Armor/Anti-Armor

ARMOR anti- ARMOR materials by design by Donald J. Sandstrom

magine armor that grains aligned in a sheet of ura- chews up a high-velocity nium that allow it to stretch into a on impact . . . or long, lethal jet of unbroken . I composites of tungsten and These examples illustrate how that lend an antitank Los Alamos is using its knowl- penetrator rod the stiffness of edge of materials to design and the tungsten. the density and py - fabricate new and stronger com- rophoric property of the uranium, ponents for both armor and pene- and the surprising strength of trators of armor. their mixture or tiny crystal Our interest in applying ma-

36 Los Alamos Science Summer 1989 terials research to conventional tive process of theory, design. There is also a complementarily has its origins in the fabrication, and testing used to between the applications of mate- Laboratory’s nuclear weapons develop nuclear weapons serves rials in conventional and nuclear program. To deal with the unique as the basis for a similar process weapons-one that has a syner- materials used in nuclear weap- in developing conventional ord- gistic effect on both programs, A ons, such as , special ce- nance. The attention to detail in nuclear releases so much ramics, polymers, and so forth, material properties required for energy so rapidly that materials the Laboratory had to develop nuclear weapons is, perhaps, even behave much like isotropic fluids significant expertise in materi- more important for conventional and can usually be described by als research. Further, the itera- weapons. hydrodynamic equations. In addi-

Los Alamos Science Summer 1989 37 Armor/Anti-Armor

tion, the performance of a nuclear de- A KINETIC-ENERGY PENETRATOR vice is more dependent on the nuclear and atomic properties of its constituents Fig. 1. These x-ray pictures are orthogonal views of the U. S. Army’s M-833 standard round (a fin- than on material properties, In contrast, stabilized, -discarding projectile for ) taken after the round had traveled about two and a a conventional munition subjects ma- half meters from the muzzle of the tank . The central rod, or core, is a kinetic-energy penetrator terials to less severe deformation rates, made from a dense, hard alloy of and titanium, and the tip is hardened steel. and the deformation processes are more The sabot is a device that allows the pressure of the expanding gas from the burning dependent on the chemistry and prior to accelerate the core and sabot assembly out the barrel of the gun. The sabot is discarded after fabrication history of its constituents. the core exits. These pictures show the beginning of the sabot-core separation. Also, note that For example, the behavior of an armor- the lower view reveals a bent fin on the core. piercing projectile is strongly affected by variations in the chemical composi- tion. processing history. microstructure, Two Orthogonal Views and mechanical properties of the materi- als from which it was formed. Further, nuclear reaction times are extremely short, whereas the reaction times for conventional munitions are of the order of microseconds-sufficiently long to allow for many types of mea- surements. And generally. very little, if any, material is recoverable from a test of a , whereas a test of a conventional weapon frequently leaves a considerable amount of material for post-mortem analysis, The philosophy underlying the design of nuclear weapons at Los Alamos is traditionally conservative (in the most positive sense), especially in regard to reliability and ease of production. Our approach to conventional weapons follows the same philosophy and pays the same close attention to detail. We strive to use well-characterized, wel1- understood starting materials, we care- fully control the synthesis and manu- facturing processes. and wc work to develop a complete understanding of the Kinetic-Energy Penetrators the simple spin-stabilized slug of a 30- experimental results. Only in this way mm to fin-stabilized are we able to relate the performance of Weapons designed to penetrate armor that consist of a long, steel-tipped pen- armor and anti-armor systems to slight generally fall into two classes: kinetic- etrator rod and a sabot that falls free of and often subtle variations in material energy penetrators and chemical-energy the penetrator after it is tired (Fig. 1 ). properties or device design and fabri- penetrators. I will discuss the first class If the material strength and kinetic en- cation. I will point out many of’ those now and return to the second later, ergy of the projectile are sufficient, it subtleties as I discuss advances made A kinetic-energy penetrator is a solid penetrates the armor, In addition, the at Los Alamos in the design of armor projectile, usually fired from a gun, that shock wave generated by the impact penetrators and armor, including some uses high-velocity impact (typically, at may travel through the armor plate and surprising properties of a new type of about 1 to 2 kilometers per second) to blow off a portion of its backside. Frag- ceramic armor. defeat the armor. Examples range from ments both from this spall and from the

