Transparent Materials Safeguard the Army's Vision - Advanced ...

Regular readers of the AMPTIAC Quarterly will have noticed thatwe’ve published several ‘special issues’ over the past few years. Thecommon aim of these publications has been to highlight topics ofspecial interest to targeted technological communities. Examplesinclude our issues on nanotechnology (May 2002) and blast mitigation(Protecting People at Risk, February 2003). Both issues werewell received by our readers but for different reasons: Nanotechnologyrepresents an exciting andunexplored frontier with intensescientific interest; while protectingpeople and structures fromexplosions has gained national attention, especially since 9/11.This current issue addresses perhaps an even more important topic:the development of new technologies to enable our ground troopsto become more effective in the war against terror as well as otheremerging global threats.All one has to do is follow the news reports to appreciate themajor technological hurdles now facing the Army. Gone are thedays when we faced large standing armies, consisting of heavilyarmored units employing traditional tactics much like our own.Today, we face adversaries that seemingly have no qualms at sacrificingtheir lives or those of innocent bystanders in an attempt toinflict damage on our troops. Through a mix of conventional andunconventional weapons (such as improvised explosive devicesor IEDs) these fanatics have forced us to adopt new tactics whilerelying upon our existing weaponry and equipment.To be totally effective against our new and other possible futureenemies, the Army must transform from a force relying on heavyarmor to one employing a broad spectrum of lightweight, yet survivablesystems and equipment that will enhance their ability tofight. In this context, the word ‘transform’ means to change doctrine,tactics, and assets to respond rapidly to the environments ofthe new battlefield. The challenge for our community is to developthe advanced materials that will provide the Army improved effectivenessacross the full spectrum of operational environments. Tomake things even more complex, researchers must address additional21st Century requirements beyond mere system performance.They must give greater consideration for ‘green’ solutions thatreduce the generation or introduction of toxic materials into theenvironment during production, training, deployment, or othermilitary operations.Editorial:Adapting to a Changing BattlefieldMuch recent work has been undertaken to improve the survivabilityof vehicles and their occupants subjected to fire from ballisticweapons as well as blast and fragmentation from mines, Rocket-PropelledGrenades (RPGs), and IEDs. Discussed in thispublication are several of the emerging materials that will enableimproved yet lighter armor for future systems. Included are ceramic,metal, and composite material research programs that showtremendous promise. Pastarmor research has yielded theeffective but heavy systems weemploy today. Becoming moreeffective against insurgents requires lighter armored vehiclesemploying innovative materials including transparent armor forwindshields and visors. Armor research has been and continues tobe a significant activity at the Army Research Laboratory.Other subjects of significant interest are those related to ordnancematerials, including propellants, projectiles, and even the systemsused to shoot them. One area of concern lately has been to findreplacements for lead bullets and depleted uranium (DU) kineticenergy penetrators. Environmental concerns are the primary reasonsfor finding alternative materials for these applications, and several ofthe articles here discuss the programs addressing the problem.One approach to reduce the weight and complexity of systems isto develop multifunctional materials that perform two or more primaryfunctions. Army researchers have many programs underwaythat are leading to technologies that exploit this concept and severalof them are mentioned here. A multitude of other technologydevelopment efforts are also being examined to develop the newgeneration of lighter, higher performance materials needed toimprove warfighting effectiveness.The twenty separate articles contained in this issue of theAMPTIAC Quarterly will provide you with a glimpse at some of thetechnologies that will enable the Army to transform into a moremobile, survivable, and lethal force while simultaneously becominga better steward of the environment. There are numerous technicalchallenges yet to be overcome, but as the reader will notice the ArmyResearch Laboratory’s Weapons and Materials Research Directorate(ARL/WMRD) is actively pursing those technologies necessary forthe Army to transform the face of the new battlefield.David H. RoseAMPTIAC DirectorEditor-in-ChiefChristian E. Grethlein, P.E.Creative DirectorCynthia LongContent EditorsRichard A. LaneBenjamin D. CraigInformation ProcessingJudy E. TallarinoPatricia McQuinnInquiry ServicesDavid J. BrumbaughProduct SalesGina NashThe AMPTIAC Quarterly is published by the Advanced Materials and Processes Technology Information AnalysisCenter (AMPTIAC). AMPTIAC is a DOD-sponsored Information Analysis Center, administratively managed bythe Defense Technical Information Center (DTIC). Policy oversight is provided by the Office of the Secretary ofDefense, Director of Defense Research and Engineering (DDR&E). The AMPTIAC Quarterly is distributed tomore than 15,000 materials professionals around the world.Inquiries about AMPTIAC capabilities, products, and services may be addressed toDavid H. RoseDirector, AMPTIAC315-339-7023EMAIL: amptiac@alionscience.comURL: http://amptiac.alionscience.comWe welcome your input! To submit your related articles, photos, notices, or ideas for future issues, please contact:AMPTIACATTN: CHRISTIAN E. GRETHLEIN201 Mill StreetRome, New York 13440PHONE: 315.339.7009FAX: 315.339.7107EMAIL:amptiac_news@alionscience.com

