Armor

At the present time, the only commonly-encountered ceramic armor materials are alumina, boron carbide, and silicon carbide.  All of these materials were introduced in the 1960s and saw service in Vietnam-era armor plates.  They haven’t changed since.  The current state of the art — the monolithic, multi-curved boron carbide armor ceramic tile — was developed by scientists at the US Army Natick Research and Development Laboratories and Picatinny Arsenal in 1965.  If there has been any innovation in the many years since, it has been to manufacturing processes; these innovations have brought boron carbide tile production costs down and have facilitated high-volume production, but, pointedly, they have not improved performance.  (And, indeed, in many cases these innovations have actually reduced performance — e.g. in reaction-bonded boron carbide — RBB4C — that is softer, denser, and ultimately just less protective than the 1965 product.)

All three of these old ceramic armor materials have their shortcomings: Alumina is far too heavy, to such an extent that even steel armor is now giving it a run for its money; silicon carbide is, although much lighter, still heavy enough to be burdensome; the best grades of boron carbide, although nearly 40% lighter than alumina and more than 20% lighter than silicon carbide, are rather expensive and underperform against bullets with cemented carbide (WC-Co) cores.

Yet by no means are these three ceramic materials the only ones suitable for armor applications.  As I’ve mentioned in older posts, many other ceramic materials have been successfully tested towards armor applications, and there is an ongoing search, at Diamond Age and elsewhere, for a better ceramic armor material — something which out-performs boron carbide on a weight-basis, and maintains this high level of performance against extra-potent threats such as WC-Co-cored rounds.

To that end, one material which has captured the DoD’s imagination of late is boron suboxide, B6O.  It mirrors boron carbide in its low density (2.52 gm/cc) and its extremely high hardness (>3000 HV1) and current thinking is that it should perform well against even the toughest threats — unlike boron carbide, which, as mentioned previously, can be less than perfectly reliable.  The trouble with B6O is that it’s even harder to process and sinter than boron carbide, already a very difficult material to work with.  Even worse, B6O is staggeringly expensive to synthesize as a raw material.  For these reasons, its commercial development has never gotten off the ground — and boron suboxide armor may never enter mass production.

The aluminum dodecaboride AlB12 and the aluminium-magnesium-boron compound AlMgB14 are also attractive materials — both as standalone pure ceramic phases for armor plate development, and as candidates for better cermet development.  AlB12 and AlMgB14 are very similar to each other, and to boron carbide:  Their density is very low at roughly 2.5 gm/cc, they are all of similar hardness, and, unless something like the Lanxide process is employed, they’re best densified with the application of moderate pressure, e.g. in a hot-press or SPS/FAST furnace.  AlB12 and AlMgB14 have never been tested against WC-Co cored projectiles, but have shown good efficacy against steel-cored threats.  The Achilles heel of these ceramic materials is the fact that they’re very expensive to synthesize from the commercially-available materials that are the product of boron mining.  (Boron carbide, in contrast, is relatively easy to produce from cheap raw materials such as boric acid.)

Another option, mentioned in a few scientific papers over a decade ago, is a composite material made from roughly 60% boron carbide and 40% calcium hexaboride (CaB6).  This multiphase ceramic material has some inherent ductility, is slightly lighter than boron carbide, is slightly easier to densify, and appears to outperform pure boron carbide in head-to-head ballistic tests.  CaB6, however, is not stable in the presence of water — and degrades severely even upon exposure to moderately humid air — which is a trait that may limit its use in armor systems where, by definition, stability and reliability are of key importance.  Diamond Age has worked with CaB6-B4C composites that, quite literally, turned to dust following several months of exposure to ambient conditions.  These test tiles started exhibited spalling corrosion within days of their densification, which is certainly an extremely undesirable trait in an armor material.  Coatings which prevent the decomposition of CaB6 may, however, eventually enable the utilization of B4C-CaB6 composite plates… but not without some degree of legal liability and reputational risk.

Yet as this CaB6-B4C composite makes apparent, one doesn’t need to replace boron carbide entirely — strengthening it with a dopant or secondary phase is also a viable strategy, and a very promising one.  At Diamond Age, we’ve gotten very impressive results with B4C-TiN composites, which have a slightly higher density than pure boron carbide, but which display enhanced fracture toughness and better ballistic performance.  The addition of very small amounts of silicon or aluminum dopants to boron carbide, or tailoring the material’s stoichiometry so that it’s more boron-rich than usual (> B6.5C), have also been proposed as methods for producing a boron carbide ceramic product that’s more resistant to failure via shear amorphization, and would thus exhibit more predictable performance against WC-Co-cored ballistic threats.

