ARMOR anti- ARMOR materials by design

Armor/Anti-Armor

ARMORanti-

ARMORmaterialsby design

by Donald J. Sandstrom

36

Imagine tank armor thatchews up a high-velocityprojectile on impact . . . orcomposites of tungsten and

uranium that lend an antitankpenetrator rod the stiffness ofthe tungsten. the density and py -rophoric property of the uranium,and the surprising strength oftheir mixture or tiny crystal

grains aligned in a sheet of ura-nium that allow it to stretch into along, lethal jet of unbroken metal.These examples illustrate howLos Alamos is using its knowl-edge of materials to design andfabricate new and stronger com-ponents for both armor and pene-trators of armor.

Our interest in applying ma-

Los Alamos Science Summer 1989

terials research to conventionalweapons has its origins in theLaboratory’s nuclear weaponsprogram. To deal with the uniquematerials used in nuclear weap-ons, such as actinides, special ce-ramics, polymers, and so forth,the Laboratory had to developsignificant expertise in materi-als research. Further, the itera-


tive process of theory, design.fabrication, and testing used todevelop nuclear weapons servesas the basis for a similar processin developing conventional ord-nance. The attention to detail inmaterial properties required fornuclear weapons is, perhaps, evenmore important for conventionalweapons.

There is also a complementarilybetween the applications of mate-rials in conventional and nuclearweapons-one that has a syner-gistic effect on both programs, Anuclear weapon releases so muchenergy so rapidly that materialsbehave much like isotropic fluidsand can usually be described byhydrodynamic equations. In addi-

37

Armor/Anti-Armor

tion, the performance of a nuclear de-vice is more dependent on the nuclearand atomic properties of its constituentsthan on material properties, In contrast,a conventional munition subjects ma-terials to less severe deformation rates,and the deformation processes are moredependent on the chemistry and priorfabrication history of its constituents.For example, the behavior of an armor-piercing projectile is strongly affectedby variations in the chemical composi-tion. processing history. microstructure,and mechanical properties of the materi-als from which it was formed.

Further, nuclear reaction times areextremely short, whereas the reactiontimes for conventional munitions are ofthe order of microseconds-sufficientlylong to allow for many types of mea-surements. And generally. very little, ifany, material is recoverable from a testof a nuclear weapon, whereas a test of aconventional weapon frequently leavesa considerable amount of material forpost-mortem analysis,

The philosophy underlying the designof nuclear weapons at Los Alamos istraditionally conservative (in the mostpositive sense), especially in regardto reliability and ease of production.Our approach to conventional weaponsfollows the same philosophy and paysthe same close attention to detail. Westrive to use well-characterized, wel1-understood starting materials, we care-fully control the synthesis and manu-facturing processes. and wc work todevelop a complete understanding of theexperimental results. Only in this wayare we able to relate the performance ofarmor and anti-armor systems to slightand often subtle variations in materialproperties or device design and fabri-cation. I will point out many of’ thosesubtleties as I discuss advances madeat Los Alamos in the design of armorpenetrators and armor, including somesurprising properties of a new type ofceramic armor.

38

A KINETIC-ENERGY PENETRATOR

Fig. 1. These x-ray pictures are orthogonal views of the U. S. Army’s M-833 standard round (a fin-

stabilized, sabot-discarding projectile for tanks) taken after the round had traveled about two and a

half meters from the muzzle of the tank gun. The central rod, or core, is a kinetic-energy penetrator

made from a dense, hard alloy of depleted uranium and titanium, and the tip is hardened steel.

The sabot is a device that allows the pressure of the expanding gas from the burning propellant

to accelerate the core and sabot assembly out the barrel of the gun. The sabot is discarded after

the core exits. These pictures show the beginning of the sabot-core separation. Also, note that

the lower view reveals a bent fin on the core.

Two Orthogonal Views

Kinetic-Energy Penetrators

Weapons designed to penetrate armorgenerally fall into two classes: kinetic-energy penetrators and chemical-energypenetrators. I will discuss the first classnow and return to the second later,

A kinetic-energy penetrator is a solidprojectile, usually fired from a gun, thatuses high-velocity impact (typically, atabout 1 to 2 kilometers per second) todefeat the armor. Examples range from

the simple spin-stabilized slug of a 30-mm cannon to fin-stabilized projectilesthat consist of a long, steel-tipped pen-etrator rod and a sabot that falls free ofthe penetrator after it is tired (Fig. 1 ).If the material strength and kinetic en-ergy of the projectile are sufficient, itpenetrates the armor, In addition, theshock wave generated by the impactmay travel through the armor plate andblow off a portion of its backside. Frag-ments both from this spall and from the


Armor/Anti-Armor

penetrator itself can cause considerabledamage to people and equipment behindthe armor.

Depleted uranium. Materials researchhas made particularly noteworthy con-tributions to the design and develop-ment of the kinetic-energy penetrator.The most effective armor-piercing ma-terial to date is an alloy developed atLos Alamos—an alloy of depleted ura-nium (most of the fissionable isotopehas been removed) and a small amountof titanium (0.75 per cent).

Depleted uranium was considered anattractive material for kinetic-energypenetrators for a number of reasons. Itshigh density (almost twice that of steel)makes it easy to produce a penetratorthat delivers high momentum and ki-netic energy to a small volume of targetarmor. Uranium is highly pyrophoric,and its impact against steel targets atvelocities as low as 30 meters per sec-ond produces burning fragments that canignite fuel or propellants. In addition,depleted uranium is readily availablein large quantities and is considerablycheaper than alternative materials.