James M. SandsParimal J. PatelPeter G. DehmerAlex J. HsiehWeapons and Materials Research DirectorateArmy Research LaboratoryAberdeen Proving Ground, MDMary C. BoyceDepartment of Mechanical EngineeringMassachusetts Institute of TechnologyCambridge, MABack in the fall of 2000, AMPTIAC printed an article on transparent armor in our quarterly journal (known then as the AMPTIACNewsletter). It was written by four gentlemen from ARL – Parimal Patel, Gary Gilde, Peter Dehmer, and James McCauley. Note thattwo of these men, Dr. Patel and Mr. Dehmer, have returned to help prepare this update (please also note that Dr. McCauley has writtenthe leadoff article in this issue). From the time of its publication to the present day, it has been, by an overwhelming margin, the singlemost popular article in our history (as measured by reprint requests and downloads off our website). There was some initial hesitation towrite this piece, as we tend to shy away from revisiting old ground unless some significant advance in the state of the art has taken placesince initial publication. However, we pressed ahead for two very important reasons: First, we could not dedicate a section of the Quarterlyto Survivability Materials without covering Transparent Armor. Second, we were (and are) confident that our readers, both new andold, will find this a fascinating treatment of the subject. Readers familiar with our earlier article will find some familiar portions in thispiece, but will also be gratified to learn of the great advances made in the past four years. It will quickly become apparent to all that the“torch” that is transparent armor has been successfully passed from one team to the next. Enjoy! - EditorINTRODUCTIONWhile many of the hazards faced by our soldiers in the field areapparent, a more thorough assessment of a typical soldier’s“theater of risk” reveals some suprising findings. A significantportion of a soldier’s duties involve protection of facilities andpersonnel in confined environments. Thesetasks are akin to providing guard duty atmilitary posts and check-points in strategiclocations. The soldiers posted at these positionsbecome the front lines of defenseagainst impending attack; however, theyare often also the target of terrorist-basedactions because of their relatively exposedposition. One manner of reducing the riskto the soldier is to provide enhanced protectionin equipment that allows the soldierto perform the assigned duties efficiently,while offering some ballistic safety. Currentsystem ballistic safety is limited by the massefficiency of existing materials designs. Oneof the key protection capabilities for successfulimprovements in mission safety istransparent ballistic glass that enablessoldiers to observe the potentially hostileenvironment through a protective shield.Therefore, transparent armoring technologiesare a significant component of military effectiveness.Transparent materials are a subsection of materials that aretransparent to certain wavelengths of energy. Window glass,for example, is transparent in the visible frequencies, while aradome material, such as fused silica, is transparent to radarFigure 1. Face Shield and Body Shield.frequencies. The Army Research Laboratory (ARL) has investedconsistently to bring the best technical advancementsin polymers, glasses, ceramics and adhesives to transparentsystem designs. These materials have application not only inballistic glass but also in infrared (IR) domes, radomes, sensorprotection, and personnel protection. Thispaper will provide an overview of the technologyand applications, give specificexamples of materials of interest, and relatethe challenges that have been overcomeduring the past decade while also discussingthose that remain.Ballistic glass is a material or system ofmaterials designed to be optically transparent,yet protect against ballistic impacts,and resist fragmentation. This class of materialsis used in such diverse applications asprotective visors for non-combat usage,including riot control (Figure 1) or explosiveordinance disposal (EOD) (Figure 2)actions, to protect sensors from debris, andto protect vehicle occupants from terroristactions or other hostile conflicts. Each ofthese systems is designed to defeat specificthreats; however, there are general requirementscommon to most. The primaryrequirement for a transparent armor system is to provide amulti-hit defeat capability while retaining visibility in the surroundingareas. Land and air platforms of the future have severalparameters that must be optimized, such as weight, volume,and cost. Often, these ballistic protection materials must28The AMPTIAC Quarterly, Volume 8, Number 4

e compatible with night vision equipment to allow the soldiersto be effective in all environments. One potential solutionto increase the ballistic performance of a window materialis to increase the thickness. However, this solution isimpractical in most applications, as it will increase the weightand impose space limitations or impact other systems in thefielded environment. In addition, thick sections of transparentmaterials often experience greater optical distortion than thinnersections, which reduces the transparency.The development of modern armor systems is driven by thedoctrine of fire and maneuver. Thedemand is for lightweight solutionsthat enable soldiers and vehicles tobe highly mobile, destroy their targets,and return home safely. Thearmor must provide protection froma wide variety of bullets and fragments,and must not hinder the soldier’sability to do their job. Themodern battlefield has evolved tothe point that there are no definedbattle fronts, and therefore everyoneis at risk and must be protected.Each of these issues must be consideredwhen designing any armor system.As opposed to conventionalarmor, transparent armor has anFigure 2. EOD Helmet.additional requirement in that the material must be transparentto visible light, which dramatically limits the material choicesfor an armor system.Among the transparent materials available, new material systemsbeing explored to meet the requirements for ballisticapplications include crystalline ceramics, new polymer materials,new interlayer technologies, and new laminate designs. Thefundamentals of transparent ballistic materials are discussedhere, along with insights for future designs and potentialapproaches to advanced technologies.APPLICATIONS AND REQUIREMENTSCommon military applications for transparent armor includeground vehicle protection, air vehicle protection, personnelprotection, and protection of equipment such as sensors. Commercialapplications requiring transparent armor include itemssuch as riot gear, face shields, security glass, armored cars, andarmored vehicles.VisorsWith the onset of many new peacekeeping roles within the military,it is necessary to provide a greater degree of protection tothe individual soldier. Facial protection via the use of transparentarmor is one area of interest within the Army, marked by arecent program within the Army Research Laboratory toimprove the current visor design.