An entirely different approach, which is perhaps the most promising approach, lies with high-pressure chemistry and engineering, for these enable the production of metastable high-pressure bulk materials, thereby opening up new frontiers in armor material development.  Diamond Age has been experimenting with HPHT polycrystalline diamond ceramics for some years and has recently been issued a patent on the technology.  Bulk diamond is much harder, much tougher, and much better-performing than any known ceramic material.  We’re now at a stage where we’re beginning to make large diamond tiles on a large enough scale for commercialization, and we’re actively working on R&D, so that even larger, stronger, and superlatively high-performance diamond tiles are available for next-generation armor applications.  For instance, by utilizing fullerenes as a starting material, instead of carbon black, it seems possible to make bulk diamond tiles that are structured, comprised of aggregated diamond nanorods, and which are so hard and tough that they may be the truly ideal and ultimate armor material — unsurpassable.

Plates made with our current diamond tiles are already capable of stopping threats such as the M855 at as little as 2.2 pounds per 10×12″ shooter’s cut plate.  In the very near future, we believe that diamond-faced armor plates should be able to stop the APM2 at well under 4 pounds per body armor plate.

For more information on our diamond armor plates, please click here.

Amorphous metals and bulk metallic glasses for armor applications

Steel has many merits as a body armor material, foremost of which are its durability and its ability to be used in very thin sections. Sadly, we’re all familiar with its drawbacks, too — its high density and its near-total uselessness against armor piercing rounds — and not just those made with tungsten carbide cores, but also those rounds with cores of hardened tool steel. But what if steel’s merits could be enhanced, and its drawbacks reduced or eliminated? Recent advances in amorphous metal alloys, also known as bulk metallic glasses, might be just the thing.

Metals and alloys are generally considered crystalline, which means that their constituent atoms are arranged in a regular, periodic, granular manner. Granted, some have extremely complex crystal structures, but they are nevertheless regular and structured. Solids which do not exhibit periodicity — that is, which do not exhibit any regular arrangement of their constituent atoms, and which are fundamentally unstructured — are considered “noncrystalline,” “amorphous,” or “glassy.” Broadly speaking, the arrangement of atoms in an amorphous metal is no different from that disordered or chaotic arrangement of atoms observed in an oxide or silicate glass. And, like these ubiquitous glasses, amorphous metals can generally only be prepared via rapid solidification from a melt — which, particularly for metals as opposed to oxide glasses, involves extremely high cooling rates.

The non-crystalline atomic structure of amorphous alloys lends them unique properties; they are a novel class of material, not quite an “alloy,” not quite a glass, and certainly not a ceramic — but with traits that borrow from all of the above. Broadly speaking, their strength and hardness are greatly superior to crystalline metal alloys, their ductility is reduced, their resistance to abrasion and wear is far higher, their resistance to corrosion is higher. Their impact resistance and toughness vary, with some compositions behaving much like ceramics and glasses, whereas others are as tough as maraging steel. Generally speaking, they tend towards brittleness, as their atomic structure prevents them from arresting growing cracks. Work-hardening is impossible in amorphous alloys, and heat-treatment can turn an amorphous alloy into an inferior crystalline one — so, once made, amorphous alloys are not thermally or mechanically treated.

Amorphous alloys are generally complex from a chemical and metallurgical standpoint. One of the most popular commercially-available amorphous alloys, known as Vitreloy 105, has the following composition: Zr52.5Ti5Cu17.9Ni14.6Al10 (In other words: Zr: 52.5% Ti: 5% Cu: 17.9% Ni: 14.6% Al:10%.) It is therefore characterized as a zirconium-based amorphous alloy, as zirconium is the primary element of its composition. Vitreloy’s complexity is typical of commercial bulk amorphous alloys. But they’re not all complex; the simple ferroboron alloy Fe80B20 can be amorphous, and the first discovered amorphous alloy was a similarly simple Au-Si alloy. As these examples demonstrate, the least complex amorphous alloys are generally comprised of a transition metal alloyed with a metalloid.

Although many amorphous alloys are softer and more brittle than tool steel, certain bulk amorphous alloys — particularly those based upon a foundation of nickel, tantalum, or iridium — have displayed incredible hardness values of 1200 to 2000 HV, which means that the very softest of these amorphous alloys is nearly twice as hard as the best ultra-high-hardness armor steel, and the hardest is significantly harder than armor ceramics such as alumina and reaction bonded boron carbide! What’s more, these amorphous alloys display reasonably good elasticity, stiffness, damage tolerance, and compressive strength. Alloys based upon lightweight scandium, in particular, display outstanding elastic properties combined with good hardness and low density.

The mechanical properties of these aforementioned amorphous alloys, taken together, strongly indicate that they should be capable of defeating armor piercing rounds in thin sections, and may be even more durable and damage-resistant than armor steel.