38 Los Alamos Science Summer 1989 Armor/Anti-Armor

penetrator itself can cause considerable can be heat-treated easily (by water- STRESS-STRAIN CURVE damage to people and equipment behind quenching and subsequent aging in a the armor. high-vacuum furnace) to eliminate the cracking problem, and its properties are Depleted uranium. Materials research not sensitive to precise composition. Yield has made particularly noteworthy con- These last two features help give the Plastic Flow tributions to the design and develop- alloy low manufacturing costs. Point ment of the kinetic-energy penetrator. The alloy was originally developed The most effective armor-piercing ma- and evaluated at Los Alamos for the terial to date is an alloy developed at U.S. Air ’s GAU-8 system, a 30- Los Alamos—an alloy of depleted ura- mm gatling gun system mounted on the nium (most of the fissionable isotope A-10 close support aircraft. The gun has been removed) and a small amount can fire a thousand armor-piercing pen- of titanium (0.75 per cent). etrator rounds per minute and is said to Depleted uranium was considered an be the most effective antitank system o 20 attractive material for kinetic-energy in the world. The uranium-titanium al- Strain (per cent) penetrators for a number of reasons. Its loy was so successful that it has been high density (almost twice that of steel) adopted as the standard for large- Fig. 2. Many material properties, such as hard- makes it easy to produce a penetrator penetrators (such as the one shown in ness and strength, are determined from the re- that delivers high momentum and ki- Fig. 1). lationship between stress (the force per unit netic energy to a small volume of target area applied to the material) and strain (the re- armor. Uranium is highly pyrophoric, sulting deformation of the material). The ini- Dynamic Deformation and its impact against steel targets at tial, approximately linear part of a stress-strain and Fracture velocities as low as 30 meters per sec- curve is called the elastic region because ma- ond produces burning fragments that can The penetrating ability of armor- terial stressed in this region will not suffer any ignite fuel or . In addition, piercing rounds improves with the hard- permanent deformation when the stress is re- depleted uranium is readily available ness and strength of the material used. laxed (in other words, the stress-strain curve in large quantities and is considerably Mechanical properties of this nature are returns to the origin). The point at which the cheaper than alternative materials. normally determined from the stress- curve leaves the elastic region by bending to- Uranium, however, is more reactive strain curve for that material (Fig. 2). ward the horizontal indicates the onset of per- than most other penetrator materials, Stress is the force per unit area applied manent deformation and is a measure of the and its reactivity can result in corro- to a sample, and strain is the relative material’s yield strength. Beyond that point sion problems, particularly in moist deformation of the sample as a result is the inelastic, or plastic-flow, region of the air. In addition, some uranium alloys of that stress. Various kinds of defor- curve. The slope of the curve in the elastic re- are susceptible to delayed cracking due mation can occur (elongation, compres- gion is the elastic modulus, a measure of the to residual stresses induced by fabri- sion, bending, etc.) depending on the material’s stiffness. The slope in the plastic- cation and heat treatment of the rods. nature of the applied force. If stress to flow region is a measure of work hardening The cracking can be avoided if care is the material is kept below the so-called since a steeper slope means more stress must taken in the heat treatment to reduce yield point, or proportional limit, the be applied to create a given amount of defor- such stresses and to reduce entrapped material will spring back to its origi- mation. hydrogen gas to levels less than a few nal undeformed state—in other words, parts per million. the response is elastic. Once this yield Extensive testing at Los Alamos of strength has been exceeded, however, stress needed to achieve a given amount uranium alloyed with various at plastic flow occurs, and the material re- of plastic flow). different concentrations and processed in mains permanently deformed. The slope Generally, it is desirable for a pen- a number of ways showed that the alloy of the initial elastic region, called the etrator to have a high elastic modulus with 0.75 per cent titanium had the best elastic modulus, is a measure of the ma- (high stiffness), high yield strength, and combination of properties. The alloy terial’s stiffness; the slope of the later high work hardening. For instance, any has both reasonable corrosion resistance inelastic region is a measure of work energy lost to plastic flow in the pene- and high penetration effectiveness. It hardening (since it is the amount of trator is unavailable for destruction of

Los Alamos Science Summer 1989 39 Armor/Anti-Armor

the armor. Similar considerations are also true of armor materials. HIGH STRAIN RATES The values of these material prop- erties, however, depend on the rate at which the material is strained, and real- istic analyses of armor-penetrator impact require knowing both static and dynamic Bar Stopper material properties. Static properties are easily measurable. Moreover, they can Bore Scope serve as a starting point for an analysis of the material since dynamic proper- ties often scale in the same direction as the static properties. Nevertheless, it is the dynamic deformation and failure processes that are of paramount inter- est, and these can only be understood by measuring properties at high strain rates. The Materials Science and Tech- nology Impact Facility at Los Alamos ace Controller includes a wide variety of test equip- Strain-Gage Amplifier’? ment for determining material properties Striker Bar Incident Bar Sample Transmitter Bar over a broad range of extreme condi- (b) tions. Several gas are used for high-velocity impact research, and two split Hopkinson pressure bars (Fig. 3), measure the stress-strain behavior of materials at strain rates up to 104 per second. Figure 4 is illustrative of the influ- Recorders ence of strain rate on the strength and behavior of a material—in this case, of depleted uranium. Comparing the high (dynamic) and low (static) strain- rate curves of Fig. 4 shows that at high strain rates the material has significantly higher 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 characterized Fig. 3. (a) The split Hopkinson pressure bar can measure the stress-strain behavior of materials if one is to fully understand the per- up to strain rates of about 104 per second. Such measurements are performed, as shown formance of a material during ballistic schematically in (b), by placing the sample between two pressure bars made from high-strength impact. steel, then firing a striker from the gas gun on the left. The impact of the striker with the incident bar generates an elastic compression wave that travels into the sample, causing plastic Shock waves. Another factor of great deformation of the softer material. A strain gage in the incident bar measures the strain due to interest for the design of armor and pen- the incident and reflected waves, and another gage in the transmitter bar measures strain due to etrators is the response of materials to the wave that passed through the sample. These measurements are used to calculate the strain imposed shock. It turns out that shock rate within the sample and the stress-strain curve, such as the one show in red in Fig. 4. This waves generated by the ballistic im- Hopkinson bar facility is unique in that it can test samples at temperatures as high as 1000°C.