[1] Two types of visors weremarked for improvement, the riot visor, and an explosive ordnancedisposal (EOD) visor.Riot visors are typically made from injection-molded polycarbonatethat has an areal density of 1.55 lb/ft 2 (0.25” thick).The riot visor is a piece of equipment that is designed to defeatthreats from large, low-velocity projectiles, such as rocks andbottles, and small, high velocity fragments. Recent research anddevelopment has focused on replacing the baseline polycarbonatedesigns with improved polymer materials. Among the candidatematerials for riot visor improvements are advancedpolyurethane polymers. Polyurethanes possess a wide range ofproperties that can be exploited to improve performance orreduce weight. However, due to the limited size of the windowelement in the riot visor configuration, the decision was madeto keep the existing platform design weight and improve theballistic performance. A target performanceenhancement of 30%improved fragment protection wasselected. In addition, the improvedballistics element means that riotvisors achieve new standards for 9mm handgun protection.A second application for lightweightarmors is in the EOD visor.Because the EOD visor covers a significantfacial area, the contributionof the transparent laminate to theoverall system mass is significant.Therefore, ballistic programs toimprove performance in EODdesigns sought to reduce the weightof the overall application. For anequivalent protection baseline, a 30% reduction in total masswas desired as a success metric.Unlike the monolithic riot visors discussed previously, theEOD visors are composed of laminated plastics. ARL attemptedto reduce the weight of EOD visors by varying both laminateconstruction and material selection. Laminate designsinvestigated included plastic/plastic, glass/plastic, and glassceramic/plastic.[1]Ballistic testing of these material systemsparticularly encompassing polyurethane showed a markedreduction in areal density from the current laminated design.Among the laminates tested, those possessing hardened designs,e.g., those with Vycor fused silica and TransArm, providedthe best ballistic performance.Electromagnetic WindowsMany ceramic materials of interest for transparent armor solutionsare also used for electromagnetic (EM) windows. Theseapplications include radomes, IR domes, sensor protection, andmulti-spectral windows. Optical properties of the materialsused for these applications are very important, as the transmissionwindow and related cut-offs (UV, IR) control the electromagneticregime over which the window is operational. Notonly must these materials possess abrasion resistance andstrength properties common of most armor applications, butbecause of the unique high-temperature flight environment ofaircraft and missiles, they must also possess excellent thermalstability.Artillery ProjectilesEM window materials are also currently being investigated bythe Army for use in artillery projectiles. While the optical trans-The AMPTIAC Quarterly, Volume 8, Number 4 29

Figure 3. Army Ground Vehicles.parency is not important for this application, material propertiessuch as low dielectric constant and low loss tangent are imperative.[2]Future artillery projectiles will be subjected to muchhigher muzzle velocities (Mach 3), where aerodynamic heatingbecomes a concern. New window materials must be capable ofwithstanding 15,000 g’s of inertial setback loads with 15,000rad/s 2 of angular acceleration. Additionally, as communicationrequirements change, the transmission and reception frequenciesare changing to accommodate the more rapid exchange ofdata. Available plastic window materials are incapable of survivingin these environments. The new operational demandsrequire new polymeric and complex laminate constructionsfor the radome and EM window designs. Prototypes for newsystems utilize a glass-ceramic material known as Macor ® for thenose tip * , which was chosen for electrical properties, high temperaturecapability, and ease of machining. However, replacementceramics with a reduced dielectric constant and higheroperating temperature capabilities are still sought.Ground VehiclesGround vehicles represent one of the largest application needsfor transparent armor, including high mobility multi-wheeledvehicles (HMMWVs), tankers, trucks, and resupply vehicles(Figure 3). There are several general requirements for theapplication of transparent armor to windshields and side windowson these vehicles.[3] The first is that the armor must beable to withstand multiple hits since most threat weapons aretypically automatic or semiautomatic. The windows must alsobe full-size so that the vehicle can be operated without reducingthe driver’s field of view. One of the requirements forfuture transparent armor systems intended for vehicle use[3] isa reduction in weight. The added transparent armor weightcan be significant, often requiring enhancement of the suspensionand drive train to maintain the vehicle performance capabilityand payload capacity. Thinner armor systems are alsorequired, as thinner windows can increase the cabin volume ofthe vehicle. Future systems must also be compatible with nightvision goggle equipment and offer laser protection.Due to their size and shape, the majority of armor windowsare constructed of glass and plastic, but reductions in weightand improvements in ballistic protection are needed. Based onthe number of vehicles in service, the window dimensions, andthe associated costs, improved glasses, glass ceramics and polymersappear to be the new materials of choice. Compositionalvariations, chemical strengthening, and controlled crystallizationare capable of improving the ballistic properties of glass.Glasses can also be produced in large sizes with curved geometries,and can be produced to provide incremental ballisticperformance at incremental cost. The use of a transparentceramic as a front-ply has been shown to improve the ballisticperformance further while reducing the system weight.An excellent example of the current need for transparentmaterials is represented by the recent fielding of add-on armorkits for the military line of HMMWVs. In an effort toimprove the protection of soldiers in theater operations, theArmy designed an add-on armor capability that was developedand fielded in a very short suspense. More than 4000 of thesearmor survivability kits (ASKs) have been produced in lessthan one year. However, the kits add significant weight to thetransport platforms, and therefore, impact mission loads forthe vehicles. The transparent armor in these kits is a significantburden, contributing as much as 30% to the overall weight butonly covering 15% of the total area. Developing lighter weightsolutions with improved protection will allow transition ofthese armor upgrade kits to vehicles without dramaticallyimpacting mission capability.Air VehiclesAir vehicles include