Uranium, however, is more reactivethan most other penetrator materials,and its reactivity can result in corro-sion problems, particularly in moistair. In addition, some uranium alloysare susceptible to delayed cracking dueto residual stresses induced by fabri-cation and heat treatment of the rods.The cracking can be avoided if care istaken in the heat treatment to reducesuch stresses and to reduce entrappedhydrogen gas to levels less than a fewparts per million.

Extensive testing at Los Alamos ofuranium alloyed with various metals atdifferent concentrations and processed ina number of ways showed that the alloywith 0.75 per cent titanium had the bestcombination of properties. The alloyhas both reasonable corrosion resistanceand high penetration effectiveness. It

can be heat-treated easily (by water-quenching and subsequent aging in ahigh-vacuum furnace) to eliminate thecracking problem, and its properties arenot sensitive to precise composition.These last two features help give thealloy low manufacturing costs.

The alloy was originally developedand evaluated at Los Alamos for theU.S. Air Force’s GAU-8 system, a 30-mm gatling gun system mounted on theA-10 close support aircraft. The guncan fire a thousand armor-piercing pen-etrator rounds per minute and is said tobe the most effective antitank systemin the world. The uranium-titanium al-loy was so successful that it has beenadopted as the standard for large-caliberpenetrators (such as the one shown inFig. 1).

Dynamic Deformationand Fracture

The penetrating ability of armor-piercing rounds improves with the hard-ness and strength of the material used.Mechanical properties of this nature arenormally determined from the stress-strain curve for that material (Fig. 2).Stress is the force per unit area appliedto a sample, and strain is the relativedeformation of the sample as a resultof that stress. Various kinds of defor-mation can occur (elongation, compres-sion, bending, etc.) depending on thenature of the applied force. If stress tothe material is kept below the so-calledyield point, or proportional limit, thematerial will spring back to its origi-nal undeformed state—in other words,the response is elastic. Once this yieldstrength has been exceeded, however,plastic flow occurs, and the material re-mains permanently deformed. The slopeof the initial elastic region, called theelastic modulus, is a measure of the ma-terial’s stiffness; the slope of the laterinelastic region is a measure of workhardening (since it is the amount of

STRESS-STRAIN CURVE

YieldPoint Plastic Flow

o 20Strain (per cent)

Fig. 2. Many material properties, such as hard-ness and strength, are determined from the re-lationship between stress (the force per unitarea applied to the material) and strain (the re-sulting deformation of the material). The ini-tial, approximately linear part of a stress-straincurve is called the elastic region because ma-terial stressed in this region will not suffer anypermanent deformation when the stress is re-laxed (in other words, the stress-strain curvereturns to the origin). The point at which thecurve leaves the elastic region by bending to-ward the horizontal indicates the onset of per-manent deformation and is a measure of thematerial’s yield strength. Beyond that pointis the inelastic, or plastic-flow, region of thecurve. The slope of the curve in the elastic re-gion is the elastic modulus, a measure of thematerial’s stiffness. The slope in the plastic-flow region is a measure of work hardeningsince a steeper slope means more stress mustbe applied to create a given amount of defor-mation.

stress needed to achieve a given amountof plastic flow).

Generally, it is desirable for a pen-etrator to have a high elastic modulus(high stiffness), high yield strength, andhigh work hardening. For instance, anyenergy lost to plastic flow in the pene-trator is unavailable for destruction of

Los Alamos Science Summer 1989 39

Armor/Anti-Armor

the armor. Similar considerations arealso true of armor materials.

The values of these material prop-erties, however, depend on the rate atwhich the material is strained, and real-istic analyses of armor-penetrator impactrequire knowing both static and dynamicmaterial properties. Static properties areeasily measurable. Moreover, they canserve as a starting point for an analysisof the material since dynamic proper-ties often scale in the same directionas the static properties. Nevertheless, itis the dynamic deformation and failureprocesses that are of paramount inter-est, and these can only be understood bymeasuring properties at high strain rates.

The Materials Science and Tech-nology Impact Facility at Los Alamosincludes a wide variety of test equip-ment for determining material propertiesover a broad range of extreme condi-tions. Several gas guns are used forhigh-velocity impact research, and twosplit Hopkinson pressure bars (Fig. 3),measure the stress-strain behavior ofmaterials at strain rates up to 104 persecond.

Figure 4 is illustrative of the influ-ence of strain rate on the strength andbehavior of a material—in this case,of depleted uranium. Comparing thehigh (dynamic) and low (static) strain-rate curves of Fig. 4 shows that at highstrain rates the material has significantlyhigher yield strength and higher ini-tial work hardening. But as strain in-creases the material thermally softens—the slope of the curve, in this case, ac-tually becomes negative, Such factors,of course, must be well characterizedif one is to fully understand the per-formance of a material during ballisticimpact.

Shock waves. Another factor of greatinterest for the design of armor and pen-etrators is the response of materials toimposed shock. It turns out that shockwaves generated by the ballistic im-

HIGH STRAIN RATES

Bar Stopper

Bore Scope

ace Controller Strain-Gage Amplifier’?

