Transparent polycrystalline cubic spinels protect and defend (Credit: Surmet.) Left: Spinel crystal structure. Right: Transparent aluminum oxynitride window manufactured by Surmet Corp. (Burlington, Mass.).

By Mohan Ramisetty, Suri Sastri, cusp of new commercialization opportunities in terms of vol- Uday Kashalikar, Lee M. Goldman, and umes and costs. This article reviews the unique properties of Nagrendra Nag these materials that make solving the production challenges a worthwhile endeavor. With their manufacturing problems solved, the many desirable Optical properties properties of the polycrystalline transparent ceramics -AlON and γ Properties drive applications, and, obviously, optical prop- magnesium-spinel have led to many military and commercial erties are among the most important for transparent polycrys- applications. talline ceramics. However, the combination of mechanical properties (and, for some applications,4 other properties, too) makes these spinel materials uniquely suitable for a range of he first reported work on trans- applications in defense and aerospace systems. Many defense and aerospace applications require materials T parent polycrystalline ceramics that are transparent in the ultraviolet, visible, and through goes back to the 1950s. For example, some the mid-infrared wavelength ranges. High transparency means low scattering losses, low reflectance, and low absorp- of the literature on materials discussed in tion. For cubic spinel polycrystalline materials, several factors this article, magnesium aluminate spinel determine optical quality. Cubic materials are isotropic, so and aluminum oxynitride spinels, traces they have no inherent birefringence. However, secondary phases, such as pores, impurities, and inclusions, typically 1–3 back to late 1950s and early 1960s. The lead to low transmittance. Controlling material purity and advantages of these materials over current processing conditions minimizes defects, and increases trans- parency up to theoretical limits. In the absence of absorption state-of-the-art materials include and scatter, reflection losses from the material's inherent • Ease of manufacturing; refractive index determine transmittance.Consequently, • Superior mechanical properties, such as modulus, hard- transmittance can be increased significantly via antireflec- ness, and strength; tion coatings. • Performance at high-temperature environments; and Figure 1 shows transmittance of γ-AlON and magnesium- • Chemical durability. spinel optical ceramics. Even though it is polycrystalline, Candidate polycrystalline transparent ceramic composi- γ-AlON is one of the best currently available materials tions include yttrium aluminum garnet and yttria, but alumi- in terms of optical quality. The transmittance of γ-AlON num oxynitride (γ-AlON )and magnesium aluminate spinels approaches its theoretical values in the near-UV, visible seem to have established themselves as leading candidates through mid-IR wavelengths, but starts dropping around 4.5 in multiple market segments, such as the military, aerospace, micrometers and cuts off at mid-IR range wavelengths of and lasers, mainly because of their durability, availability in about 6 micrometers because of intrinsic (phonon) absorp- large sizes, and cost. tion. Additionally, it drops to zero at about 0.22 micrometers Today, γ-AlON and magnesium-spinel are manufactured in the short wavelength range. In comparison, magnesium- in large sizes and in large volumes. However, high cost spinel transmits further in the mid-IR range—transmit- remains a barrier to their deployment as replacements for tance starts dropping around 5 micrometers and stops at 6.5 glasses and some opaque ceramics. Growing demand in cur- micrometers wavelength. The transmittance of magnesium- rent and emerging markets position these materials at the spinel also drops to zero at about 0.2 micrometers. This is an

20 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2 advantage for spinel applications that IR range. Certain require high transmission in the 4.5–5 GRIN lenses can micrometer range. have flat surfaces, In addition to excellent transmit- because the “lens tance, commercial γ-AlON products curve” is built into have exceptional optical clarity the material via the (exceeding 98 percent) and very low refractive index gra- haze (less than 2 percent) in the visible dient. Also, grading wavelengths. (Optical clarity relates to the composition can the amount of light scattered at small eliminate the aberra- (percent) Transmittance angles. In contrast, haze relates to tion associated with the amount of light scattered at large spherical lenses. angles.) Additionally, GRIN Wavelength (micrometers)