It is worth noting here that the high-performance glassy alloys mentioned above were not optimized for use as light armor materials. Thus it should almost go without saying that a modern research program could identify and develop better materials; the best light amorphous alloys for armor use; characterized by low density and reasonable materials costs in addition to extremely high hardness, good elasticity, and so forth.

It is highly plausible that thin sections of these hard amorphous alloys would out-perform both steel and ceramics as an armor material. Preliminary work has already done much to confirm this assumption.

If there are hurdles, they’re in the development and bulk production of these unusual metallic glasses. The production methods used for steel and other metals are, in general, wholly inappropriate for amorphous alloy production. Presently-available commercial amorphous alloys like Vitreloy are completely inappropriate for use in armor systems, for reasons which go beyond the fact that they were never optimized for use in armor.

Diamond Age is actively researching bulk metallic glasses and amorphous alloys for armor applications, and is working to overcome the issues associated with their production on commercial scales. We anticipate that our light armor amorphous alloy shall find its way into commercial armor products within the next few years. What’s more, we believe that there’s a sizable market for ultra-high-performance bulk metallic glasses in aerospace, optical devices, and jewelry.

Please contact us for more details.

Armor: Future

Boron Nitride Nanotubes for Armor Composites

Carbon nanotubes have been of broad engineering interest for 25 years. Practical applications, however, are still very few and far between — due primarily to the fact that CNTs like to form aggregates and clumps, which can dramatically impair the mechanical properties of composite materials. It can be extremely difficult to disentangle the nanotubes, and homogenize them within a matrix or spin them into a yarn, without damaging them. Where their incorporation into armor materials is concerned, there’s another problem: Carbon nanotubes are not oxidatively stable, they’re not chemically stable when used alongside certain elements such as iron, and they’re not thermally stable — in fact, they begin to degrade at just 400°C. For these reasons, it is very difficult, if not impossible, to meaningfully incorporate carbon nanotubes into ceramic and metal matrices. Even if chemical compatibility is assured, processing temperatures are always just too high.

Here’s an SEM image we’ve taken of carbon nanotubes at 99% purity:

…As you can see, particularly in the top left, they form particles that look like balls of yarn. These yarnlike particles can be very difficult to disentangle completely.

Boron nitride nanotubes are structurally identical to carbon nanotubes, but are made up of boron and nitrogen at a 1:1 ratio. The mechanical properties of BNNTs are essentially identical to those of CNTs, in that they have an elastic modulus of up to 1.3TPa and a tensile strength of up to 33GPa for individual nanotubes. The thermal properties of BNNTs are also similar to those of CNTs, with, for instance, thermal conductivities measured at 350W/m K and 300W/m K, respectively.

BNNTs differ from CNTs in two key respects:

1. Stability.   High-quality BNNTs are stable to temperatures in excess of 1000°C — in fact, it’s not entirely clear at which temperatures they begin to degrade — so they are highly thermally stable. They are also almost completely inert from a chemical perspective, so are suitable for use in the presence of reactive elements such as oxygen and iron. Their great stability enables and recommends their use in ceramic and metal composite materials.

2.  Ease of use. BNNTs are significantly easier to process than carbon nanotubes. They don’t form aggregates or clumps as readily; they’re not as susceptible to the Van der Waals forces which induce carbon nanotubes to stick together, so are therefore easier to disperse in composite material powders and melts. It is extremely easy to disperse them in ceramic powder slurries with the utilization of common techniques such as ultrasonication or high-shear homogenization.

For example, here are some of our boron nitride nanotubes at 5% loading in a boron carbide powder matrix, where the average particle size (D50) of the boron carbide is in the low nanometers:

At loading volumes generally around 1-5%, BNNTs have already been utilized, experimentally, in ceramic and glass composites — such as in silicon nitride, alumina, and barium calcium aluminosilicate glass. These BNNT composites generally exhibit increased strength and hardness, alongside reduced density, for extraordinary improvements in specific strength. Yet it’s difficult to draw any firm conclusions, as few ceramic-BNNT composites have been reported, and very few metal-BNNT composites have heretofore been reported. This is an area on the frontiers of materials science — and, therefore, of armor science — where much work remains to be done.

Diamond Age is the leading producer of high-density BNNTs suitable for ceramic and metal reinforcement, and is actively researching their use and utilization in the armor systems of the very near future. We have supplied BNNTs to some of the world’s largest electronics and consumer goods companies, and have several patents pending. Please contact us to learn more.