Striker Bar Incident Bar(b)

Sample Transmitter Bar

Recorders

Fig. 3. (a) The split Hopkinson pressure bar can measure the stress-strain behavior of materialsup to strain rates of about 104 per second. Such measurements are performed, as shownschematically in (b), by placing the sample between two pressure bars made from high-strengthsteel, then firing a striker from the gas gun on the left. The impact of the striker with theincident bar generates an elastic compression wave that travels into the sample, causing plasticdeformation of the softer material. A strain gage in the incident bar measures the strain due tothe incident and reflected waves, and another gage in the transmitter bar measures strain due tothe wave that passed through the sample. These measurements are used to calculate the strainrate within the sample and the stress-strain curve, such as the one show in red in Fig. 4. ThisHopkinson bar facility is unique in that it can test samples at temperatures as high as 1000°C.

4 0 Los Alamos Science Summer 1989

Armor/Anti-Armor

DYNAMIC VERSUS STATIC

Fig. 4. Stress-strain curves for depleted ura-nium at strain rates of 5000 (red) and 0.001per second (black). The dynamic, or high-strain-rate, curve shows a higher yield pointand, initially, higher work hardening, followedby lower work hardening as the material ther-mally softens. As such, the curve illustratesthe influence of strain rate on the strength andbehavior of the material. Both samples wereinitially at room temperature (300 kelvins), butthe dynamically deformed specimen reacheda temperature of 470 kelvins at 100 per centstrain.

pact affect the microstructure and thestrength of the components—that is, the“as fabricated” properties of the mate-rials are altered by the passage of theshock waves. The massive structuraldeformations that occur during armorpenetration take place in shock-deformedmaterial with transformed properties.

To study those changes, we use an80-mm-diameter gas gun (Fig. 5) toshoot a projectile called a flyer plateat a target of the same material. Afterimpact the shock-deformed sample isrecovered, examined for microstructuralchanges with a transmission electronmicroscope, and tested for changes inmaterial properties.

Figure 6 displays static stress-straincurves for an aluminum alloy in itsas-received state and after being shock

deformed at 2, 8, and 13 gigapascals.All four curves were measured usinga slow strain rate (0.001 per second).The data show that yield strength in-creases with increasing shock deforma-tion, but work hardening decreases. Bythe time the sample has been strained 20per cent, the decrease in work harden-ing has compensated for the higher yieldstrength, and the curves for as-receivedand shock-deformed material intersect.

As it turns out, the effect of shockdeformation on this alloy is relativelysmall. Other materials, such as uraniumand copper, show much larger changesin their stress-strain curves. In general,we find some materials are very rateand shock sensitive, whereas others arenot. Shock-induced changes to materialsproperties illustrate why it is importantto characterize materials carefully andthoroughly.

Dynamic fracture. Fracture at highstrain rates is another important consid-eration in armor and anti-armor perfor-mance. Although fracture is generallydetrimental to penetrators, certain typesof armor may, in fact, turn fracture toan advantage.

Because dynamic fracture is a com-plex process dependent on structure,processing history, strain rate, and stressstate, it cannot be fully characterizedby a single parameter or measurement.Our approach to a more fundamentalunderstanding is a combined experi-mental and theoretical effort based on

computer modeling. We incorporate intothe models the factors influencing dy-namic fracture, and then compare codepredictions of deformation and fracturewith those that actually occur during ar-mor penetration (see “Modeling ArmorPenetration”).

We are currently studying the dy-namics of how voids are initiated, howthey grow, and how the generation ofsuch voids leads to ductile fracture—forexample, span failure in armor plate.Using the 80-mm-diameter gas gun, thespan strength of a material can be de-termined from axial stress (measuredby noting changes in the resistance ofmanganin gages embedded in the backof the target) or from particle motion atthe back surface of the target (by mea-suring Doppler shifts with a recentlyinstalled laser interferometer). Severalmetals have been studied, including cop-per, rolled homogeneous armor, andcarbon steel. Now that we have mas-tered the experimental techniques, aninvestigation of dynamic brittle fracturein ceramic materials is under way.

Fig. 5. One of the teat devices of the MaterialsScience and Technology Impact Facility at LosAlamos, an 80-mm-diameter, single-stage, gasgun. In this gun, pressurized gas shoots a pro-jectile, or flyer plate, down the launch tube at astationary target in the experimental chamber.The flyer plate and target are typically made ofthe same material, which is the material beingtested for changes due to imposed shock.

I Breech Flyer PlateExperimentalC h a m b e r / T a r g e t

Catch Tank

Launch Tube

THE 80-MM DIAMETER GAS GUN


Armor/Anti-Armor

SHOCK-DEFORMED ALUMINUM

0.4

0.0

2 GPa. . . . . . . . . . . . . . . . . .. . - - - - -

\

13 GPa

As Receivedf 8 GPa

6061-T6 Aluminum Alloy

o 10 20

Strain (per cent)

Fig. 6. The static stress-strain curves of 6061-T6 aluminum alloy as received (black) and afterhaving been shock-deformed (red) at 2, 8, and 13 gigapascals with the gas gun in Fig. 5. Theshock-deformed samples show higher yield strengths but less work hardening. The strain ratefor ail samples was 0.001 per second.

One of our main goals in the work ondynamic processes is to develop consti-tutive relations that describe the stress-strain behavior of materials over a widerange of strain rates, strains, and tem-peratures. Such relations will increaseour ability to predict the behavior ofparticular systems at a variety of condi-tions.