Pores, secondary phases, inclusions, lenses can reduce (Credit: Surmet.) defects, and inhomogeneous grain significantly the size, Figure 1. Calculated transmittance of γ-AlON and magnesium- boundaries greatly affect the clar- weight, and com- spinel (at 2 millimeter thickness). The calculation includes ity and haze properties of transparent plexity of the optical Fresnel reflective losses that can be eliminated through use of polycrystalline ceramic materials. High train for defense an antireflection coating. clarity and low haze requires nearly applications, such as image systems for durability are critically important prop- 100 percent density and no secondary laser range finders, night vision goggles, erties in the military context. phases, which are very challenging from and unmanned aerial vehicles. Although γ-AlON and magnesium- a processing perspective. In addition, Night vision technologies improve spinel are cubic spinel lattices, their impurities can impart a tint to the final visibility (transmission) in low light bonding and bond strength differences component, whereas less than full den- conditions. Most night vision devices make γ-AlON mechanically superior sity causes haze. Thus, the production (NVD) are for military applications, to magnesium-spinel, giving it a higher of high-quality transparent ceramics but some are used in the civilian hardness and elastic modulus than demands both careful powder synthesis arena. NVDs require transmission in magnesium-spinel. In fact, the hardness and close control during densification. the 0.4–0.92 micrometer range. In of γ-AlON approaches that of single- Another important optical property this range, γ-AlON and magnesium- crystal sapphire, making it the hardest is the refractive index. In γ-AlON, it spinel transmit better than glasses. As polycrystalline transparent material varies between 1.81 and 1.67 over the Figure 3 shows, γ-AlON-based armor currently available commercially. The 0.4–5.0 micrometer wavelength range, offers significantly more night vision combination of high hardness and with normal dispersion as shown in capability (about 40–50 percent more elastic modulus makes γ-AlON a lead- Figure 2. (Dispersion is the change in transmission) over glass laminates in ing candidate material for transpar- refractive index with wavelength for a low-light conditions. More transmission ent armor applications, followed by material and is denoted by the dimen- means a higher signal-to-noise ratio magnesium-spinel and single-crystal sionless "Abbe number.") A typical and higher-resolution imaging, which sapphire. Table 2 compares the impor- Abbe value for γ-AlON and magne- improve awareness for the warfighter in tant mechanical properties of the two sium-spinel is about 60, which means low-light situations. transparent polycrystalline materials as that the dispersion is much lower than well as some of their thermal properties. some of the glasses with similar refrac- Mechanical properties Although lower elastic modulus and tive indices, making the spinels strong Transparency candidate materials for lenses with low alone is not Table 1. Important optical properties of transparent polycrystalline spinels chromatic aberration. enough for warfare Property γ-AlON Mg-spinel Unit Also, the cubic spinel phase of alu- situations. These Refractive index (at wavelength 0.5 μm)6 1.80 1.723 minum oxynitride exists across a wide materials must dn/dT (in 3–5 μm wavelength range)6 3 3 10–6 K–1 composition range. Therefore, proper- endure stresses Absorption coefficient (at 3.39 μm wavelength)6 0.1 0.018 cm–1 ties, such as refractive index, can be encountered in Total integrated optical scatter (at 0.64 μm; ~5 mm thick 2.1 7.2 % tailored without losing transparency. manufacturing, sample)6 Components like graded refractive transport to the- Transmission wavelength range* 0.22–6 0.25–6.5 μm Optical homogeneity achieved in 15 in. 3 25 in. part with ~5 N/A ppm index lenses (GRIN) are fabricated, for ater, deployment 3.4 in. aperture example, by controlling composition or in service, or ulti- Typical transmittance without AR coatings (in the visible >84 75–80 % by adding dopants. Unlike many glass- mately, under bal- range)* es, which are transparent only in visible listic conditions. Typical haze (in the visible range)* <2 <10 % wavelengths, γ-AlON GRIN lenses are Ballistic properties Typical clarity (in the visible range)* >98 >95 % transparent in the visible through mid- and environmental *Varies depending on thickness and processing conditions.