helicopters, anti-tank aircraft, fixed wingaircraft, and other aircraft that are used in combat and supportroles. Transparent armor applications in these vehicles includewindshields, blast shields, lookdown windows, and sensor protection.Requirements for aircraft systems are similar to thosefor ground vehicles, and systems are designed for use against7.62 mm, 12.7 mm projectiles, and 23 mm High ExplosiveIncendiary (HEI) threats.The Army Aviation Applied Technology Directorate has anAdvanced Lightweight Transparent Armor Program (JTCC/AS)to develop advanced transparent armor with an areal densityno greater than 5.5 lb/ft 2† . The goal of this program is to defeata 7.62 mm PS Ball M 1953 threat. This constitutes a 35%reduction in weight over currently fielded systems. Opticalrequirements include a minimum 90% light transmission witha maximum haze of 4%. A second goal of the program is todefeat the blast and fragments from a 23 mm HEI projectiledetonated 14 inches from the barrier, without exceeding a6 lb/ft 2 areal density limit.[4] Many of these transparent armorsystems utilized for military applications would also have usein commercial systems, such as law enforcement protectionvisors, riot gear, and windows in cars, trucks, and buses, as wellas structural hardening in buildings. The cost/performancetrade-off is not as critical in the commercial arena since VIP protectionsystems can use more exotic and expensive materials toprotect against significant threats.DESIGNING A TRANSPARENT ARMOR SYSTEMPolymeric MaterialsAmorphous glassy polymers are used in a wide variety ofapplications in which transparency is critical; these includelenses, goggles, and face shields for soldier, law enforcement,and medical personnel; ballistic shields for explosive ordnancedisposal personnel; windows and windshields for vehicles; and30The AMPTIAC Quarterly, Volume 8, Number 4

canopies for aircraft and helicopters. The vital considerationfor materials selection is the behavior of the material inresponse to mechanical deformation, chemical exposure,ultraviolet irradiation, heat, humid environments, and otherpotential in-service hazards.Two distinct groups of glassy polymers are classified in relationto their physical and thermo-mechanical properties asthermoplastics and thermosets. Thermoplastics are linear orbranched polymers that become soft and deformable uponheating, while thermosets, on the other hand, are rigid and possessan interconnected three-dimensional network that limitsflow under elevated temperatures. Both types of polymers havea subset of materials that are visibly transparent.Transparent polymers can be fabricated with sufficientlyhigh strength and stiffness and developed as lightweight, lowcostalternatives to traditional glass components. Unlike glass,the physical properties of amorphous polymers vary significantlywith temperature and rate of deformation. In general,material characteristics of a polymer change from being a rigidglass to an entangled rubbery-like structure once heated abovea critical temperature known as the glass transition temperature.This critical temperature is indicative of an upper limitfor the service temperature applicable to these amorphouspolymeric materials.ThermoplasticsAs a thermoplastic material, poly (methyl methacrylate)(PMMA) has better impact resistance than most types of glassand is commonly used as a substitute for glass housings orenclosures, where hardness, optical clarity, and ultraviolet (UV)stability requirements are essential. The use of PMMA for militaryapplications dates back to World War II. PMMA was thematerial of choice (really the only material available) for lightweightdomes and canopies on aircraft of that era.PMMA is manufactured in sheet form via casting or extrusion,and the product sheets can then be thermally formed intocomplex shapes. Casting is used to produce the thicker sheetsusually used in transparent armor applications. As casting technologyhas improved, PMMA has found wide use as bulletresistant glazing for protecting against handgun threats. MonolithicPMMA is nevertheless brittle, and polycarbonate (PC)has been used as a substitute in applications where impact performanceis most critical. PC has outstanding impact toughness(almost 300-times stronger than single-strength glass), and ithas a higher glass transition temperature and better flame andfire resistance than PMMA. However, one of the drawbacks ofPC is its susceptibility to degradation upon exposure to organicsolvents, UV-irradiation, scratches, and abrasion. To be usedin outdoor applications, PC requires UV-stabilizers and surfacemodification with hard coatings to ensure long-term durability.Despite these limitations however, polycarbonate (PC) hasbeen the material of choice for both military and commercialeye protection since its introduction nearly 40 years ago.For thermoplastics including PC and PMMA, extrusionmolding and injection molding are the predominant processesfor making an end product. The choice of proper molecularweight of polymers is critical in these processes to ensuredesired rheological characteristics at elevated temperatures.Alternatively, some commercially available PMMA are fabricatedby casting the material between two glass plates to achieve acast sheet with excellent optical clarity at a desired thickness.An advantage of this casting process is the ability to produce aPMMA sheet with a significantly higher molecular weight andenhanced mechanical properties, which are not attainable inthermo-molding processes due to the practical limits anddegradation of polymers.Polycarbonate is the most common plastic used for transparentarmor applications. It is an inexpensive thermoplastic materialthat is easily formed or molded, and offers excellent ballisticprotection against small fragments. PC has been used by the USArmy for aircrew visors and sun, wind, and dust (SWD) gogglessince the early 1970’s and spectacles since the mid 1980’s. Thisequipment provides protection from small (1 gram or less), slowmoving (650 ft/sec) fragments, but does not provide full-facecoverage. It is currently used in applications such as goggles,spectacles, visors, face shields, laser protection goggles, and isalso used as a backing material for enhanced protection frommore advanced threats. It has been found to be more effective inthe thinner dimensions required for individual protection thanin the thicker sections required for vehicle protection.Several investigations have been undertaken to develop newthermoplastic polymers for improved ballistic protection. Theefforts uncovered several candidate materials, including transparentnylons. However, many