As an example, to model deformationand plastic flow we need relations foryield stress and work hardening. The

described by using an equation of theform

a parameter (or combination of param-eters) that represents the current stateof the material. This equation reflectsthe fact that a material’s yield stresschanges, both because of what is hap-pening to the sample (s) and because of

have been affected, say, by the previoushistory of stress loading.

We can then go further by describing

of the form

rate and F is a function of the ratio ofthe current yield stress to a saturation

siderable working of the material at aparticular strain rate and temperature.In other words, the slope of the stress-strain curve beyond the yield point de-pends, among other things, on the cur-rent stress history of the sample com-pared to a state in which further stressloading of a particular type has no ef-fect.

The advantage of the above type ofanalysis is that the kinetics of workhardening are separated from the con-ditions that determine the yield stressfor a given state. This procedure allowspredictions for complex strain-rate andtemperature histories, such as are typi-cally found in dynamic impact events.We have developed constitutive relationsfor model metals and are now extend-ing this work to armor and penetratormaterials.

Composite Penetrators

The Department of Defense has aneed for gun-launched kinetic-energypenetrators with length-to-diameter ra-tios sufficiently high that the rods willpenetrate modern armor steel configu-rations. However, such rods must havehigh stiffness (that is, high elastic mod-ulus) to resist bending during launchand flight because slight bending maylead to yaw during flight and a glanc-ing blow off the target. The uranium-titanium alloy described above is amarginal candidate for use in the pro-posed penetrator rods because its elasticmodulus is not high enough. Designanalysis shows that composites of de-pleted uranium and of tungsten (whoseelastic modulus for bending is threetimes that of uranium) improve the stiff-ness of the rod and thus, potentially, itsperformance. The stiffness of the com-posite rod is directly related to the ge-ometric placement of the high-modulus

42 Los Alamos Science Summer 1989

Armor/Anti-Armor

material in the rod. It is possible to ar-range the composite so that maximumstiffening is achieved with the leastchange in penetrator density.

Early in the development of the com-posite penetrator, we realized that the

difference between the coefficients ofthermal expansion of the two materi-als was sufficiently large that the tung-sten either fractured or buckled slightly,causing it to lose collinearity with thepenetrator axis. Both these effects, ofcourse, are detrimental to the propertiesof the composite as a penetrator. Weadded various metal powders to the ura-nium component and found, for some.that the coefficients were matched moreclosely. In fact, both the thermal-expan-sion coefficient and the elastic moduluswere altered according to the “rule ofmixtures” (the value of a property of amixture is the sum of component values,each weighted by the relative concentra-tion of the component).

We tested tungsten-uranium compos-ite rods in which the uranium was re-inforced with metallic particles. Therewas both an expected slight increasein elastic modulus (25 per cent) and anunexpected but significant increase inyield strength. For example, the ten-sile (stretching) yield strength increasedfrom the 25,000 psi (pounds per squareinch) typical of cast unalloyed uraniumto 110.000 psi in the cast composite, anincrease of more than 400 per cent.

The significant jump in yield strengthwas an exciting bonus. Penetrators castfrom the uranium-titanium alloy are brit-tle and therefore must be heat treated.but heat treatment is expensive, timeconsuming, and prone to formationof voids in the uranium. Compositepenetrators can simply be cast with-out heat treatment, producing rods withyield strengths in the same range as foruranium-titanium alloy penetrators thathave been heat-treated. The results todate have identified an optimum compo-sition of metallic powders that produces

rods with both high strength and highstiffness.

Another alloy. Our research on thesecomposites has concentrated on devel-oping material with the highest strengthcompatible with a low enough pow-der content to preserve ease of cast-ing. Optical micrographs of both theoriginal powder and a cast uranium-metallic powder material (Fig, 7) showthat part of’ the powder, after casting, ispresent in the uranium as a dispersionof coarse particles. However, the par-ticles are smaller and less angular thanthose found in the starting powder itself,which indicates that part of the metaldissolves in the uranium, forming an-other alloy. Significantly,, regions of fineparticles are also observed: apparently.some of the dissolved metal reprecip-itates during the cooling process. Ourstudies indicate that the precipitation isthe principal cause of the strengtheningof the material.

The addition of metallic powder touranium has been so effective in mini-mizing the mismatch of thermal expan-sion coefficients in the composite thatfabrication of full-scale penetrators haveyielded crack-free rods that require nofurther heat treatment before rnachining(Fig. 8). The simplicity of processing isa significant advantage for manufacture,Further. subscale ballistic tests haveshown that uranium-tungsten compositerods can penetrate targets at relativelylow velocities, whereas pure uraniumrods failed to penetrate the same targetsat any velocity.

Our work to date on the mixturesof uranium and metallic powder alsohints at the possible development of anew high-strength uranium alloy withother highly desirable features not pos-sessed by, say, the heat-treated uranium-titanium alloy. Weldability of’ the mate-rial is quite good, and bend tests showit to have significantly enhanced ductil-

ity’ (the ability to be deformed without

METAL-POWDERMORPHOLOGY

I

ium-Metal Powder I

Fig. 7. These optical micrographs show the

changes in morphology that occur when metal-

lic powder is mixed with uranium and then cast

at about 1350°C. The fact that the occasional

sharply angular regions in the original powder

have disappeared in the cast material indicates

that part of the metal dissolved in the uranium,

and the presence of finer particles in the cast

material indicates that part of that dissolved

metal reprecipitated on cooling.

fracture).

