ARMOR anti- ARMOR materials by design

15 Pages·1999·1.35 MB·English

Armor/Anti-Armor
ARMOR
anti-
ARMOR
materials
by design
by Donald J. Sandstrom
I
magine tank armor that grains aligned in a sheet of ura-
chews up a high-velocity nium that allow it to stretch into a
projectile on impact . . . or long, lethal jet of unbroken metal.
composites of tungsten and These examples illustrate how
uranium 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
weapons 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 actinides, special ce- nance. The attention to detail in nuclear weapon 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, sabot-discarding projectile for tanks) taken after the round had traveled about two and a
a conventional munition subjects ma- half meters from the muzzle of the tank gun. The central rod, or core, is a kinetic-energy penetrator
terials to less severe deformation rates, made from a dense, hard alloy of depleted uranium 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 propellant
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 nuclear weapon, 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 cannon to fin-stabilized projectiles
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.
has made particularly noteworthy con- These last two features help give the Yield
Plastic Flow
Point
tributions to the design and develop- alloy low manufacturing costs.
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 Force’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-caliber 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 propellants. 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 metals 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
Strain-Gage Amplifier’?
includes a wide variety of test equip-
ment for determining material properties
Striker Bar Incident Bar Sample Transmitter Bar
over a broad range of extreme condi- (b)
tions. Several gas guns 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 motion 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 I Experimental
impact the shock-deformed sample is Breech Flyer Plate 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
siderable working of the material at a
2 GPa
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)
Composite Penetrators
Fig. 6. The static stress-strain curves of 6061-T6 aluminum alloy as received (black) and after
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-
ium-Metal Powder I
added various metal powders to the ura- ticles are smaller and less angular than
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 n 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- n 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 n 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 n 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-
n 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
n 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
n 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
n 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 explosive 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 shell 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.
The key to these desired mechani- Powder
Argon Feed Pump
cal properties is the production of an
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
most of the kinetic energy is absorbed that are light but difficult to penetrate.
Ceramic Armor
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 (A10),
2 3
armor packages that will defeat a wide high-hardness projectiles, such as armor- silicon carbide (SiC), boron carbide
range of penetrators. piercing bullet cores and tungsten rods. (BC), and titanium diboride (TiB), all
4 2
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