of these promising materials arenot available in commercial quantities which limits their use forfuture designs.ThermosetsIn the ophthalmic industry, CR-39 ® allyl diglycol carbonatemonomer is sometimes used for casting plastic lenses for prescriptioneyewear that require high quality optical properties.During World War II, CR-39 ® resin was used to produce transparenttubes that were embedded in fuel lines to function as avisible gage that indicated fuel flow to each engine. These newplastic tubes replaced glass tubes, which often shattered duringcombat, spraying gasoline throughout the cockpit. Plastics madefrom CR-39 ® exhibit excellent chemical resistance and thermalproperties, yet are thermosets in nature and do not possess highimpact strength. A new series of thermoset polyurethane-basedpolymers are currently commercially available, which offerexcellent chemical resistance and impact strength and can beformulated to meet the desired physical and mechanicalproperties. Lenses or other forms of plastics fabricated fromcastings of either CR-39 ® or polyurethane-based thermosetpolymers are commercially available.Polyurethanes (PU) have a unique morphology, possessing acombination of hard and soft domains. The properties of a PUcan be tailored to specific applications by adjusting the size andordering of these domains, yielding materials that range frombeing rigid and brittle, like a glass, to flexible and ductile, likean elastomer. It is becoming increasingly common to use anumber of specially formulated urethanes in transparent armordesigns. Thermoset PU’s can be processed via casting or liquidinjection molding. They are clear with a very light tint anddemonstrate very good impact resistance, even when fabricatedin thick sections.The AMPTIAC Quarterly, Volume 8, Number 4 31

Table 1. Typical Polymer Properties for Materials Found in Military Ballistic Systems.Lexan Simula Plexi Glass GPolycarbonate Polyurethane PMMADensity, g/cm 3 1.2 1.104 1.19Areal Density at 1” thick lb/ft 2 6.2 5.7 6.2Tensile Strength MPa 66 62 72Tensile Modulus MPa 2208 689 3102Shear Strength MPa 45 – 62Shear Modulus MPa 1000 – 1151Compressive Strength MPa 83 72 124Compressive Modulus MPa 1660 1241 3030Flexural Strength MPa 104 89 104Flexural Modulus MPa 2586 2020 3280Max Operating Temperature °C 121 149 95Glass Transition Temperature °C 145 -75 100The result of ballistic testing an all-polyurethane visorshowed that it performed better than both polycarbonate andPMMA, on an equal weight basis. Because of its physical properties,this PU shows promise as a replacement for PC formonolithic eye protection and as the backing plies in all-plasticand glass/plastic laminated armor systems. Thermosetpolyurethanes have also demonstrated promise in mechanicaland ballistic screening and are an example of a research areawith a broad horizon for future applications. The specific characterof urethanes can be specifically tailored by selecting theconcentration of backbone monomers, resulting in a verydiverse set of material parameters. A wide range of transparenturethanes have demonstrated improved fracture performancecompared to polycarbonate but with improved durability andimproved scratch resistance. Some basic properties of thesepolymeric materials are shown in Table 1.Material Characteristics and Design of TransparentPolymeric MaterialsAs pointed out, monolithic PC has outstanding impact toughnessparticularly at low temperatures, while PMMA has betterhardness and environmental durability. The ductility of PC isreported to be associated with the molecular motion of mainchain molecules at low temperatures[5]. The molecular motionis presumably present even upon exposure to high-rate impact,and can therefore provide efficient dissipation of impact energy.This molecular mechanism is not prevalent in PMMA; andin fact, monolithic PMMA has significantly lower impact energyabsorption capability than PC, particularly in the thicknessrange of interest for eye/face protection applications. As a consequence,the potential of monolithic PMMA has not beent 1t 2historically realized in the ophthalmic industry due to the concernof spall upon impact, and thus PC is the predominantchoice of material for eye protection.Recent experimental results revealed that monolithic PMMAexhibits a greater increase in energy absorption when the platethickness is increased compared to PC.[6] Furthermore, PMMAand PC plates with an equivalent thickness of about 12 mmhave displayed similar impact performance against 0.22-caliberfragment-simulating projectiles, albeit absorbing the energy bydifferent deformation and failure mechanisms. The challenge isto choose an adequate transparent armor from the numerouscommercially available products that are claimed to be capableof withstanding a level of ballistic impact according to theNational Institute of Justice (NIJ) specifications and standards.In general, the material characteristics of most concern tosystem engineers include the overall weight (or areal density),optical clarity, and cost. However, from a material scientist’sperspective, a better understanding of molecular mechanismson high-rate mechanical deformation is important to facilitatethe synthesis and design of next generation transparent polymericmaterials with desired strength and toughness.[7]Recently, Dr. Boyce’s team at the Massachusetts Institute ofTechnology’s Institute for Soldier Nanotechnologies hasdemonstrated a new approach to design novel hierarchicalassembly materials with significantly improved ballistic impactresistance against a fragment simulating projectile[8,9]. Thenew macro-composite material assembly, shown in Figure 4,encompasses a distribution of discrete lightweight components,such as platelets, discs, tablets, etc., dispersed in a continuousmatrix of another lightweight material possessing contrastingand complementary mechanical behavior (e.g., hardness, stiffness,ductility, and strain-hardening). Inthis macro-scale demonstration, thedimensions (thickness, t 2 and diameter,d) of the discrete components are smallUniform, Graded orRandom DistributionFigure 4. Hierarchical Material Assembly for Macro-Scale Demonstration.d(but still macro-scale) in comparison tothe overall sample thickness (t 1 ). In addition,the geometrical parameters such asthe size and distribution of discrete discscan be tailored.Preliminary findings, obtained for asimplified materials assembly design32The AMPTIAC Quarterly, Volume 8, Number 4