40 Los Alamos Science Summer 1989 Armor/Anti-Armor

DYNAMIC VERSUS STATIC deformed at 2, 8, and 13 gigapascals. computer modeling. We incorporate into All four curves were measured using the models the factors influencing dy- a slow strain rate (0.001 per second). namic fracture, and then compare code The data show that yield strength in- predictions of deformation and fracture creases with increasing shock deforma- with those that actually occur during ar- tion, but work hardening decreases. By mor penetration (see “Modeling Armor the time the sample has been strained 20 Penetration”). per cent, the decrease in work harden- We are currently studying the dy- ing has compensated for the higher yield namics of how voids are initiated, how strength, and the curves for as-received they grow, and how the generation of and shock-deformed material intersect. such voids leads to ductile fracture—for As it turns out, the effect of shock example, span failure in armor plate. deformation on this alloy is relatively Using the 80-mm-diameter gas gun, the small. Other materials, such as uranium span strength of a material can be de- and copper, show much larger changes termined from axial stress (measured Fig. 4. Stress-strain curves for depleted ura- in their stress-strain curves. In general, by noting changes in the resistance of nium at strain rates of 5000 (red) and 0.001 we find some materials are very rate manganin gages embedded in the back per second (black). The dynamic, or high- and shock sensitive, whereas others are of the target) or from particle at strain-rate, curve shows a higher yield point not. Shock-induced changes to materials the back surface of the target (by mea- and, initially, higher work hardening, followed properties illustrate why it is important suring Doppler shifts with a recently by lower work hardening as the material ther- to characterize materials carefully and installed laser interferometer). Several mally softens. As such, the curve illustrates thoroughly. metals have been studied, including cop- the influence of strain rate on the strength and per, rolled homogeneous armor, and behavior of the material. Both samples were Dynamic fracture. Fracture at high carbon steel. Now that we have mas- initially at room temperature (300 kelvins), but strain rates is another important consid- tered the experimental techniques, an the dynamically deformed specimen reached eration in armor and anti-armor perfor- investigation of dynamic brittle fracture a temperature of 470 kelvins at 100 per cent mance. Although fracture is generally in ceramic materials is under way. strain. detrimental to penetrators, certain types of armor may, in fact, turn fracture to an advantage. Fig. 5. One of the teat devices of the Materials pact affect the microstructure and the Because dynamic fracture is a com- Science and Technology Impact Facility at Los strength of the components—that is, the plex process dependent on structure, Alamos, an 80-mm-diameter, single-stage, gas “as fabricated” properties of the mate- processing history, strain rate, and stress gun. In this gun, pressurized gas shoots a pro- rials are altered by the passage of the state, it cannot be fully characterized jectile, or flyer plate, down the launch tube at a shock waves. The massive structural by a single parameter or measurement. stationary target in the experimental chamber. deformations that occur during armor Our approach to a more fundamental The flyer plate and target are typically made of penetration take place in shock-deformed understanding is a combined experi- the same material, which is the material being material with transformed properties. mental and theoretical effort based on tested for changes due to imposed shock. To study those changes, we use an 80-mm-diameter gas gun (Fig. 5) to shoot a projectile called a flyer plate at a target of the same material. After Experimental Flyer Plate impact the shock-deformed sample is I Breech Chamber /Target recovered, examined for microstructural Catch Tank changes with a transmission electron microscope, and tested for changes in material properties. Launch Tube Figure 6 displays static stress-strain curves for an aluminum alloy in its THE 80-MM DIAMETER GAS GUN as-received state and after being shock

Los Alamos Science Summer 1989 41 Armor/Anti-Armor

SHOCK-DEFORMED ALUMINUM rate and F is a function of the ratio of the current yield stress to a saturation

2 GPa siderable working of the material at a 0.4 .. ----- ...... particular strain rate and temperature. In other words, the slope of the stress- \ strain curve beyond the yield point de- 13 GPa pends, among other things, on the cur- As Received rent stress history of the sample com- f 8 GPa pared to a state in which further stress loading of a particular type has no ef- fect. The advantage of the above type of analysis is that the kinetics of work hardening are separated from the con- ditions that determine the yield stress for a given state. This procedure allows 6061-T6 Aluminum Alloy predictions for complex strain-rate and temperature histories, such as are typi- cally found in dynamic impact events. We have developed constitutive relations for model metals and are now extend- 0.0 ing this work to armor and penetrator o 10 20 materials. Strain (per cent)