American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 21 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

Index of refraction Index 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) 1 – v) k *R’ = σ––––––– ( E Wavelength (micrometers) a (Credit: Surmet.) Figure 2. Measured dispersion of γ-AlON optical ceramic over a Ballistics: tection levels similar to glass laminates range of wavelengths. Projectiles and at smaller armor thicknesses, leading to explosives lower areal densities and lighter weights, Any transpar- as shown in Figure 5. ent armor system Ballistic tests conducted at the must defeat bal- Army Research Laboratory (ARL) in listic threats and be Aberdeen, Md., compared current glass- optically transpar- based transparent armor with three ent. Glass-based transparent ceramic armor materials: transparent armor γ-AlON, magnesium-spinel, and single- systems can meet crystal sapphire. Details for this work these requirements, are classified and unavailable for pub- lication. However, ARL tests showed

Transmittance (percent) Transmittance but there are draw- backs. Glasses and that γ-AlON ceramic armor resisted polymers are usually penetration 10 percent better than less hard than armor magnesium-spinel armor, 20 percent piercing (AP) core better than sapphire armor, and 150 Wavelength (micrometers) percent better than conventional glass-

(Credit: Surmet.) materials, so lami- Figure 3. Comparison of night vision performance of γ-AlON nates are made thick based laminate armor. In separate tests laminate to glass laminates. (heavy) to stop pen- conducted by Surmet, γ-AlON trans- parent armor successfully withstood sin- hardness values of magnesium-spinel etration. Spinels, however, are two- to gle-hit and multihit projectile threats, compared with those of -AlON are three- times harder than glasses, and γ including 30 caliber and 50 caliber AP related to structure and bonding, the the laminates are less bulky. threats. It outperformed glass armor lower flexure strength is also strongly Lightweight, high-performance, transparent armor is a system of materi- with less than half the thickness and affected by processing influences. reduced weight by about 60 percent. Lithium fluoride sintering aids are als that includes ceramics, glass, and polymers. Usually, the design consists added to magnesium-spinel during the of multiple layers separated by thin Environment: Rocks and weather hot press/HIP process (uniaxial hot polymeric sheets. Typically, the front In addition to ballistic threats, materi- pressing followed by hot isostatic press- layer is made of hard ceramic (known als for defense applications must resist ing). However, the grain-boundary as the front face) and is capable of environmental damage threats in the phase formed during liquid-phase sin- destroying the projectile on impact. field. For example, tank windows must tering is weaker than the bulk and can Transparent ceramics, such as resist abrasion from airborne dust and lead to intergranular fracture, as shown γ-AlON, magnesium-spinel, and sap- sand. Similarly, electromagnetic windows in Figure 4. Use of a sinter/HIP process phire, are much harder than AP core must resist abrasion and wear resistance, eliminates sintering aids and yields materials (typically steel, tungsten as well as chemical stability erosion. a stronger microstructure, however, carbide, or tungsten). When the AP Sand erosion and rock strikes inclusions are more likely. core of the projectile hits the strike-face In the field, glass-based armors suffer The sinter/HIP process can produce of a hard ceramic material, it erodes severe loss of transparency from ero- inclusion-free, optical quality γ-AlON and disintegrates during penetration.8 sion by wind-swept sand, dust storms, parts, provided that the starting pow- The fractured and eroded core debris is and scratches from rock strikes. In ders are high quality. The hot pressing/ then stopped efficiently by a polymeric simulated environmental sand erosion HIP process offers no advantage when layer on the back face of the laminate. tests, the optical transmission of glass- high-quality starting powders are used. Moreover, ceramic armor achieves pro- based armor fell by 23 percent, whereas