Figure 5. Cracks Arrested at the Interface of PC Matrix andPMMA Disc ( ~ 1” Dia.).consisting of PMMA discs distributed in a PC sample, demonstratethat the overlapping of discs increases the interactionzone between the projectile and the target by forming a networkof interacting energy absorbing components. Experimentsand computational simulations indicate that this magnificationin the interaction zone results in a greater energyabsorption and increased penetration resistance. This newdesign also demonstrates an enhanced multi-hit defeat capability.Figure 5 shows the impact zone of a recovered hierarchicalassembly sample. The brittle failure of PMMA discs facilitatesthe impact energy dissipation, yet it is confined locally and thecracks are arrested at the matrix-platelet interface, thus inhibitingcatastrophic failure.The above configuration is an example of how engineeringcomposite designs can improve energy absorption by inducingdesired failure criteria into the polymer matrix. Future effortsseek to extend this knowledgeof polymer failure duringballistic defeat intodesigning nanostructuredpolymer matrix materials.The proposed outcomefrom such research is toincrease the multi hit performanceof polymer matrixtransparent armor solutionsby reducing the probabilityof catastrophic failure foreach impact.Regardless, the performanceparameters of boththermoplastic and thermosettingpolymer materialsare being advanced, andComparablePerformancecan be exploited to improve ballistic protection limits in militaryand commercial applications. There is significant work tobe performed, however, to transform ideas into fieldable andreliable designs.Glasses and Glass-CeramicsGreater requirements for optical properties and ballistic performancehave generated the need for new armor materials. Themajor challenges for these materials are cost, available sizes, andthe ability to produce curved products at reasonable deliverycosts. Chemical or thermal treatments can increase the strengthof glasses, as can the controlled crystallization of certain glasssystems to produce transparent glass-ceramics. AREVA, Ltd. §currently produces a recrystallized lithium silicate-based glassceramicknown as TransArm, for use in transparent armorsystems. It has all the workability of an amophorous glass, but itdemonstrates properties similar to a ceramic after it has beencrystallized. Vycor is a 96% fused silica glass, which is waterclear,high-strength, and shows promise as an armor material,especially because of its low specific gravity.There are several inherent advantages of glasses and glassceramics.First, compared to more traditional ceramics, the costof glass-ceramics is lower. Glass-ceramics can be processed toproduce curved shapes that are often achieved only by costlymachining for traditional ceramics. Finally, the fabricationmethods of glass-ceramics allow large material shapes to beachieved, since much of the processing is akin to glass manufacturing.All of these advantages lead to an improved readinesslevel for inclusion in window designs.BAL 38 Plus -3.62” thick 41.45 lb/ft 2BAL 38 -3.07” thick 34.68 lb/ft 2BAL 31 -2.48” thick 27.8 lb/ft 2ALON Laminate1.33” thick 16.7 lb/ft 2ALON Laminate0.921” thick 10.5 lb/ft 2Image furnished courtesy of Surmet CorporationFigure 6. Comparison of ALON to Standard Armor Systems**.Transparent Crystalline CeramicsTransparent crystalline ceramics are used to defeat advancedthreats. Three major transparent candidates currently exist:aluminum oxynitride (Al 23 O 27 N 5 ) (ALON ‡ ), magnesiumaluminate spinel (MgAl 2 O 4 ) commonly referred to as justspinel, and single crystal aluminum oxide (sapphire).ALON, one of the leading candidates for transparent armor,is patented by the US Army and its production and developmentwas advanced by the Raytheon Corporation. Figure 6provides a comparison between representative sections ofALON and some glassbasedballistic standards(BAL 31 and BAL 38).Thicknesses of comparableballistic performance arehighlighted.The incorporation ofnitrogen into aluminumoxide stabilizes the matrix,and results in a cubic crystalstructure that is isotropicand can be produced as atransparent polycrystallinematerial. Polycrystallinematerials can be producedin complex geometriesusing conventional ceramicforming techniques such aspressing and slip casting. Table 2 lists some properties ofALON compared with other ceramics and glass-ceramics.Although becoming commercially viable, ALON still isavailable only in limited sizes and at relatively high costs, owingin large part to the post manufacturing polishing costs, particularlyfor armor based needs where optics are important.The Surmet Corporation has acquired Raytheon’s ALONbusiness and is currently building a market for the technologyin the areas of point of sale scanner windows and as alternativesto quartz and sapphire in the semiconductor market. TheThe AMPTIAC Quarterly, Volume 8, Number 4 33

Table 2. Selected Mechanical Properties of Transparent Glasses and Ceramics.ALON Fused Silica Sapphire Spinel Zinc SulfideDensity g/cm 3 3.69 2.21 3.97 3.59 4.08Area Density (at 1” thickness) lb/ft 2 19.23 11.44 20.68 18.61 21.20Young’s Elastic Modulus GPa 334 70 344 260 10.7Mean Flexure Strength MPa 380 48 742 184 103Fracture Toughness MPa√⎺m 2.4 - - 1.7 -Knoop Hardness (HK 2 ) GPa 17.7 4.5 19.6 14.9 2.45Figure 7. Three Product Stages of a Saphikon ® EFG SapphireWindow; Including As-Grown, Rough Cut and Optically Polished.high hardness of ALON provides a scratch resistance thatexceeds even the most durable coatings for glass scanner windows,such as those used in supermarkets, thus leading toenhanced life cycles. Leveraging ALON into new applicationsis a mechanism to increase ALON production andcapabilities, which will facilitate the fielding of armor designsfor military applications.Surmet has successfully produced a 15″ x 18″ curvedALON window and is currently attempting to scale-up thetechnology and reduce the cost. Through government smallbusiness innovative research (SBIR) and other investmentfunding, the US Army and US Air Force are pushing the envelopeof development into next generation applications, includingdomes for advanced missile targeting systems and armorfor commercial and military vehicles.Ceramic magnesium aluminate spinel (spinel) is transparentin its polycrystalline form andpossesses a cubic crystal structure.Transparent spinel hasbeen produced by sinter/hotisostatic pressing (HIP), hotpressing, and hot-press/HIPoperations. The use of a HIPcan improve optical and physicalproperties of spinel byincreasing density and reducingvoids resulting from powderconsolidation and bindervolume. Some typical propertiesof spinel are listed inTable 2.Spinel offers some processingadvantages compared toALON, especially sincespinel powder is availablefrom commercial powdermanufacturers in bulk quantities, while ALON powders areproprietary. Spinel is also capable of being processed at muchlower temperatures than ALON and has been shown to possesssuperior optical properties within the IR region.