Fig. 6. The static stress-strain curves of 6061-T6 aluminum alloy as received (black) and after Composite Penetrators having been shock-deformed (red) at 2, 8, and 13 gigapascals with the gas gun in Fig. 5. The The Department of Defense has a shock-deformed samples show higher yield strengths but less work hardening. The strain rate need for gun-launched kinetic-energy for ail samples was 0.001 per second. penetrators with length-to-diameter ra- tios sufficiently high that the rods will penetrate modern armor steel configu- One of our main goals in the work on rations. However, such rods must have dynamic processes is to develop consti- a parameter (or combination of param- high stiffness (that is, high elastic mod- tutive relations that describe the stress- eters) that represents the current state ulus) to resist bending during launch strain behavior of materials over a wide of the material. This equation reflects and flight because slight bending may range of strain rates, strains, and tem- the fact that a material’s yield stress lead to yaw during flight and a glanc- peratures. Such relations will increase changes, both because of what is hap- ing blow off the target. The uranium- our ability to predict the behavior of pening to the sample (s) and because of titanium alloy described above is a particular systems at a variety of condi- marginal candidate for use in the pro- tions. have been affected, say, by the previous posed penetrator rods because its elastic As an example, to model deformation history of stress loading. modulus is not high enough. Design and plastic flow we need relations for We can then go further by describing analysis shows that composites of de- yield stress and work hardening. The pleted uranium and of tungsten (whose of the form elastic modulus for bending is three described by using an equation of the times that of uranium) improve the stiff- form ness of the rod and thus, potentially, its performance. 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- rods with both high strength and high range the composite so that maximum stiffness. METAL-POWDER stiffening is achieved with the least MORPHOLOGY change in penetrator density. Another alloy. Our research on these I Early in the development of the com- composites has concentrated on devel- posite penetrator, we realized that the oping material with the highest strength difference between the coefficients of compatible with a low enough pow- thermal expansion of the two materi- der content to preserve ease of cast- als was sufficiently large that the tung- ing. Optical micrographs of both the sten either fractured or buckled slightly, original powder and a cast uranium- causing it to lose collinearity with the metallic powder material (Fig, 7) show penetrator axis. Both these effects, of that part of’ the powder, after casting, is course, are detrimental to the properties present in the uranium as a dispersion of the composite as a penetrator. We of coarse particles. However, the par- added various metal powders to the ura- ticles are smaller and less angular than ium-Metal Powder I nium component and found, for some. those found in the starting powder itself, that the coefficients were matched more which indicates that part of the metal closely. In fact, both the thermal-expan- dissolves in the uranium, forming an- sion coefficient and the elastic modulus other alloy. Significantly,, regions of fine were altered according to the “rule of particles are also observed: apparently. mixtures” (the value of a property of a some of the dissolved metal reprecip- mixture is the sum of component values, itates during the cooling process. Our each weighted by the relative concentra- studies indicate that the precipitation is tion of the component). the principal cause of the strengthening We tested tungsten-uranium compos- of the material. ite rods in which the uranium was re- The addition of metallic powder to Fig. 7. These optical micrographs show the inforced with metallic particles. There uranium has been so effective in mini- changes in morphology that occur when metal- was both an expected slight increase mizing the mismatch of thermal expan- lic powder is mixed with uranium and then cast in elastic modulus (25 per cent) and an sion coefficients in the composite that at about 1350°C. The fact that the occasional unexpected but significant increase in fabrication of full-scale penetrators have sharply angular regions in the original powder yield strength. For example, the ten- yielded crack-free rods that require no have disappeared in the cast material indicates sile (stretching) yield strength increased further heat treatment before rnachining that part of the metal dissolved in the uranium, from the 25,000 psi (pounds per square (Fig. 8). The simplicity of processing is and the presence of finer particles in the cast inch) typical of cast unalloyed uranium a significant advantage for manufacture, material indicates that part of that dissolved to 110.000 psi in the cast composite, an Further. subscale ballistic tests have metal reprecipitated on cooling. increase of more than 400 per cent. shown that uranium-tungsten composite The significant jump in yield strength rods can penetrate targets at relatively was an exciting bonus. Penetrators cast low velocities, whereas pure uranium fracture). from the uranium-titanium alloy are brit- rods failed to penetrate the same targets Among the many aspects of the alloy tle and therefore must be heat treated. at any velocity. that we of interest and that need to be but heat treatment is expensive, time Our work to date on the mixtures studied are the following: consuming, and prone to formation of uranium and metallic powder also of voids in the uranium. Composite hints at the possible development of a ■ confirmation of the alloy phase dia- penetrators can simply be cast with- new high-strength uranium alloy with gram. especially the solid volubility of out heat treatment, producing rods with other highly desirable features not pos- the metal in uranium: yield strengths in the same range as for sessed by, say, the heat-treated uranium- ■ determination of the precipitation uranium-titanium alloy penetrators that titanium alloy. Weldability of’ the mate- mechanism; have been heat-treated. The results to rial is quite good, and bend tests show ■ variation of the metal grain size with date have identified an optimum compo- it to have significantly enhanced ductil- thermomechanical processing; sition of metallic powders that produces ity’ (the ability to be deformed without ■ effect of size and size distribution of