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

American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 23 Transparent polycrystalline cubic spinels protect and defend (Credit: Surmet.) Figure 6. Performance of γ-AlON (upper) and glass (lower) in a simulated rock strike test. The glass tiles experience severe cracking, whereas the γ-AlON remains intact. ics are being evaluated. Designs for same ballistic threat (Figure 2). NV laser data links, Counter Manpads an electro-optic defense system under performance is expected to be increas- (shoulder launched missiles), laser development, the Common Infrared ingly important for next-generation rangefinders, laser target designators, Counter-Measures, also may include systems. and laser radars. Military optics systems mid-IR transparent ceramic hyper- • GRIN optics This new and highly must meet the optical specification for hemispherical domes. advanced application area for transpar- transmitting laser light with high effi- • Reconnaissance and sensor windows ent ceramics is still in the development ciency, low absorption and scatter, and Reconnaissance systems with imaging stage. This technology reduces the minimal distortion. The extremely high capabilities for surveying and sensing number, weight, field conditions, such as terrain or heat, and complexity of Table 3. Other useful properties of γ-AlON and magnesium-spinel have stringent optical requirements. optical train com- Property Description and potential application areas A typical specification for transmitted ponents in military rf transparency γ-AlON is transparent to radio frequencies, which is useful for selective wavefront uniformity allows less than systems, such as rf communications and microwave related applications. one-tenth of a wavelength of error image systems for γ-AlON has a high dielectric constant (>9), low loss tangent, and high Dielectric constant and break-down strength. Its use as a bulk ceramic in the electronic and over the size of the sensor aperture. laser range finders, strength semiconductor industry has been very limited. However, amorphous Some helicopter-based sensors and NV goggles, and γ-AlON coatings are used as dielectrics because of their unique combina- aircraft-based targeting pods now have unmanned aerial tion of mechanical and electrical properties.9 Laser windows transmit laser beams efficiently while protecting the laser γ-AlON windows installed. vehicles. from the outside world. The thermal shock resistance, hardness, and • Night vision systems The current • Windows for Laser damage threshold strength, as well as high optical quality (low absorption, low scatter, and technology NV systems, Generation laser communica- minimal distortion) of γ-AlON make it suitable for as laser windows. III, sense signals over wavelength range tions Transparent Ability to dope, optical transparency, and low phonon energy (low nonradiative transition) make γ-AlON an excellent host material for of 0.4–0.92 micrometers. Transparent windows or domes Upconversion phosphors. Several studies reported upconversion luminescence behavior γ-AlON-based armor has a high trans- protect laser sys- phosphorescence when doped with rare-earth ions. Broad emissions in the visible wave- mission over this wavelength range and tems from the out- lengths were recorded under UV excitation. Similar behavior also was reported for Mg-spinel.10 has shown 40–50 percent improvement side world for many Mg-spinel is a potential scintillator host for γ-ray detection for in NV transmittance performance com- airborne laser-based Radiation detection via medical imaging. When doped with cerium, Mg-spinel shows promising pared with glass armor designed for the systems, such as scintillation luminescence behavior and better optical transparency compared with 11 materials such as LaBr3:Ce and LaCl3:Ce.

24 www.ceramics.org | American Ceramic Society Bulletin, Vol. 92, No. 2 (a) (b) References 1K.A. Wickersheim and R.A. Lefever, “Optical Properties of the Synthetic Spinel,” J. Opt. Soc. Am., 50, 831–32 (1960). 2G.R. Rigby, G.H.B. Lovell, and A.T. Green, “Some Properties of the Spinels Associated with Chrome Ores,” Br. Ceram. Soc. Trans., 45, 137–48 (1946). 3G. Yamaguchi and H. Yanagida, “Study on the