Among the many aspects of the alloy

that we of interest and that need to bestudied are the following:

■ confirmation of the alloy phase dia-gram. especially the solid volubility of

the metal in uranium:

■ determination of the precipitationmechanism;

■ variation of the metal grain size with

thermomechanical processing;■ effect of size and size distribution of


Armor/Anti-Armor

particles in the powder on mechanicalproperties:■ dependence of fracture toughness andother mechanical properties on tempera-ture;■ large-strain behavior and work-harden-ing characteristics:■ resistance to chemical and stress-ill-duced corrosion; and■ relationships between the microstruc-ture and material properties.

I.ow-pressure plasma spray. Cost isa major consideration in the develop-ment of any armor or anti-armor com-ponent. Generally. but not always, thecost of the raw material is only a smallfraction of the overall cost of a com-ponent. and significant savings can berealized by reducing fabrication costs.In general, we have found that simplematerials coupled with reliable engineer-ing and assembly lead to cost-effectivecomponents. With that approach inmind, we have investigated low-pressureplasma spraying as a possible fabrica-tion technique for such things as com-posite penetrators.

The plasma-spray process that wehave developed uses a DC-arc plasma-spray torch in a chamber filled with in-ert gas at a low pressure (Fig. 9). Ahigh-velocity stream of high-temperature

plasma melts injected powder particlesand propels the molten droplets againsta substrate. The result is a rapidly solid-ified deposit of fine-grained material.

Our facility features a single DC-arcplasma-spray torch with two powder-feed inlets. The two inlets allow us todeposit two materials simultaneously.Four axes of manipulation are availablebetween the spray torch and the sub-strate. Plasma spraying should proveto be faster and cheaper than any othermeans of fabricating composite penetra-tors.

Chemical-Energy Penetrators

As mentioned earlier, the second classof penetrators is the chemical-energypenetrator. This weapon defeats ar-mor by using the chemical energy ofa shaped explosive charge, ignited onimpact, to propel a metal liner at thetarget, Typically, the liner is a conicalshell bonded to a machined hollow inthe charge opposite the detonator withthe base of the cone pointing outwardtoward the target (Fig. 10). The shapeof the charge focuses much of its explo-sive force onto the metal liner. turningit inside out and stretching it to form along jet of solid material, (In other ver-sions of the weapon, a compact, high-

Fig. 8. Crack-free composite penetrator rods of tungsten and uranium have been successfully

formed by more closely matching the thermal coefficients of the two materials. The match was

achieved by adding metal powder to the uranium.

4 4

velocity slug is formed.) In effect, theliner becomes a kinetic-energy penetra-tor but with typical impact velocitiesof about 7 kilometers per second com-pared to 1 or 2 kilometers per secondfor normal kinetic-energy penetrators.Although a kinetic-energy penetratortravels from gun to target at high ve-locity. a chemical-energy weapon canwork even if the device is simply placed against the armor and ignited.

Los Alamos has applied much of itsknowledge about materials to the devel-opment of liners for the chemical-energyweapon, find liners made from unalloyeduranium represent the most effectivesuch penetrator currently available. Thefact that the physical and mechanicalproperties of materials are importantdeterminants of the performance of amunitions component is nowhere moreevident than in the case of those lin-ers. For example, the ability of a linerto form a long, stable jet depends in anextraordinary way on both the physi-cal properties of the material and theprocess-induced mechanical properties.

To achieve ideal performance, a pre-cisely fabricated shell of depleted ura-nium bonded into the machined cavityof high explosive must, upon detonation.produce a long, thin, unbroken jet ofmetal traveling at a high velocity. Thejet elongates in flight and must havesufficient dynamic ductility to preventbreakup before striking the target, Suchductility depends strongly on the metal-lurgical history of the liner.

When we recognized that jet breakupwas highly dependent on the material”sprocess history as well as on its phys-ical properties, we undertook a pro-gram. sponsored primarily by the AirForce Armaments Laboratory at EglinAir Force Base, to gain a better under-standing of how metallurgy affects jetformation. To achieve this understand-ing, we studied uranium and other met-als with different crystal structures. Anumber of metallurgical factors emerged

Armor/Anti-Armor

that have an important bearing on linerperformance.

The key to these desired mechani-cal properties is the production of anappropriate crystalline microstructurein the formed liner blanks. To achievethe correct microstructure, we first se-lect a material whose properties arehighly sensitive to mechanical defor-mations and then subject that materialto a series of carefully manipulated de-formations and heat treatments. To learnmore about formation of the preferredmicrostructure, we monitor our materi-als carefully during the various stagesof deformation. Mechanical propertiesof the fabricated sheet are measured inthree orthogonal directions in the ma-terial, crystallographic orientation ofthe grains are determined using x-raydiffraction, and the development of themicrostructure is followed using variousmetallographic techniques.

In addition to our success with de-pleted-uranium jets, we have shown thatliners with reproducible characteristicscan be formed from other metals. Infact, some of our experimental metalliners produce particularly long ductilejets with very late breakup times. Thesame careful attention to processing his-tory and development of the appropriatecrystalline microstructure are criticallyimportant for these metals also.

Ceramic Armor

The opposite side of the coin frompenetrators, of course, is armor. Herealso knowledge of material properties isof critical importance to the design ofarmor packages that will defeat a widerange of penetrators.