Los Alamos Science Summer 1989 43 Armor/Anti-Armor

particles in the powder on mechanical plasma melts injected powder particles velocity slug is formed.) In effect, the properties: and propels the molten droplets against liner becomes a kinetic-energy penetra- ■ dependence of fracture toughness and a substrate. The result is a rapidly solid- tor but with typical impact velocities other mechanical properties on tempera- ified deposit of fine-grained material. of about 7 kilometers per second com- ture; Our facility features a single DC-arc pared to 1 or 2 kilometers per second ■ large-strain behavior and work-harden- plasma-spray torch with two powder- for normal kinetic-energy penetrators. ing characteristics: feed inlets. The two inlets allow us to Although a kinetic-energy penetrator ■ resistance to chemical and stress-ill- deposit two materials simultaneously. travels from gun to target at high ve- duced corrosion; and Four axes of manipulation are available locity. a chemical-energy weapon can ■ relationships between the microstruc- between the spray torch and the sub- work even if the device is simply placed ture and material properties. strate. Plasma spraying should prove against the armor and ignited. to be faster and cheaper than any other Los Alamos has applied much of its I.ow-pressure plasma spray. Cost is means of fabricating composite penetra- knowledge about materials to the devel- a major consideration in the develop- tors. opment of liners for the chemical-energy ment of any armor or anti-armor com- weapon, find liners made from unalloyed ponent. Generally. but not always, the uranium represent the most effective Chemical-Energy Penetrators cost of the raw material is only a small such penetrator currently available. The fraction of the overall cost of a com- As mentioned earlier, the second class fact that the physical and mechanical ponent. and significant savings can be of penetrators is the chemical-energy properties of materials are important realized by reducing fabrication costs. penetrator. This weapon defeats ar- determinants of the performance of a In general, we have found that simple mor by using the chemical energy of munitions component is nowhere more materials coupled with reliable engineer- a shaped charge, ignited on evident than in the case of those lin- ing and assembly lead to cost-effective impact, to propel a metal liner at the ers. For example, the ability of a liner components. With that approach in target, Typically, the liner is a conical to form a long, stable jet depends in an mind, we have investigated low-pressure bonded to a machined hollow in extraordinary way on both the physi- plasma spraying as a possible fabrica- the charge opposite the detonator with cal properties of the material and the tion technique for such things as com- the base of the cone pointing outward process-induced mechanical properties. posite penetrators. toward the target (Fig. 10). The shape To achieve ideal performance, a pre- The plasma-spray process that we of the charge focuses much of its explo- cisely fabricated shell of depleted ura- have developed uses a DC-arc plasma- sive force onto the metal liner. turning nium bonded into the machined cavity spray torch in a chamber filled with in- it inside out and stretching it to form a of high explosive must, upon detonation. ert gas at a low pressure (Fig. 9). A long jet of solid material, (In other ver- produce a long, thin, unbroken jet of high-velocity stream of high-temperature sions of the weapon, a compact, high- metal traveling at a high velocity. The jet elongates in flight and must have sufficient dynamic ductility to prevent breakup before striking the target, Such ductility depends strongly on the metal- lurgical history of the liner. When we recognized that jet breakup was highly dependent on the material”s process history as well as on its phys- ical properties, we undertook a pro- gram. sponsored primarily by the Air Force Armaments Laboratory at Eglin Air Force Base, to gain a better under- standing of how metallurgy affects jet formation. To achieve this understand- Fig. 8. Crack-free composite penetrator rods of tungsten and uranium have been successfully ing, we studied uranium and other met- formed by more closely matching the thermal coefficients of the two materials. The match was als with different crystal structures. A achieved by adding metal powder to the uranium. number of metallurgical factors emerged

