by Benjamin Leever Accomplishment: The Air Force Research Laboratory (AFRL) completed baseline optical testing of small spinel windows as a part of its effort to scale-up window sizes for potential use on Air Force infrared targeting and imaging systems. These systems provide aircraft with essential capabilities such as missile warning, laser spot tracking, and air-to-surface forward looking infrared (FLIR) tracking. Payoff: Spinel windows can be fabricated using conventional low-cost ceramic processing technologies. Success in this area has led to a renewed interest in spinel for military applications such as targeting pods, missile domes and transparent armor. Substituting the baseline window material with spinel for some applications could save the Air Force millions. Background: The infrared window material commonly used for weapons systems is sapphire. Sapphire provides the properties required for high performance windows and other military applications, but it is very expensive.
This testing was conducted by Surmet Corporation of Burlington, Massachusetts, in cooperation with AFRL’s Materials and Manufacturing Directorate, Manufacturing Technology Division (MLM). Spinel is a polycrystalline cubic material that exhibits properties similar to sapphire and polycrystalline aluminum oxynitride (ALON™). In its polycrystalline form, spinel can be processed into transparent, optically isotropic components. It provides approximately 20 percent greater range than sapphire during air-to-air encounters, and is expected to save approximately 50 percent over sapphire (hundreds of millions of dollars, depending on the number of applications). The spinel technology is transferable to myriad other military technologies and has the potential to result in additional savings across the Air Force inventory. Spinel window technology also offers improved transmission over currently used sapphire and ALON™. Spinel is transparent to electromagnetic radiation from the ultraviolet through the mid-infared , and has a distinct transmission advantage over sapphire and ALON™ from 4.5 to 5.5 microns. This is of particular importance for seeker and electro-optic imaging systems. Testing conducted by the Surmet/MLM team indicated that these spinel samples met standard electro-optical system specifications for transmittance, optical homogeneity, and haze. The test confirmed that spinel possesses the required optical properties for many Air Force systems. This preliminary demonstration provides confidence in the material to continue with planned development. Spinel is strong enough to resist the environment but still provide optical properties equal to sapphire. Testing of the technology’s strength properties is ongoing, and all the demonstrated properties will be closely tracked by engineers from AFRL/MLM as they scale-up to the window sizes needed for future electro-optical systems.
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