[10] Theimproved optical characteristics make spinel attractive in sensorapplications where effective communication is impacted by theprotective dome’s absorption characteristics. Opening thetransparent frequency range implies that spinel-based sensorprotection may offer enhanced performance capability. Thespinel products business is being pursued by two key manufacturersin the United States, Technology Assessment and Transfer(TA&T) and the Surmet Corporation. Despite significantinvestments in the technology, spinel products are still availableonly in research applications at this time.Polishing the finished ceramic products is an essential processto achieve optical clarity and low haze. Whether for scanner orarmor applications, windows require a high degree of mechanicalpolishing with diamond pastes to achieve an optical finish.The number of processing stages and length of processing timedrives up final production costs and limits the supply rates formany of the advanced polished ceramic designs. Additionally, ascurvature is introduced into the formulation of new armor platforms,more complex and automated polishing equipmentbecomes essential to keeping distortions low, allowing parallelsurface machining in curved structures. New approaches intendedto reduce finishing costs are underway and may lead toimproved capability for fielding large-dimensional transparentceramics. Clearly opportunitiesto produce opticallytransparent ceramics withminimal polishing wouldreduce overall product costssignificantly.Sapphire is a transparentceramic possessing a rhombohedralcrystal structure.From a production andapplication perspective,sapphire remains the mostmature transparent ceramicand is available from severalmanufacturers, but thecost is high due to thenecessary high processingtemperatures and machiningand polishing steps.Sapphire has a very highstrength, but clarity and transparency are still highly dependenton the surface finish. Limitations to larger area sapphiresare often business related, in that larger induction furnaces andcostly tooling dies are necessary to increase beyond currentfabrication limits. However, as an industry, sapphire manufacturershave endured significant competition from coatinghardened glass and new ceramic alternatives, such as ALONand spinel, and still managed to offer advanced capabilitiesand expand business markets.The high level of maturity in sapphire can be attributed totwo business areas, EM windows and electronic/semiconduc-Image Furnished Courtesy of Rob Nash Studios, LLC34The AMPTIAC Quarterly, Volume 8, Number 4

tor industries. One producer, Saint Gobain Group, producestransparent sapphire using an edge-defined growth technique(Saphikon ® EFG Sapphire) that offers unique potential.Sapphire grown by this technique produces an optically inferiormaterial to single crystal sapphire, but is much less expensiveand retains much of the hardness, transmission, andscratch resistant characteristics. With optical polishing, largearea windows can be fabricated to meet commercial demands.Saint Gobain is currently capable of producing 0.43″ thick(as grown), by 12″ x 18.5″ sheets (Figure 7), as well as thick,singly-curved sheets. They have commercialized the capabilityto meet requirements for flight on the F-35 Joint Strike Fighterand F-22 Raptor next generation fighter aircraft. Saint Gobain,however, has not expanded production to make sapphireplates larger than 12″ x 18″. ARL has investigated edgedefined growth sapphires for ballistic window applicationsand determined that sapphire is a competitor to ALON andspinel if product demand can drive production. In a free market,sapphire producers are limited in production volumebecause of the growth methods and product demand, and businessneeds and commercial value drive production decisions.There are some challenges that must be overcome for thesematerials to be viable for window applications. The majorchallenge is in manufacturing large plates (curved anduncurved) that can be made reproducibly with high yields.Another challenge is in finishing the final part. This encompassesthe grinding steps to get the correct geometry and moreimportantly, the final polishing. As the size of the plates getlarger, the equipment available to polish these windows isscarce and is currently, a limiting step in the production ofwindows. Novel techniques need to be developed to grind andpolish windows in a timely, cost efficient manner. Still, thefuture of these technologies offers great promise in dramaticallyimproving soldier protection and in reducing system weightfor future fighting platforms.CONCLUSIONSProtection of all vehicles in the combat theater has become arealized need over the past couple of years. The realization thatfuture business of the United States military will involve regularcombat actions in hostile environments, where single vehiclesand supply convoys are as great a target as organized troopformations, brings with it the realization that all military personnelare at great risk. Coupled with the need to reduce thelogistic burden in theater environments, the military leadershipcontinues to strive for weapons and transportation systems thatpossess reduced weight and operational costs, and increasedmaneuverability and transportability.The approach discussed here involves reducing the weight oftransparent armor systems by incorporating the most advancedtechnical capabilities available from a wide range of materialstypes, specifically polymers and ceramics. Transparent ceramicswere shown to offer significant ballistic protection at reducedweights over conventional glass/plastic systems. Although significantadvances in production capability for advanced ceramicshas been realized over the past five years, several major issuesremain, such as availability, the shapes and sizes available, andcost. Although they are now capable of meeting size demandsfor flat plate ceramics, with transparent areas greater than 12inches by 18 inches, low demand and high production costshave prevented businesses from investing in putting largerdimensions into production. Furthermore, producing transparentceramics that possess compound curvatures remains predominatelya research and development program for all of theceramics industry.Costs also remain high for ceramic armors due to the highpurity powder requirements, the high processing temperatures,long processing times, complex processing