44 Armor/Anti-Armor

that have an important bearing on liner PLASMA-SPRAY DEVICE performance. Powder The key to these desired mechani- Argon cal properties is the production of an Feed Pump appropriate crystalline microstructure I \\’ in the formed liner blanks. To achieve the correct microstructure, we first se- lect a material whose properties are highly sensitive to mechanical defor- mations and then subject that material to a series of carefully manipulated de- formations and heat treatments. To learn Plasma more about formation of the preferred Stream microstructure, we monitor our materi- als carefully during the various stages of deformation. Mechanical properties Chamber of the fabricated sheet are measured in three orthogonal directions in the ma- Power Supply terial, crystallographic orientation of Fig. 9. This schematic depicts the major components of a low-pressure plasma-spray device being the grains are determined using x-ray used at Los Alamos to explore the low-cost fabrication of such objects as composite penetrators. diffraction, and the development of the An 80-kilowatt arc is generated in a mixture of argon and helium gases by applying a DC voltage microstructure is followed using various across the gap between a tungsten cathode and a cylindrical water-cooled copper anode. The metallographic techniques. arc creates a high-temperature, high-velocity plasma stream moving to the right. Powder fad into In addition to our success with de- this region collides with the stream, melts, and is propelled as molten droplets onto a substrate, pleted-uranium jets, we have shown that where it quickly solidifies, producing a fine-grained deposit. A second powder feed (not shown) liners with reproducible characteristics allows one to run the feeds simultaneously, producing a layer of mixed material. The whole device can be formed from other metals. In operates under a reduced pressure of argon, and the powder feeds operate by being pressurized fact, some of our experimental metal with argon. liners produce particularly long ductile jets with very late breakup times. The same careful attention to processing his- jectile, which is how steel armor deals stroy the projectile. ) However, one of tory and development of the appropriate with a steel projectile, and 3) gross de- the key problems facing armor designers crystalline microstructure are critically formation of the projectile. The last is weight—a well-armored tank may, important for these metals also. mechanism is the most efficient way in the end, be too heavy to move. As a for armor to defeat projectiles because result, there is a need for armor systems Ceramic Armor most of the is absorbed that are light but difficult to penetrate. in the destruction of the projectile itself One approach to weight reduction The opposite side of the coin from and, with little rebound of the projec- has been the use of ceramics, which of- penetrators, of course, is armor. Here tile, momentum transfer to the armor is fer exceptional protection for very light also knowledge of material properties is minimized. Unfortunately, conventional weight. Some of the relevant ceramic of critical importance to the design of steel armor is not capable of defeating materials are aluminum oxide (A1203), armor packages that will defeat a wide high-hardness projectiles, such as armor- silicon carbide (SiC), boron carbide range of penetrators. piercing cores and tungsten rods. (B4C), and titanium diboride (TiB2), all Any material used to defeat a high- in this way. of which have high hardness with an as- velocity projectile must deal with the As a result, a variety of armors have sociated abrasiveness, high compressive kinetic energy and momentum of that been developed, including multilayered and tensile strengths, and good elastic projectile with some combination of composites and reactive armor. (Re- properties to high stress values. three mechanisms: 1) absorption of the active armor has a layer of explosive energy as heat and deformation in the material that ignites on impact, blowing Microwave processing. High cost is target material, 2) rebound of the pro- a facing plate outward to deflect or de- currently one of the disadvantages of

Los Alamos Science Summer 1989 45 Armor/Anti-Armor

ceramic armor, and, as pointed out ear- mechanical properties, such as greater to shapes close to those required for the lier, cost is a major consideration in the strength and higher resistance to ballistic ultimate use. development of’ any weapons compo- penetration. Although microwave sintering of ce- nent. A significant portion of the cost Microwave processing also offers ad- ramics is not new, we took the process of ceramic armor lies in the fabrication vantages in the final fabrication steps. a step further by combining precise po- of monolithic ceramic plates with the Hot pressing can produce only simple sitioning in the microwave oven with required high density. Here, again, we shapes that must then be machined into insulation techniques that reflect and have attempted to reduce fabrication the desired forms. Depending on the concentrate the radiated energy on the costs—in this case, by using microwave density and eventual application of the sample. much as snow or sand reflect radiation to process the ceramic, ceramic, the machining may require sunlight back to the skin. The resulting The ceramics of interest for armor many extra hours and the use of ex- greater thermal efficiency of the pro- materials are currently processed us- pensive diamond-tipped cutting tools. cess improved the sinterability of diffi- ing hot pressing (in which graphite dies Microwave processing can be applied cult materials such as aluminum oxide. apply high uniaxial pressure while the boron carbide, and titanium diboride. material is slowly heated) or using hot We have. for example, been able to sin- isostatic pressing (in which an inert gas THE CHEMICAL-ENERGY ter boron carbide to 95 per cent theoret- applies high isotropic pressure to the PENETRATOR ical density (Fig. 11). The time required material in a heated chamber). These to heat the material from room temper- techniques generate the high densities Casing Metal ature to over 2000 degrees centigrade is needed for ceramic armor but are expen- (a) / Liner under 12 minutes. whereas conventional sive and slow. hot pressing takes several hours. The Microwave processing, using the capital costs for the Los Alamos mi- commonly employed frequency of com- crowave facility were less than $35,000, mercial [microwave ovens (2.45 giga- whereas a 3-inch-diameter hot press, the hertz), achieves the required high den- Detonator Explosive equipment needed to density a boron sities by starting with cold-pressed ce- carbide sample of the same size, costs ramic powder and rapidly sintering it between $120,000 and $200,000. Fur- (b) Accelerating (heating without melting until the mate- ther, energy costs were cut about 18 per rial forms a dense homogeneous mass). cent. Microwave processing is much faster, We are also working on a new com- and therefore less energy-consumptive. than conventional hot pressing, and the High-Pressure \ Ignition equipment needed is considerably less Gases Front Fig. 10. (a) The conical shape of a typical expensive. chemical-energy penetrator is designed to fo- Microwave processing also produces cus the explosive energy of the charge onto a a superior material because the heating metal sheet (red) that lines the conical hollow. occurs rapidly throughout the entire vol- (b) Because the explosive force in the charge ume of material. Traditional processing reaches the center of the liner first, this region methods, which depend upon conduction is accelerated before the outer regions. (c) As from surface to interior, promote growth a result, the liner turns inside out, stretching of large crystal grains in the material into a long jet of material. If the metal liner because of prolonged heating, much as has the proper materials properties, it will form overbaking creates a rough, crumbly an unbroken jet and will impact the target at texture in bread. Microwave sinter- a velocity much higher than that of a typical ing couples energy rapidly throughout kinetic-energy penetrator. (d) This doubly ex- the material and thereby favors den- posed radiograph of a chemical-energy pen- sification of the material over grain etrator shows the on the left growth. The end result is a ceramic with, in this case, a hemispherical liner. The with a finer grain size, fewer voids, image to the right is the solid jet formed when and fewer stress cracks and thus better the charge was fired.