Reductive Spinel—A New Spinel Formula AlN–Al2O3

Instead of the Previous One Al3O4,” Bull. Chem. Soc. Jpn., 32, 1264–65 (1959). (c) (d) 4J.A. Salem, “Transparent Armor Ceramics as Spacecraft Windows,” J. Am. Ceram. Soc., 96 [1] 281–89 (2013). 5D.C. Harris, “Durable 3–5 mm Transmitting Infrared Window Materials,” Infrared Phys. Technol., 39, 185–201 (1998). 6P.J. Patel, G.A. Gilde, P.G. Dehmer, and J.W. McCauley, “Transparent Armor,” The AMPTIAC Newsletter, 4 [3, Fall] (2000). 7“Army Materials Research: Transforming Land Combat though New Technologies,” AMPTIAC Quarterly, 8 [Nov 4] (2004). (Credit: Surmet.) Figure 7. Example transparent polycrystalline ceramics components: (a) γ-AlON panel for 8E. Strassburger, “Ballistic Testing of Transparent Armor Ceramics,” J. Eur. Ceram. Soc., 29, 267–73 (2009). armor laminate; (b) magnesium-spinel lens for sensor pod systems; (c) γ-AlON reconnaissance 9 window for aircraft; (d) -AlON hyper-hemispherical dome for IR countermeasure systems. K.R. Bray, R.L.C. Wu, S. Fries-Carr, and J. Weimer, γ “Aluminum Oxynitride Dielectrics for Multilayer Capacitors with Higher Energy Density and Wide hardness that makes γ-AlON good for About the authors Temperature Properties,” Thin Solid Films, 518, 366–71 ballistic protection also provides field Mohan Ramisetty, Uday Kashalikar, (2009). durability for optical components. Lee Goldman, and Nagendra Nag are 10F. Zhang, L. An, X. Liu, G. Zhou, X. Yuan, and S. Wang, “Upconversion Luminescence in • Laser igniter windows At present, in Surmet's advanced materials R&D γ-AlON:Yb3+,Tm3+ Ceramic Phosphors,” J. Am. Ceram. high-current electrical pulses ignite the group. Suri Sastri is founder, CEO, and Soc., 92 [8] 1888–90 (2009). propellant of small- and medium-caliber chairman. Contact: Mohan Ramisetty, 11C.-F. Chen, F.P. Doty, R.J.T. Houk, R.O. Loutfy, H.M. cannons, but the technology is fraught Volz, and Pin Yang, “Characterizations of a Hot-Pressed [email protected]. Polycrystalline Spinel:Ce Scintillator,” J. Am. Ceram. with problems, such as premature igni- Soc., 93 [8] 2399–402 (2010). n tion from stray electromagnetic fields and hazardous compositions. Laser igni- tion may solve some of these issues, and Manufacturing transparent ceramics and components at Surmet initial tests show that γ-AlON main- Surmet was founded in 1982 and entered the advanced ceramics business in 2002 when it tains optical and mechanical stability licensed and subsequently bought the γ-AlON technology from Raytheon Co. Here are a few of during the high pressures and tempera- the company’s accomplishments since its founding: tures experienced with multiple firings. • Powder synthesis. Powder synthesis requires high-temperature furnaces capable of reaching close to 2,000ºC, with atmosphere control and uniform temperature control. Careful blending, Beyond defense: Nonmilitary crushing, and milling processes all play a role in powder preparation. Over the past 10 years, Surmet has developed processes for manufacturing -AlON powders in tonnage quantities. applications γ Just as defense applications exploit • Fabrication. Parts larger than about 4 inches by 4 inches are susceptible to fracturing during the optical and mechanical proper- forming and densification because of considerable shrinkage during sintering. Through careful control of processing protocols, Surmet fabricates parts with areas of several square feet. ties of γ-AlON and magnesium-spinel, these materials also could solve many • Components. Highlight achievements for Surmet components for military applications include nondefense and industrial-materials- – Qualified and FAA certified transparent γ-AlON-based armor windows installed in produc- tion commercial armored aircraft and related problems. For example, there helicopter systems; are energy-related applications for – γ-AlON windows measuring oil and gas drilling, phosphors, LED approximately 14 inches by 25 inches technology, solid-state lasers, and lamp for reconnaissance pods; and envelopes. In the medical arena, they – γ-AlON GRIN lenses with the can be used for prostheses, scintillator required gradients (in development hosts, and medical equipment sensors with DARPA support). Further work in the mid-IR transmission range. will increase magnitude and size of gradients using materials and pro- cesses compatible with large-volume Acknowledgement

manufacturing. n (Credit: Surmet.) Sreeram Balasubramanian contrib- Lee Goldman in front of AlON heat treating furnaces. uted to this article.

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