steps, and highmachining and polishing costs. Several programs continue toreduce these costs. However, expectations to reach currentglass/plastic systems costs are unrealistic.Polymeric material advancements, such as the improvementof the optical properties of polyurethane, have led to a renewedinterest in these materials to reduce the overall weight of armorsystems. It has been shown that polyurethanes offer superiorballistic performance at a reduced weight, as compared to currentpolycarbonate backing materials.Numerous polyurethane materials are currently beingexplored as direct replacements for polycarbonate. In addition,there are significant research and development activities ondesign, synthesis and processing of advanced, high performancehierarchical assembly or nano-engineered polymeric matrixmaterials among government laboratories, industry, and academia.With successful insertion of these new materials intotransparent armor systems, a significant weight reduction couldbe realized, along with an increase in ballistic performance andability to defeat future threats. Still, the road ahead has dangerlurking in the unseen byways and beyond the next ridge.Therefore, transparent materials for armor applications mustcontinue to improve and increase the protection at the individual,vehicle, convoy, and battalion levels.NOTES & REFERENCES* Corning Inc., One Riverfront Plaza, Corning, NY 14831† Areal density, in units of weight/area, is the typical methodof normalizing ballistic performance of materials of variedconstruction. In general, this can be converted to a traditionaldensity by summation of component densities; however, this isnot usually reported. For a monocoque design, areal densitydivided by thickness is the density§ AREVA T&D UK Ltd., Registered Office. St. LeonardsAvenue, Stafford ST17 4 LX‡ Raytheon Electronic Systems, Lexington Laboratory, 131Spring Street, Lexington MA 02421. Registered trademark No.2554362. March 2002** Property of Surmet Corporation. Used with permission.http://www.surmet.com/alonArmor.html[1] P. Dehmer and M. Klusewitz, Proceedings of 8th DODElectromagnetic Windows Symposium at the USAF Academy,24-27 April 2000[2] M.J. Hollis and F.J. Brandon, Design and Analysis of a Fuze-Configurable Range Correction Device for an Artillery Projectile,ARL-TR-2074, Army Research Laboratory, Aberdeen ProvingGround, MD, December 1999The AMPTIAC Quarterly, Volume 8, Number 4 35

[3] R. Gonzalez, and G.J. Wolfe, Ballistic Transparencies forGround Vehicles, Proceedings of DARPA/ARL/ARO TransparentArmor Materials Workshop, Annapolis MD, November16-17, 1998[4] R.A. Huyett and F.S. Lyons, Advanced Lightweight TransparentArmor (ALTA), USAAMCOM TR 02-D-18[5] D.J. Williams, Polymer Science and Engineering, Prentice-Hall, pp. 333-334 (1971)[6] A.J. Hsieh, D. DeSchepper, P. Moy, P.G. Dehmer, and J.W.Song, The effects of PMMA on Ballistic Impact Performance ofHybrid Hard/Ductile All-Plastic- and Glass-Plastic-Based Composites,ARL Technical Report, ARL-TR-3155, February 2004[7] A.D. Mulliken and M.C. Boyce, Understanding the HighRate Behavior of Glassy Polymers, Proceedings, 24th Army ScienceConference, Orlando FL, 2004[8] M.C. Boyce, A.J. Hsieh, A.D. Mulliken, and S. Sarva,Transparent Lightweight Composite Armor for Protection againstProjectile Impact, MIT Technology Disclosure Case #11256,July 7, 2004[9] S. Sarva, A.D. Mulliken, M.C. Boyce, and A.J. Hsieh,Mechanics of Transparent Polymeric Material Assemblies underProjectile Impact: Simulations and Experiments, Proceedings,24th Army Science Conference, Orlando FL, 2004[10] D.C. Harris, Infrared Window and Dome Materials,SPIE, Washington, pp. 32, 1992Dr. James M. Sands is currently the Leader of the Transparent Materials Technology Team in the SurvivabilityMaterials Branch of the Weapons and Materials Research Directorate at Aberdeen Proving Grounds, MD.Dr. Sands began his research career at the US Army Research Laboratory in 1998, working as a contractor,until joining as a civil servant in 2000. He earned his doctorate in Materials Science and Engineering from thePennsylvania State University; and a BA degree in Chemistry and Mathematics from Augustana College (SD).Dr. Sands has more than 20 combined refereed publications, conference proceedings, and Army technical reportsto his credit.PhotoNotAvailableDr. Parimal J. Patel has fourteen years of experience in processing of ceramics including high modulus oxynitrideglasses and fibers, Si-N-O dome materials, and aluminum oxynitride (AlON). He is currently investigating theprocessing of AlON as well as testing and evaluation of ceramics and glasses for transparent armor applications.He received his BS in Ceramic Engineering from Rutgers University in 1990, with a focus on processing of oxideand non-oxide glass optical waveguides. He received his PhD in 2000 from Johns Hopkins University. His dissertationtopic was “Processing and Properties of Aluminum Oxynitride Ceramics.”Mr. Peter G. Dehmer is currently conducting work in the development, processing and testing of transparent armorfor the both the individual soldier and vehicles. He has over 25 years of work experience in this area. He holds aBS in Plastics Engineering from Lowell Technological Institute (now the University of Massachusetts at Lowell).Dr. Alex J. Hsieh is a Materials Research Engineer in the Survivability Materials Branch at the US Army ResearchLaboratory, Aberdeen Proving Ground, MD. He is experienced in the design, development, characterization andballistic testing of high performance polymeric materials for transparent visors and ballistic shields applications. Hisresearch interests include studies of molecular mechanisms on high-rate mechanical deformation and evaluation ofnanocomposite hardcoatings. Currently, he serves as a visiting research scientist at the MIT’s Institute for SoldierNanotechnologies.Professor Mary C. Boyce is the Kendall Family Professor of Mechanical Engineering at the Massachusetts Instituteof Technology. Her research areas focus primarily on the mechanics of elastomers, polymers, and polymeric-basedmicro- and nano-composite materials, with emphasis on identifying connections among microstructure, deformationmechanisms, and mechanical properties. She has published over 100 technical papers in the field of mechanicsand materials. Professor Boyce has received numerous awards and honors recognizing her research and teachingefforts; among them are the NSF Presidential Young Investigator Award, Fellow of the American Academy ofMechanics, Fellow of the ASME, and Fellow of the American Academy of Arts and Sciences.36The AMPTIAC Quarterly, Volume 8, Number 4