Ceramic Armor It’s said that the development of ceramic armor can be traced back to 1918, to the final days of WWI, when British Army Major Neville Monroe Hopkins discovered that a plate of rolled homogeneous armor steel was much more resistant to penetration if covered with a thin layer of hard enamel. Over twenty-five years later, as Doron was being studied by the Monsanto Chemical Company in the final days of World War II, it was discovered that a solid plate of glass in front of a hard Doron plate was more effective than an equal weight of Doron plate alone. But the first truly modern ceramic armor system dates back to early 1963, and a patent filed by Richard Cook on behalf of the Goodyear Aerospace Corporation. This patent, entitled “Hard faced ceramic and plastic armor,” describes a piece of armor of essentially modern construction: A plate of alumina, or a mosaic of alumina tiles, over a backing of Doron fiberglass

It is unknown to what extent the Cook patent was commercialized or licensed, but it is known that the Army Materials and Mechanics Research Center (AMMRC) started experimenting with ceramic armors almost immediately after that patent was filed. And indeed the Army’s first experimental systems perfectly mirrored the one mentioned in the Cook patent: They were made of alumina ceramic, with a backing material of Doron or aluminum alloy. Coors Ceramics manufactured a small number of these alumina-Doron breastplates for use in the early days of Vietnam, which likely represents the first use of ceramic body armor in combat. The armor was effective — but, as the front and back plates weighed 28 pounds (12.7 kilograms) in total and didn’t dissipate body heat, they were very uncomfortable to wear in the sweltering jungles of Vietnam; thus, like the gleaming steel breastplates of WWI, they were utilized only by aircrews, sentries, and stationary troops. The vast majority of the ceramic armor plates produced over the course of the Vietnam war were issued solely to aircrews; even so, and in spite of the fact that long-range physical mobility is not something frequently demanded of aircrewmen, they found the early alumina plates — colloquially referred to as “chickenplates” — excessively burdensome. Needless to say, infantrymen would have thrown such breastplates away at the first possible opportunity!

The DoD sought out lighter and more comfortable solutions. Small quantities of silicon carbide (“Class II”) and boron carbide (“Class III”) ceramic vests were subsequently produced and issued to aircrews. The class-threes were originally made of hot-pressed boron carbide tiles, arranged in a mosaic-type array of small plates, and were extremely expensive to produce. In 1965, monolithic, multi-curved boron carbide armor plates were developed by scientists at The US Army Natick Research and Development Laboratories and Picatinny Arsenal — an extraordinary engineering achievement at the time. These highly advanced, essentially modern plates were issued to aircrews in very small quantities.

Dr. John Foster, Deputy Director of Defense Research and Engineering in the DoD in the mid-1960s, realized the potential of ceramic armor, and wanted to see how far his department could advance the military applications of ceramic armor. He asked his friend, Dr. Mark Wilkins at the Lawrence Livermore National Laboratories, to spearhead a project which became known as the “Light Armor Materials Program.” The primary goal of this project was to determine the optimal materials and configurations for military light armor — that is, for aircraft, light vehicles, and body armor purposes — and it’s quite impossible to see how the LLNL-based team could have done a better job; the exactingly rigorous and pioneering work of Dr. Wilkins and his colleagues stands out as the most impressive body of work in the field of armor science.

The LLNL team started by preparing a database of all known low-density ceramic materials with desirable mechanical properties for armor purposes, such as high hardness and compressive strength. This list included all of the ceramic materials that are in use today: Alumina, silicon carbide, titanium diboride, and boron carbide. It also included a number of materials that have never seen use, such as the beryllium borides Be2B and Be4B, the aluminum boride AlB12, and the intermetallic titanium beryllide TiBe12, among many others. Wilkins and his colleagues then combined real ballistic experiments with numerical simulations. The experiments included flash radiography, microstructural analysis, the measurement of residual velocities, the analysis of failure modes, and the development of a standardized .30-caliber monolithic hardened-steel test bullet. The 2D numerical simulations were the first of their kind, and the “HEMP” model developed by Wilkins himself and used by the LLNL team was the basis for a great deal of subsequent work in the field.

Following dozens of experiments, Wilkins and his team were able to make a number of general observations which later became effectively axiomatic:

1) The armor needs to be as hard as the bullet (or harder) to defeat the bullet; and the converse is true, thus the reason for tungsten carbide core bullets;

2) The armor layer needs to be at least 1/2 the bullet diameter in thickness to successfully defeat the bullet;

3) The front ceramic needs to be about 1/3 of the thickness of the total plate and the backing needs to be about 2/3 of the total;

4) The basic characteristics of the composite plate are that the front armor material needs to break and turn the bullet and the backing material needs to be ductile to catch the penetrating products without failing;

5) Understanding the fracture/cracking/breakup of the ceramics is only important for thick ceramics, less so for thin sections of armor plate. [1]

These axioms guided the next few decades of armor research. They are only just starting to show their age, as will be discussed here in the future.