46 Armor/Anti-Armor

posite material for armor applications— aluminum oxide reinforced with platlets of silicon carbide. The platlets, being single crystals, have exceptional tensile strength and can be used to increase the fracture toughness of ceramics, metals, and perhaps even polymeric. Less than 10 minutes of microwave processing are required to produce the new composite at 94 per cent of theoretical density, and we expect that material to have very good resistance to ballistic penetration.

Ceramic-Filled Polymer Armor Ceramic armor for, say, lightweight fighting vehicles and armored personnel carriers currently consists of an out- side layer of high-density ceramic tile bonded to a backing plate. Conventional wisdom about such armor had suggested that the ceramic should have high im- pact strength and hardness so it can help break up a sharp, hard projectile. That requirement implies the ceramic should possess high elastic impedance combined with high hardness and high compressive strength. Another property that had been felt to be important for ceramic armor is high tensile strength. The impact load trans- mitted through the ceramic produces compressive stress on the backing plate and a corresponding tensile stress on the rear surface of the ceramic tile. The re- sult is plastic yield in the ceramic and ceramic-filled polymer armor, and the importance of the entire design of an the development of a fracture conoid. A results were exceptional. Our new ma- armor package. One of the important ceramic with high tensile strength would terial typically consists of a ceramic properties of this material may be its resist such fracture. aggregate (about 85 per cent ceramic dilatancy, that is, its tendency to read- However, research by Mark Wilkins by weight) mixed with a binding poly- ily expand into any free volume when at Lawrence Livermore National Labo- mer or other carrier. Such a material fractured. But whether dilatancy works ratory indicates that the most important possesses essentially none of the me- to advantage in the erosion process may mechanism for defeat of a projectile by chanical properties deemed important depend critically on how the material is ceramic armor is abrasion. The frac- for ceramic armor. In fact, the primary confined. ture conoid in the ceramic spreads from mechanism for defeat—erosion of the The effect of packaging on dilatancy the point of impact and generates sharp penetrator—depends upon the tendency can easily be demonstrated by using rice fragments that are instrumental in help- of the new material to fragment fully. to represent the ceramic-tilled armor and ing to abrade or erode the projectile. a pencil to represent the projectile. If We recently performed a series of Design and fabrication. The ceramic- a pencil is pressed down into a beaker ballistic tests on a new type of armor, filled polymer serves to illustrate the filled with rice, resistance will be slight.

Los Alamos Science Summer 1989 47 Armor/Anti-Armor

TESTING CERAMIC-FILLED POLYMER

But if the rice is confined to a flask with These properties, of course, are quite Fig. 12. The before and after of a test of a narrow neck, resistance to the pencil different from those usually thought of the of ceramic-filled polymer. will be much larger because the rice as ideal for ceramic armor. In fact, the (a) The various pieces of the test configura- is unable to move out of the way of ultimate tensile strength of’ ceramic- tion in the order in which they are put to- the pencil. Free volume is available for filled polymer armor is limited by the gether, including polymer plates (white), the expansion in the first case but not in the strength of the polymer binder, which target holder that constrains the polymer (the second. typically is much lower than that of metal pieces on the left and at the center), and Although a complete explanation of monolithic ceramic, Another prop- the armor plate being protected by the poly- the excellent results of ceramic-filled erty of the aggregate limits compres- mer (the metal piece on the far right). (b) The polymer armor has not yet been ob- sive strength—the polymer bonding same pieces after the plates have stopped a tained. it appears that dilatancy is in- agent becomes fluid at low applied projectile without significant damage to the ar- volved, A chunk of unconstrained poly- mor plate. mer simply blows away on impact with little or no effect on the projectile. A properly designed armor package, how- ever, totally constrains the ceramic-filled polymer (Figs, 12 and 13), say with a backplate and surrounding layers of a high-performance polymeric fiber like Kevlar©. On impact the only free vol- CERAMIC-FILLED POLYMER ARMOR ume is the hole generated by the pro- jectile itself’ as the armor is hit and frac- Fig. 13. This sample of polymeric armor has tures. The resulting expansion of the been cut open to reveal the various layers ceramic-filled composite generates a of ceramic-filled polymeric plates confined be- very large number of highly erosive ce- neath Kevlar©. The ceramic used in the front ramic particles that may be forced out plate (black) is boron carbide; the ceramic between the sides of the hole and the used in the other plates (white) is aluminum penetrator, eroding the projectile. oxide.

