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LiF-doped magnesium-spinel (hot pressing/HIP process) | Fine-grained magnesium-spinel (sinter/HIP process. Figure 4. Comparison of hot-pressed sintered microstructure with sinter/HIP process. the optical transmission of γ-AlON transparent armor remained unchanged under the same test conditions. Similar performance also can be extrapolated to γ-AlON used in sensor and electromagnetic window applications. Laboratory rock strike tests show similar differences between γ-AlON and armor glass tiles. Figure 6 shows what happens when a granite projectile is fired at two monolithic window targets—γ-AlON and N-BK7 optical glass—at a speed exceeding 270 miles per hour. (These are not laminates. The test simulates field conditions for sensor and electromagnetic windows, where laminates cannot be used.) The top series shows a rock impacting a γ-AlON window, pulverizing, and leaving behind an intact window. The bottom series shows a rock impacting a glass window: The rock also pulverizes, but the damage to the glass window is severe. Delamination Delamination brought on by high interlaminar residual stresses and thermal cycling stresses is a significant source of nonballistic failure of transparent armor. Tests show that γ-AlON-based (Credit: Surmet.) transparent armor delaminates less during thermal cycling tests owing to thinner laminate sections and less thermal expansion mismatch with both the glass and polycarbonate sections. Thus, γ-AlON transparent armor delaminates less than glass-based armor under thermal cycling in field conditions. Extreme environments Beyond ballistic properties, γ-AlON transparent armor tolerates vibration, mechanical shock, g-loading, and sudden pressure release. Also, it endures extreme environmental stresses, such as solar radiation, humidity, temperature ranges from –67°F to +185°F, and thermal cycling. A recent study even considered the material for spacecraft windows.4 Table 3 lists other useful properties of transparent polycrystalline spinels that could add to their utility as armor or lead to new applications. Military applications for transparent ceramics Military and civilian security forces use transparent armor extensively for ground vehicle protection. Examples of ground vehicles equipped with transparent armor include high-mobility multipurpose wheeled vehicles (“Humvees”) and mine-resistant ambushprotected (MRAP and M-ATV) vehicles and trucks. Although most systems currently use glass-based armor, γ-AlON-based transparent armor is under evaluation for future vehicles because it weighs up to 65 percent less, can sustain multiple strikes, offers increased field-of-view to the driver (less weight means larger windows can be installed), and larger cabin volume (because of thinner panels). In addition, vehicle designers can incorporate γ-AlON’s other properties into systems, such as night vision capability and laser protection. Some military aircraft need armoring, and for these applications, weight, mechanical integrity, and transparency are of paramount importance. Typical aviation applications include windshields, blast shields, windows for sensor protection, and armored "lookdown" windows for helicopters, combat aircraft, and other airborne systems. Most of the property requirements for these applications are similar to those of ground vehicles. However, the optical specifications require a minimum of 70-80 percent transmission and less than 4 percent haze. Transparent γ-AlON-based armor windows that have successfully completed qualificatgion testing and obtained FAA certification are starting to be installed in production commercial armored aircraft and helicopters. More applications are under evaluation, and some applications of polycrystalline transparent ceramics are in the early stages of adoption, such as helicopter, aircraft, and ground vehicle windows. Several other military optical applications require windows, however, most or these applications are domes or lenses. An important advantage of polycrystalline ceramics over single crystals is that the shapes can be made standard powder-processing techniques, such as pressing, injection molding, slip casting, and cold isostatic pressing. Optics applications include • Domes IR-guided missile systems use IR-transparent domes. For example, one of the Joint Air-to-Ground Missile system designs uses tri-mode seeker domes (near IR, MWIR or LWIR, and millimeter wave). Optical-quality γ-AlON and magnesium-spinel ceram- (Credit: Surmet.) Figure 5. Dimension and weight comparison of armor laminates with similar ballistic protection. Left: γ-AlON laminate, 1.6 inches thick and density of 18.9 pounds per cubic foot. Right: Glass laminate, 3.6 inches thick and density of 43 pounds per cubic foot. American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 23


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