Wilkins and his colleagues made a few other noteworthy observations. First, they determined that “functionally graded materials” — that is, materials where the transition from hard to ductile proceeds in a gradual, graded manner, are potentially highly effective as armor materials. What’s more, unlike ceramics and ceramic composites, graded materials can be used for structural, load-bearing purposes.

The LLNL team also determined that inherent ductility in ceramic materials improves performance markedly. Unfortunately, only a few low-density ceramics (the beryllium compounds BeO, Be2B, and Be4B, and magnesium oxide, MgO,) exhibit any ductility, and most of them proved completely unsuitable for armor use. Experiments in ceramic-metal hybrids — cermets — which would presumably exhibit the high hardness and low density properties of ceramics, yet retain some of the ductility of metals, were subsequently pursued. These experiments met with limited success. Cermets with desirable mechanical properties were made — particularly in the titanium carbide-nickel system — but these were relatively high-density materials, at over 5g/cm3. Low-density cermets were not successfully fabricated at that time. Experiments in lightweight systems of presumably high potential — such as, for instance, aluminum-boron carbide — were generally unsuccessful.

The best-performing ceramics in the LLNL’s multi-year survey were the beryllium borides Be2B and Be4B. They exhibited astonishingly low density (<2g/cm3), some measure of ductility, and were hard enough to defeat the steel-cored ammunition of the test. (Though, at roughly 1300HB, likely not hard enough to fare so well against tungsten carbide cored ammunition!) What’s more, they were easy to manufacture; for, on account of their unusually low melting points, they could be cast like aluminum or steel! Despite these merits, and despite the exhaustive and costly evaluations they were subject to, beryllium-based armor materials were never given serious consideration by the US Military. For one thing, inhalation of beryllium dust can be fatally toxic, which fact alone was enough to disqualify it from use in service. For another thing, beryllium is a scarce element and extremely expensive — far too expensive for broad adoption as an armor material. [2]

As boron carbide and silicon carbide armors were in use prior to the establishment of the Light Armor Materials Program, and as no useful material was found that surpassed them, the LLNL’s program had little impact on the course of the Vietnam war. But Wilkins and his colleagues shaped the fields of armor engineering, terminal ballistics, and ceramic fracture mechanics, for decades to follow.

Where armor engineering is concerned, this was not wholly positive. Over the 80s and 90s, staggering efforts were invested in producing silicon carbide and boron carbide cermets. Although some early silicon carbide cermets — with over 60% ceramic content — were eventually used in aircraft armor systems, the material was never issued as part of a body armor system, likely on account of antiballistic inferiority to pure boron carbide and silicon carbide. These cermets were produced by Lanxide Armor Products Co., and their fabrication involved the creation of porous ceramic sponges that were infiltrated with molten metal at atmospheric pressure and relatively low temperature. (Around 1500°C.)

A lightweight aluminum-boron carbide cermet was eventually produced by a different US Military contractor, in a follow-up on some LLNL research on the same system. Although it performed well in ballistic experiments, it was difficult to fabricate and proved far too expensive for broad use, so only a handful of experimental plates were made.

Functionally graded materials have never been successfully produced in bulk for armor or aerospace applications. Despite their initial promise, the manufacturing difficulties associated with their fabrication have thus far been impossible to overcome.

At the time of this writing, boron carbide, silicon carbide, and alumina plates over a ductile backing material still represent the “state of the art,” so to speak, in ceramic body armor. Thus, needless to say, there has been limited technical progress in the field since the mid-1960s. Advances in light armor polymers, e.g. aramid and Dyneema, have rendered fiberglass-reinforced thermoplastics like Doron obsolete, and so the backing layers in armor plates have become lighter, thinner, and more effective — but nothing has yet been found which might replace or improve the ceramic materials which have been in use since the mid 60s.

The major development of recent decades has been the development of reaction bonded silicon carbide and reaction bonded boron carbide. (RBSiC and RBB4C, respectively.) In 1998, Lanxide Armor Products closed up shop. A few years later, their intellectual property rights were bought by M-Cubed Technologies. M-Cubed, doubtless drawing upon Lanxide’s research data and production equipment, then developed the reaction bonding process, whereby molten silicon would infiltrate sponges of either porous silicon carbide and carbon, for RBSiC, or boron carbide, for RBB4C. In the former case, the result is a Si-SiC cermet. In the latter, a composite B4C-SiC-Si cermet.