Any material used to defeat a high-velocity projectile must deal with thekinetic energy and momentum of thatprojectile with some combination ofthree mechanisms: 1) absorption of theenergy as heat and deformation in thetarget material, 2) rebound of the pro-

PLASMA-SPRAY DEVICE

Argon PowderFeed Pump

I \ \ ’

PlasmaStream

Chamber

Power Supply

Fig. 9. This schematic depicts the major components of a low-pressure plasma-spray device beingused at Los Alamos to explore the low-cost fabrication of such objects as composite penetrators.An 80-kilowatt arc is generated in a mixture of argon and helium gases by applying a DC voltageacross the gap between a tungsten cathode and a cylindrical water-cooled copper anode. Thearc creates a high-temperature, high-velocity plasma stream moving to the right. Powder fad intothis region collides with the stream, melts, and is propelled as molten droplets onto a substrate,where it quickly solidifies, producing a fine-grained deposit. A second powder feed (not shown)allows one to run the feeds simultaneously, producing a layer of mixed material. The whole deviceoperates under a reduced pressure of argon, and the powder feeds operate by being pressurizedwith argon.

jectile, which is how steel armor dealswith a steel projectile, and 3) gross de-formation of the projectile. The lastmechanism is the most efficient wayfor armor to defeat projectiles becausemost of the kinetic energy is absorbedin the destruction of the projectile itselfand, with little rebound of the projec-tile, momentum transfer to the armor isminimized. Unfortunately, conventionalsteel armor is not capable of defeatinghigh-hardness projectiles, such as armor-piercing bullet cores and tungsten rods.in this way.

As a result, a variety of armors havebeen developed, including multilayeredcomposites and reactive armor. (Re-active armor has a layer of explosivematerial that ignites on impact, blowinga facing plate outward to deflect or de-

stroy the projectile. ) However, one ofthe key problems facing armor designersis weight—a well-armored tank may,in the end, be too heavy to move. As aresult, there is a need for armor systemsthat are light but difficult to penetrate.

One approach to weight reductionhas been the use of ceramics, which of-fer exceptional protection for very lightweight. Some of the relevant ceramicmaterials are aluminum oxide (A1203),silicon carbide (SiC), boron carbide(B4C), and titanium diboride (TiB2), allof which have high hardness with an as-sociated abrasiveness, high compressiveand tensile strengths, and good elasticproperties to high stress values.

Microwave processing. High cost iscurrently one of the disadvantages of


Armor/Anti-Armor

ceramic armor, and, as pointed out ear-lier, cost is a major consideration in thedevelopment of’ any weapons compo-nent. A significant portion of the costof ceramic armor lies in the fabricationof monolithic ceramic plates with therequired high density. Here, again, wehave attempted to reduce fabricationcosts—in this case, by using microwaveradiation to process the ceramic,

The ceramics of interest for armormaterials are currently processed us-ing hot pressing (in which graphite diesapply high uniaxial pressure while thematerial is slowly heated) or using hotisostatic pressing (in which an inert gasapplies high isotropic pressure to thematerial in a heated chamber). Thesetechniques generate the high densitiesneeded for ceramic armor but are expen-sive and slow.

Microwave processing, using thecommonly employed frequency of com-mercial [microwave ovens (2.45 giga-hertz), achieves the required high den-sities by starting with cold-pressed ce-ramic powder and rapidly sintering it(heating without melting until the mate-rial forms a dense homogeneous mass).Microwave processing is much faster,and therefore less energy-consumptive.than conventional hot pressing, and theequipment needed is considerably lessexpensive.

Microwave processing also producesa superior material because the heatingoccurs rapidly throughout the entire vol-ume of material. Traditional processingmethods, which depend upon conductionfrom surface to interior, promote growthof large crystal grains in the materialbecause of prolonged heating, much asoverbaking creates a rough, crumblytexture in bread. Microwave sinter-ing couples energy rapidly throughoutthe material and thereby favors den-sification of the material over graingrowth. The end result is a ceramicwith a finer grain size, fewer voids,and fewer stress cracks and thus better

mechanical properties, such as greaterstrength and higher resistance to ballisticpenetration.

Microwave processing also offers ad-vantages in the final fabrication steps.Hot pressing can produce only simpleshapes that must then be machined intothe desired forms. Depending on thedensity and eventual application of theceramic, the machining may requiremany extra hours and the use of ex-pensive diamond-tipped cutting tools.Microwave processing can be applied

THE CHEMICAL-ENERGYPENETRATOR

Casing Metal(a) / Liner

Detonator Explosive

(b) Accelerating

High-Pressure \ IgnitionGases Front

to shapes close to those required for theultimate use.

Although microwave sintering of ce-ramics is not new, we took the processa step further by combining precise po-sitioning in the microwave oven withinsulation techniques that reflect andconcentrate the radiated energy on thesample. much as snow or sand reflectsunlight back to the skin. The resultinggreater thermal efficiency of the pro-cess improved the sinterability of diffi-cult materials such as aluminum oxide.boron carbide, and titanium diboride.We have. for example, been able to sin-ter boron carbide to 95 per cent theoret-ical density (Fig. 11). The time requiredto heat the material from room temper-ature to over 2000 degrees centigrade isunder 12 minutes. whereas conventionalhot pressing takes several hours. Thecapital costs for the Los Alamos mi-crowave facility were less than $35,000,whereas a 3-inch-diameter hot press, theequipment needed to density a boroncarbide sample of the same size, costsbetween $120,000 and $200,000. Fur-ther, energy costs were cut about 18 percent.We are also working on a new com-

Fig. 10. (a) The conical shape of a typical

chemical-energy penetrator is designed to fo-

cus the explosive energy of the charge onto a

metal sheet (red) that lines the conical hollow.

