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Transparent polycrystalline cubic spinels protect and defend Table 2: Key mechanical and thermal properties of AlON and magnesium- spinel5-7 Property AlON Mg-Spinel Unit Flexural strength 300 70–100 MPa Hardness (Knoop at 200 g load) 1850 1450–1650 kg/mm2 Young’s modulus 323 277 GPa Fracture toughness 2.4 ± -0.11 1.72 ± -0.06 MPa-m1/2 Weibull modulus 8.7 19.5 Thermal shock resistance* (figure of merit R’) 1.2 1.1 Thermal expansion coefficient (30–900°C) 7.5 N/A 10–6 K–1 Thermal conductivity (at 25°C) 12.6 25 W/(m•K) *R’ = σ– –(1– –– –v–) –k aE refraction of Index Wavelength (micrometers) armor fell by 23 percent, whereas (Credit: Surmet.) Figure 2. Measured dispersion of γ-AlON optical ceramic over a range of wavelengths. Transmittance (percent) Wavelength (micrometers) Figure 3. Comparison of night vision performance of γ-AlON laminate to glass laminates. hardness values of magnesium-spinel compared with those of γ-AlON are related to structure and bonding, the lower flexure strength is also strongly affected by processing influences. Lithium fluoride sintering aids are added to magnesium-spinel during the hot press/HIP process (uniaxial hot pressing followed by hot isostatic pressing). However, the grain-boundary phase formed during liquid-phase sintering is weaker than the bulk and can lead to intergranular fracture, as shown in Figure 4. Use of a sinter/HIP process eliminates sintering aids and yields a stronger microstructure, however, inclusions are more likely. The sinter/HIP process can produce inclusion-free, optical quality γ-AlON parts, provided that the starting powders are high quality. The hot pressing/ HIP process offers no advantage when high-quality starting powders are used. Ballistics: Projectiles and explosives Any transparent armor system must defeat ballistic threats and be optically transparent. Glass-based transparent armor systems can meet these requirements, but there are drawbacks. Glasses and polymers are usually less hard than armor piercing (AP) core materials, so laminates are made thick (heavy) to stop penetration. (Credit: Surmet.) Spinels, however, are two- to three- times harder than glasses, and the laminates are less bulky. Lightweight, high-performance, transparent armor is a system of materials that includes ceramics, glass, and polymers. Usually, the design consists of multiple layers separated by thin polymeric sheets. Typically, the front layer is made of hard ceramic (known as the front face) and is capable of destroying the projectile on impact. Transparent ceramics, such as γ-AlON, magnesium-spinel, and sapphire, are much harder than AP core materials (typically steel, tungsten carbide, or tungsten). When the AP core of the projectile hits the strike-face of a hard ceramic material, it erodes and disintegrates during penetration.8 The fractured and eroded core debris is then stopped efficiently by a polymeric layer on the back face of the laminate. Moreover, ceramic armor achieves protection levels similar to glass laminates at smaller armor thicknesses, leading to lower areal densities and lighter weights, as shown in Figure 5. Ballistic tests conducted at the Army Research Laboratory (ARL) in Aberdeen, Md., compared current glassbased transparent armor with three transparent ceramic armor materials: γ-AlON, magnesium-spinel, and singlecrystal sapphire. Details for this work are classified and unavailable for publication. However, ARL tests showed that γ-AlON ceramic armor resisted penetration 10 percent better than magnesium-spinel armor, 20 percent better than sapphire armor, and 150 percent better than conventional glassbased laminate armor. In separate tests conducted by Surmet, γ-AlON transparent armor successfully withstood single hit and multihit projectile threats, including 30 caliber and 50 caliber AP threats. It outperformed glass armor with less than half the thickness and reduced weight by about 60 percent. Environment: Rocks and weather In addition to ballistic threats, materials for defense applications must resist environmental damage threats in the field. For example, tank windows must resist abrasion from airborne dust and sand. Similarly, electromagnetic windows must resist abrasion and wear resistance, as well as chemical stability erosion. Sand erosion and rock strikes In the field, glass-based armors suffer severe loss of transparency from erosion by wind-swept sand, dust storms, and scratches from rock strikes. In simulated environmental sand erosion tests, the optical transmission of glassbased 22 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2


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