So-called “reaction bonded ceramics” are relatively easy to fabricate — RBB4C, in particular, is much cheaper and easier to manufacture than hot-pressed boron carbide — a fact which has enabled their production in large bulk quantities. Furthermore, much unlike hot-pressed materials, which tend to be flat plates or discs, reaction bonded ceramics are relatively easy to manufacture in various different geometries and curvatures. But these reaction bonded ceramic materials, which are actually silicon cermets, display inferior mechanical and ballistic properties. RBB4C, especially, is very inferior to hot-pressed boron carbide as an armor material: It is half as hard, it is much less effective at stopping tungsten carbide cored rounds, and it is significantly heavier than hot-pressed boron carbide; it often clocks in at 2.65-2.8 grams per cubic centimeter. The progress of recent years, therefore, has not been without its drawbacks; it has resulted in armor materials that are, in many key respects, actually inferior to those used in the Class II and Class III Chickenplates of the Vietnam War! And yet, with that said, reaction bonded cermets are extensively used in the US Military’s latest body armor systems.

In today’s CONUS police and civilian markets, virtually all ceramic armor systems are made of alumina over aramid. Silicon carbide plates are obtainable, but are fairly rare and relatively expensive. Boron carbide plates are very rare and can be exorbitantly expensive — likely on account of the fact that boron carbide, unlike silicon carbide and alumina, is of limited industrial use, is not produced in huge quantities, and the military’s suppliers don’t sell their armor plates to outside markets.

Mechanical properties of selected armor ceramics:

Alumina (Al2O3) is an ivory-colored ceramic of low cost and ready availability. It is the highest volume armor ceramic in the word due to its favorable cost:performance ratio. It is, however, too heavy for use in modern body armor systems. Typical properties are: Alumina purity (typical): 97-99.9% Density: 3.95g/cm3 Hardness (Vickers): 1200-1600 HV1 Fracture toughness: 3-5 MPa*m1/2 Compressive strength: >2000MPa

Silicon Carbide (SiC) is lighter in weight than alumina, and is slightly superior in terms of performance. The cost of silicon carbide armor pieces is, however, more than five times higher than equivalent alumina pieces — this is on account of the fact that silicon carbide is more difficult to work with, as far higher temperatures and tighter processes are required during sintering. It is generally utilized in high-end body armor. Its crystal structure and molecular characteristics are complex, and will be the subject of a future post. Density: 3.21g/cm3 Hardness (Vickers): 2000-2850 HV1 Fracture toughness: 3-5 MPa*m1/2 Compressive strength: >2500MPa

Nominally B4C,”boron carbide” denotes a number of possible compositions, from the carbon-rich extreme of B4.3C, to, on the other end of the spectrum, the boron-rich extreme of B10.4C.  Commercial boron carbide is generally far closer to the carbon-rich extreme, so it’s not inappropriate to simply use the common “B4C” nomenclature.

Boron carbide is a dark gray ceramic which possesses exceptionally good mechanical properties — particularly in its low density and high hardness.  It is, however, difficult to densify from powder, on account of the covalent nature of its bonding, low surface tension, poor plasticity, and high slip resistance on grain boundaries up to temperatures approaching its melting point.

Processing difficulties mean that it’s generally a very expensive material, and pure hot-pressed boron carbide is extremely rare as a body armor material, but is generally considered to be the best material that money can buy. This is true, with one caveat: Boron carbide armor under-performs against tungsten carbide cored bullets, on account of the fact that its crystal structure collapses (undergoes shear amorphization) under the high pressures typical of ballistic impact from WC-based rounds.

Density: 2.51g/cm3 (pure boron carbide), 2.6-2.8g/cm3 (reaction bonded boron carbide)
Hardness: 2900-3200 HV1 (hot-pressed boron carbide), 1500-1700 HV1 (reaction bonded boron carbide) Fracture toughness: 2.5 MPa*m1/2
Compressive strength: >2700MPa

[1] – S.R Scaggs, 27th Annual Cocoa Beach Conference on Advanced Ceramics and Composites: A: Ceramic Engineering and Science Proceedings, Volume 24, Issue3

[2]-Beryllium’s 2018 price is approximately $2000/kg. Assuming each armor plate of Be2B would require approximately 750 grams of beryllium, and assuming each soldier requires at least two plates, the cost to outfit a division of 12,500 men in beryllium armor would come out to nearly $40 million. Those same soldiers could be outfitted in (admittedly sub-par) alumina plates for significantly less than $1 million, and in boron carbide of the absolute finest quality for less than $5 million.

The first titanium alloys — including the famous Ti6Al4V, which now accounts for more than 50% of total titanium production — were developed in the USA in the late 40s. Shortly after their development, an assessment from Pitler and Hurlich noted that these new alloys showed promise at defeating small arms projectiles. Despite numerous subsequent investigations, experiments, and studies over the 1950s and 60s — which included the development of extremely hard (62HRc) alloys — titanium body armor was never issued to US or NATO soldiers. There is just one exception: River boat crews in Vietnam were issued a light titanium-nylon flak jacket, which was not intended to stop high-velocity projectiles; it was merely a lightweight analog of WWII’s steel-nylon aircrew flak jacket.