(b) Because the explosive force in the charge

reaches the center of the liner first, this region

is accelerated before the outer regions. (c) As

a result, the liner turns inside out, stretching

into a long jet of material. If the metal liner

has the proper materials properties, it will form

an unbroken jet and will impact the target at

a velocity much higher than that of a typical

kinetic-energy penetrator. (d) This doubly ex-

posed radiograph of a chemical-energy pen-

etrator shows the shaped charge on the left

with, in this case, a hemispherical liner. The

image to the right is the solid jet formed when

the charge was fired.

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Armor/Anti-Armor

posite material for armor applications—aluminum oxide reinforced with platletsof silicon carbide. The platlets, beingsingle crystals, have exceptional tensilestrength and can be used to increase thefracture toughness of ceramics, metals,and perhaps even polymeric. Less than10 minutes of microwave processing arerequired to produce the new compositeat 94 per cent of theoretical density, andwe expect that material to have verygood resistance to ballistic penetration.

Ceramic-Filled Polymer Armor

Ceramic armor for, say, lightweightfighting vehicles and armored personnelcarriers currently consists of an out-side layer of high-density ceramic tilebonded to a backing plate. Conventionalwisdom about such armor had suggestedthat the ceramic should have high im-pact strength and hardness so it canhelp break up a sharp, hard projectile.That requirement implies the ceramicshould possess high elastic impedancecombined with high hardness and highcompressive strength.

Another property that had been felt tobe important for ceramic armor is hightensile strength. The impact load trans-mitted through the ceramic producescompressive stress on the backing plateand a corresponding tensile stress on therear surface of the ceramic tile. The re-sult is plastic yield in the ceramic andthe development of a fracture conoid. Aceramic with high tensile strength wouldresist such fracture.

However, research by Mark Wilkinsat Lawrence Livermore National Labo-ratory indicates that the most importantmechanism for defeat of a projectile byceramic armor is abrasion. The frac-ture conoid in the ceramic spreads fromthe point of impact and generates sharpfragments that are instrumental in help-ing to abrade or erode the projectile.

We recently performed a series ofballistic tests on a new type of armor,


ceramic-filled polymer armor, and theresults were exceptional. Our new ma-terial typically consists of a ceramicaggregate (about 85 per cent ceramicby weight) mixed with a binding poly-mer or other carrier. Such a materialpossesses essentially none of the me-chanical properties deemed importantfor ceramic armor. In fact, the primarymechanism for defeat—erosion of thepenetrator—depends upon the tendencyof the new material to fragment fully.

Design and fabrication. The ceramic-filled polymer serves to illustrate the

importance of the entire design of anarmor package. One of the importantproperties of this material may be itsdilatancy, that is, its tendency to read-ily expand into any free volume whenfractured. But whether dilatancy worksto advantage in the erosion process maydepend critically on how the material isconfined.

The effect of packaging on dilatancycan easily be demonstrated by using riceto represent the ceramic-tilled armor anda pencil to represent the projectile. Ifa pencil is pressed down into a beakerfilled with rice, resistance will be slight.

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Armor/Anti-Armor

TESTING CERAMIC-FILLED POLYMER

But if the rice is confined to a flask witha narrow neck, resistance to the pencilwill be much larger because the riceis unable to move out of the way ofthe pencil. Free volume is available forexpansion in the first case but not in thesecond.

Although a complete explanation ofthe excellent results of ceramic-filledpolymer armor has not yet been ob-tained. it appears that dilatancy is in-volved, A chunk of unconstrained poly-mer simply blows away on impact withlittle or no effect on the projectile. Aproperly designed armor package, how-ever, totally constrains the ceramic-filledpolymer (Figs, 12 and 13), say with abackplate and surrounding layers of ahigh-performance polymeric fiber likeKevlar©. On impact the only free vol-ume is the hole generated by the pro-jectile itself’ as the armor is hit and frac-tures. The resulting expansion of theceramic-filled composite generates avery large number of highly erosive ce-

ramic particles that may be forced outbetween the sides of the hole and thepenetrator, eroding the projectile.

These properties, of course, are quitedifferent from those usually thought ofas ideal for ceramic armor. In fact, theultimate tensile strength of’ ceramic-filled polymer armor is limited by thestrength of the polymer binder, whichtypically is much lower than that ofmonolithic ceramic, Another prop-erty of the aggregate limits compres-sive strength—the polymer bondingagent becomes fluid at low applied

Fig. 12. The before and after of a test ofthe stopping power of ceramic-filled polymer.(a) The various pieces of the test configura-tion in the order in which they are put to-gether, including polymer plates (white), thetarget holder that constrains the polymer (themetal pieces on the left and at the center), andthe armor plate being protected by the poly-mer (the metal piece on the far right). (b) Thesame pieces after the plates have stopped aprojectile without significant damage to the ar-mor plate.

CERAMIC-FILLED POLYMER ARMOR

Fig. 13. This sample of polymeric armor hasbeen cut open to reveal the various layersof ceramic-filled polymeric plates confined be-neath Kevlar©. The ceramic used in the frontplate (black) is boron carbide; the ceramicused in the other plates (white) is aluminumoxide.