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shear stress. This phenomenon, called modular package to increase the capabil- Further Reading thixotropy, can be capitalized on during ity of the armor to break up penetrators. John W. Hopson, Lawrence W. Hantel, and Don- manufacture or repair of the armor be- We are currently exploring in greater ald J. Sandstrom. 1973. Evaluation of depleted- cause the aggregate-filled polymer will uranium alloys for use in armor-piercing pro- detail both the abrasion-erosion mech- flow under a constant applied forming jectiles. Los Alamos National Laboratory report anism of defeat and the exact contri- LA-5238. pressure, allowing the armor to be cast bution of packaging constraints on ar- or molded at low temperatures. Joseph E. Backofen, Jr, Kinetic energy pene- mor effectiveness. Those effects must Lightweight armor systems are cur- trators versus armor. Armor March-April 1980: be studied systematically if we are to 13–17. rently made of high-density ceramic exploit ceramic-filled polymers for fabri- tiles—a very expensive process because Joseph E. Backofen, Jr. Shaped charges versus cating inexpensive, reliable, lightweight the ceramic requires high-temperature armor. Part I: Armor July-August 1980: 60-64; armor for mobile fighting vehicles (see Part 11: Armor September-October 1980: 16– fabrication and extensive finish grinding. “ATAC and the Armor/Anti-armor Pro- 21; Part III: Armor November-December 1980: The polymeric armor requires no high- 24–27. gram”). temperature fabrication or expensive fin- A variety of other research on armor Joseph E. Backofen, Jr. Armor technology. Part ishing steps and can be easily formed to and anti-armor materials takes place I: Armor May-June 1982: 39-42; Part II: Armor any required shape, including very large September-October 1982: 35-37; Part III: Armor at Los Alamos. Those studies range and thick or very geometrically compli- March-April 1983: 18–20: Part IV: Armor May- from investigation of other alloys for June 1983: 38-42. cated shapes. Additionally, monolithic penetrators to the use of chemical va- ceramic suffers from a limited ability por deposition to infiltrate “open mesh” to withstand multiple hits because of its composite materials. The latter has a propensity to break up, whereas poly- particularly high potential for improving meric armor, although highly fractured the properties of ordnance components by the impact, mostly remains in place. such as gun barrels and sabots. Ballistic tests on an armor package We believe that materials technology containing ceramic-filled polymer tiles is the enabling—or limiting—technology have shown exceptional results. On for virtually all conventional weapons an equal-volume basis the polymer- systems. Materials science and tech- bonded material is almost equal to a nology has progressed to the point that high-density, high-purity aluminum ox- “tailored” properties of materials are a ide ceramic tile. On an equal-mass ba- reality. The effects of microstructure on sis the ceramic-filled polymer is better! liner performance for chemical-energy Ceramic-filled polymer armor can weapons, the adjustment of the coeffi- offer four important advantages over cient of thermal expansion and the ac- conventional ceramic armor: companying improvements in mechan- ■ a reduction in weight of about 10 per ical properties of the tungsten-uranium cent since more than 10 per cent of the composite penetrators, and the excep- ceramic is replaced with low-density tional protection offered by ceramic- polymer bonding agent; filled polymer armor are examples of ■ a reduction in manufacturing cost of rather straightforward applications of greater than 50 per cent due to low- developments in materials. These de- temperature fabrication and elimination velopments, though seemingly simple, of expensive grinding steps; are grounded in a thorough understand- ■ greater ease of in-field repair since ing of materials science and technology. either prefabricated, lightweight tiles or We believe the surface has barely been the ceramic and polymer constituents scratched and that the future in conven- can be stored on board the vehicle; and tional munitions belongs to innovators ■ greater ease of accommodating design and designers of new materials. ■ improvements, such as incorporation of very hard boron carbide plates in the

LOS Alamos Science Summer 1989 49 Armor/Anti-Armor

Donald J. Sandstrom is Deputy Division Leader Some of the people responsible for the work of the Material Science and Technology Di- described in this article include (from left to vision at the LoS Alamos National Laboratory. right ) Anna Zu rek ( high-strain properties of ma- He is responsible for working closely with the terials), Joel Katz (microwave processing), Phil division leader in managing all aspects of the di- Armstrong (materials properties and characteri- vision's operations including scientific and tech- zation). Noel Calkins (development of compos- nical management, people management, strategic ite aroms), Pete Shalek (ceramics processing), and tactical planning, and organizational devel- Paul Dunn (development of composite kinetic- opment. He received his B.S. in metallurgical energy penetrators), Paul Stanek (development engineering from the University of Illinois in of low-pressure plasma spraying), Don Sand- 1958 and his M.S. in the engineering science of strom, Billy Hogan (Program Manager for the materials from the University of New Mexico in kinetic-energy penetrators), and Robert Reiswig 1968. Before joining the staff in Los Alamos in ( chemical-energy penetrators and materials char 1961, he was a metallurgical engineer for ACF acterization). Industries from 1958 to 1961. At Los Alamos he helped pioneer much of the materials work in armor and anti-armor, including the development of- depleted uranium alloys for penetrators and the development of ceramic-filled polymer armor.

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