Over the ten-year period from roughly 1996-2006, titanium enjoyed a small resurgence in the US as an armor material: Several private US companies examined monolithic titanium body armor plates, the Army Research Labs investigated hot pressing titanium metal powders in 2005, and the now-defunct DragonSkin’s namesake armor vest made use of titanium-ceramic composite armor tiles. Ultimately, these efforts did not meet with much success.

The Soviets, in contrast to the Americans, made extensive use of titanium as a body armor material. In 1979, shortly after the initiation of the Soviet-Afghan war, the 6B2 armor vest was issued to troops on the ground in Afghanistan. The 6B2 consisted of an array of titanium alloy plates, each just 1.25mm thick, bonded to 30 layers of twill-weave aramid. The shell was made of nylon, with Velcro fasteners — and it’s worth noting that Velcro was a very new material at the time, so its appearance on Soviet vests led to speculation as to the vest’s origins. Total system weight was 4.8kgs, including front and back portions. Protection from shrapnel and low-velocity rounds was deemed adequate, but the 6B2 was completely incapable of stopping high-velocity aimed projectiles. In fact, it reportedly could not stop 7.62x39mm rounds fired from distances of 300-500 meters. The 6B2 was therefore much like the Vietnam-era flak jacket in many respects, even in appearance, but markedly heavier.

The 6B2 was quickly replaced in service by the 6B3TM, a still heavier version, where the thickness of the titanium plates was increased to 6.5mm. This change increased the weight of the vest to 12kg. It was then changed again, in the 6B3TM01, to a version with front plates which were 6.5mm thick, and rear plates which were 1.25mm thick. [1] This final version was roughly 9kg in total weight. Having said all of that, it is exceedingly unlikely that 6.5mm-thick plates of titanium would stop 7.62x39mm or 5.56x45mm rounds at anywhere near muzzle velocity — but they would have done the job at engagement distances of 200-500+ meters.

In 1985, midway through the war, the 6B4 was introduced. This was an armor vest made of boron carbide and aramid, similar to those issued to certain soldiers in Vietnam, doubtless of much greater protective ability than the titanium armor vests which preceded it. Ceramic strike faces feature heavily in subsequent models of Soviet and Russian armor — though, until quite recently, body armor in Russia often comprised titanium and steel portions. This appears to have been eschewed entirely in recent years; the most advanced Russian model at present, the 6B46 “Granite 5a” armor plate, appears to be made entirely of silicon carbide over aramid.

Mechanical and ballistic properties of selected titanium alloys used in modern armor systems:

Ti6Al4V is an alpha-beta titanium alloy comprised of 90% Ti, 6% Al, and 4% V. It has a density of 4.43g/cm3, a hardness of 334HB, a yield strength of 880MPa, tensile strength of 950MPa, charpy impact at 17 J, and elongation at 14% in 2″. It has a shear strength of just 550MPa.

Russian Armor-Grade Titanium is a titanium alloy also comprised of 3% Al, 5.15% V, 3.65% Cr, with trace amounts of boron, zirconium, and molybdenum. It has a hardness of 387HB, and a density of 4.62 g/cm3. Its other properties are unknown, but it is not unreasonable to assume that it possesses greater yield strength and tensile strength than Ti6Al4V, an inferior charpy impact strength, and inferior elongation.

Adjusted for weight, the US Army Research labs have determined that the ballistic performance of Ti6Al4V is superior by roughly 7% to that of the aforementioned Russian alloy. Adjusted for weight, I repeat. As Ti6Al4V is about 5% lighter, what this means is that the two alloys perform nearly identically at equivalent thicknesses. The differences in mechanical properties are apparently not of great importance; the lighter alloy performs better largely on account of its lightness; if titanium alloys are still of interest to body armor designers, I’d imagine that one priority should be the development of a lightweight titanium armor alloy — perhaps with a higher volume fraction of aluminum.

In any case, as the above paragraphs clearly show, both titanium alloys possess a great strength-to-density ratio, and both are comparable in hardness to rolled homogenous armor (RHA) steel. What is less obvious is titanium’s propensity to fail via plugging, which markedly reduces its utility as a standalone armor material. When a titanium alloy plate is struck by a high velocity projectile, shear strain, strain rate, and temperature can rise to very high values over a very small surface area. Titanium’s relatively poor shear strength, combined with its very poor heat transfer properties, make it inherently susceptible to fail catastrophically in such situations via a phenomenon known as adiabatic shear plugging. This problem becomes almost insurmountable when titanium alloy armors are used at low thicknesses — thicknesses typical of body armor! For this reason, it is our view that the further development of titanium as a body armor material is not warranted.