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Armor/Anti-Armor

shear stress. This phenomenon, calledthixotropy, can be capitalized on duringmanufacture or repair of the armor be-cause the aggregate-filled polymer willflow under a constant applied formingpressure, allowing the armor to be castor molded at low temperatures.

Lightweight armor systems are cur-rently made of high-density ceramictiles—a very expensive process becausethe ceramic requires high-temperaturefabrication and extensive finish grinding.The polymeric armor requires no high-temperature fabrication or expensive fin-ishing steps and can be easily formed toany required shape, including very largeand thick or very geometrically compli-cated shapes. Additionally, monolithicceramic suffers from a limited abilityto withstand multiple hits because of itspropensity to break up, whereas poly-meric armor, although highly fracturedby the impact, mostly remains in place.

Ballistic tests on an armor packagecontaining ceramic-filled polymer tileshave shown exceptional results. Onan equal-volume basis the polymer-bonded material is almost equal to ahigh-density, high-purity aluminum ox-ide ceramic tile. On an equal-mass ba-sis the ceramic-filled polymer is better!

Ceramic-filled polymer armor canoffer four important advantages overconventional ceramic armor:

■ a reduction in weight of about 10 percent since more than 10 per cent of theceramic is replaced with low-densitypolymer bonding agent;■ a reduction in manufacturing cost ofgreater than 50 per cent due to low-temperature fabrication and eliminationof expensive grinding steps;■ greater ease of in-field repair sinceeither prefabricated, lightweight tiles orthe ceramic and polymer constituentscan be stored on board the vehicle; and■ greater ease of accommodating designimprovements, such as incorporation ofvery hard boron carbide plates in the

modular package to increase the capabil-ity of the armor to break up penetrators.

We are currently exploring in greaterdetail both the abrasion-erosion mech-anism of defeat and the exact contri-bution of packaging constraints on ar-mor effectiveness. Those effects mustbe studied systematically if we are toexploit ceramic-filled polymers for fabri-cating inexpensive, reliable, lightweightarmor for mobile fighting vehicles (see“ATAC and the Armor/Anti-armor Pro-gram”).

A variety of other research on armorand anti-armor materials takes placeat Los Alamos. Those studies rangefrom investigation of other alloys forpenetrators to the use of chemical va-por deposition to infiltrate “open mesh”composite materials. The latter has aparticularly high potential for improvingthe properties of ordnance componentssuch as gun barrels and sabots.

We believe that materials technologyis the enabling—or limiting—technologyfor virtually all conventional weaponssystems. Materials science and tech-nology has progressed to the point that“tailored” properties of materials are areality. The effects of microstructure onliner performance for chemical-energyweapons, the adjustment of the coeffi-cient of thermal expansion and the ac-companying improvements in mechan-ical properties of the tungsten-uraniumcomposite penetrators, and the excep-tional protection offered by ceramic-filled polymer armor are examples ofrather straightforward applications ofdevelopments in materials. These de-velopments, though seemingly simple,are grounded in a thorough understand-ing of materials science and technology.We believe the surface has barely beenscratched and that the future in conven-tional munitions belongs to innovatorsand designers of new materials. ■

Further Reading

John W. Hopson, Lawrence W. Hantel, and Don-ald J. Sandstrom. 1973. Evaluation of depleted-uranium alloys for use in armor-piercing pro-jectiles. Los Alamos National Laboratory reportLA-5238.

Joseph E. Backofen, Jr, Kinetic energy pene-trators versus armor. Armor March-April 1980:13–17.

Joseph E. Backofen, Jr. Shaped charges versusarmor. Part I: Armor July-August 1980: 60-64;Part 11: Armor September-October 1980: 16–21; Part III: Armor November-December 1980:24–27.

Joseph E. Backofen, Jr. Armor technology. PartI: Armor May-June 1982: 39-42; Part II: ArmorSeptember-October 1982: 35-37; Part III: ArmorMarch-April 1983: 18–20: Part IV: Armor May-June 1983: 38-42.

LOS Alamos Science Summer 1989 4 9

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Donald J. Sandstrom is Deputy Division Leaderof the Material Science and Technology Di-vision at the LoS Alamos National Laboratory.He is responsible for working closely with thedivision leader in managing all aspects of the di-vision's operations including scientific and tech-nical management, people management, strategicand tactical planning, and organizational devel-opment. He received his B.S. in metallurgicalengineering from the University of Illinois in1958 and his M.S. in the engineering science ofmaterials from the University of New Mexico in1968. Before joining the staff in Los Alamos in1961, he was a metallurgical engineer for ACFIndustries from 1958 to 1961. At Los Alamoshe helped pioneer much of the materials work inarmor and anti-armor, including the developmentof- depleted uranium alloys for penetrators andthe development of ceramic-filled polymer armor.

Some of the people responsible for the workdescribed in this article include (from left toright ) Anna Zu rek ( high-strain properties of ma-terials), Joel Katz (microwave processing), PhilArmstrong (materials properties and characteri-zation). Noel Calkins (development of compos-ite aroms), Pete Shalek (ceramics processing),Paul Dunn (development of composite kinetic-energy penetrators), Paul Stanek (developmentof low-pressure plasma spraying), Don Sand-strom, Billy Hogan (Program Manager for thekinetic-energy penetrators), and Robert Reiswig( chemical-energy penetrators and materials characterization).

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ARMOR anti- ARMOR materials by design

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