidation state, is completed to further stabilize optical properties. As frequently discussed, oxide compounds produce the hardest, stable films, and are used when stability is essential.
A common example of exposure to potentially damaging environments includes the handling of eyewear. Before being qualified for consumer use, ophthalmic AR coatings must undergo hot / cold water soaking, salt water immersion, and moderate abrasion. After purchase, the user subjects these coatings to further abrasive abuse and cleaning in the uncontrolled conditions of daily wear. Because they are designed for careless handling, these coatings resist scratching, peeling, solvent dissolution, and crazing. Layers of metal oxides such as TiO2 and SiO2 are routinely deposited using energetic IAD processes to apply durable AR coatings onto millions of polymer eyewear, goggles, and windscreens.
Effect on Thermal Imagers in Abrasive Environments
Another application was developed particularly for the IR windows on thermal imagers. Imager windows are routinely exposed to the abrasive forces of blowing sand and high-velocity rain. Impacting particles and raindrops cause surface pitting that results in increased scatter and the weakening of the window. These windows are often coated with diamond-like carbon (DLC), which also serves to reduce reflection losses. While DLC is very hard and somewhat brittle, it extends the operational life of these imagers for jet craft, naval applications, and even automobiles. Adhesion can work well for Ge and Si windows, but ZnSe requires additional materials for adequate adhesion...