[1] This is reminiscent of earlier eras. As was noted in a previous post, Alexander the Great and the tacticians of the 16th century both recommended that little-to-no armor be worn on the back, “on the ground that it [is] unnecessary, and that its absence would discourage cavalry from turning its back to the enemy.” This is, to a certain extent, nonsensical in modern warfare: Epidemiological studies indicate that the back sustains just as many injuries as the front, from shrapnel in particular. However, the back doesn’t need as much protection from rifle fire — as it is also quite typical that 70-80% of battlefield casualties who have sustained bullet wounds were shot from the front; bullet entry wounds in the back are markedly less common.

The first truly modern woven armor material — a laminate comprised of layers of woven glass fabric, impregnated with an ethyl cellulose resin, and bonded under high pressure — was invented by the Dow Chemical Company in May 1943. Initial reports were promising, and the Military Planning Division, Office of the Quartermaster General, under the command of Colonel Georges Doriot, launched the “Doron project” to find suitable applications for this new material. (“Dor” from the Colonel’s name, +”on”, from the generic ending for fibers such as rayon, nylon, cotton, etc.)

Doron was first used in battle in August 1944. The T37 flak vest for airmen was a straightforward design where the steel plates of the ubiquitous M1 flak jackets were replaced by flat hot-pressed Doron plates approximately 50mm square and 3.3mm thick. Later modifications utilized thicker, curved Doron plates. The T37 and its derivatives remained experimental, however; they were not adopted by the Air Force, as aluminum and nylon-based solutions were deemed more suitable for use at that time. The Army, following the Air Force’s lead, did not engage in further experiments with Doron over the last days of WWII.

Which is not to say that nobody was interested. The US Navy remained keenly interested in Doron, and pioneered its research and development in armor systems.

Two Naval officers, Lt. Commander Edward Corey and Lt. Commander Andrew Paul Webster, were the first to test the personal protective ability of Doron — and they did this in an unusual and brave, if not foolhardy, way: Lt.Cdr Corey wore Doron panels, with various backing materials, over his arms and held them in his hands, as Lt.Cdr Webster fired at him with a .45 pistol. A short account of their tests follows:

“Dr Corey, with a great deal of physical courage, volunteered to be the subject of this test. The strain of this first test is understood when one considers that we did not know whether his arm would be torn off by the blow from the bullet or whether I would even hit the armor on his hand. The shots proceeded as follows. When Doron was backed with [a 1 inch layer of] sponge rubber or a heavy layer of kapok, the impact of the bullet on the hand resulted in no discomfort, and even with a few thicknesses of duck cloth no injury resulted. The bullets were literally picked out of the air and caught as if catching marbles flipped at the hand. When the backing was reduced to a single thickness of duck cloth, severe bruising resulted, with hematoma, pain and edema, but without fracture. Deep sensation did not return in the hand for a period of about six weeks. It was concluded from these experiments that since our problem of armouring ground troops and shipboard personnel was one involving maximum protection and comfort for minimum weight, no backing should be used.”

The folks who today obsess over BFD and behind-armor trauma should read that last part again.

The Navy subsequently utilized Doron in two forms: By sewing Doron panels into pockets on the outside of the Kapok lifejacket, and by sewing Doron panels into the standard-issue Marine Corps utility jacket. Roughly a thousand such jackets were prepared for use in the Battle of Okinawa, but, unfortunately for the development of body armor if not for the men involved, the Marine division outfitted with the armored jackets was not involved in the fighting.

Doron, ballistic nylon, and doron-nylon vests were again tested during the Korean War. For the first time in modern history, these body armor experiments were judged a complete operational success. The Doron vests were popular with Army troops, commanders, and battlefield physicians alike, to the extent that the body armor evaluation team wrote in its final report:

“The effect of body armor on confidence is probably best expressed in the results of the post-use interviews where over 85 percent of the men stated that they felt safer and more confident when wearing body armor.”

“Interviews with commanders, who have led troops wearing body armor in combat, have repeatedly emphasized that aggressiveness is increased and that there is more of a desire and willingness to engage the enemy at close quarters.”

“A poll of over 100 front line physicians and surgeons has resulted in the almost unanimous expression of opinion that the use of body armor would result in an increase in morale among combat troops.”

“Under certain conditions the effect of body armor on morale may not be good. For example, during the last month of the test period there were several instances where soldiers who had previously used body armor expressed a reluctance to their unit leaders to go out on patrols when body armor was not available.”

More than anything prior, this glowing evaluation of Doron and nylon armor in Korea cemented body armor’s place in the modern soldier’s kit.

Given the Navy’s interest in Doron vests during WWII, it should come as no surprise that the Marine Corps were early adopters of body armor – and, throughout the course of the Korean war, all Marine Corps frontline personnel were armored in Doron-Nylon flak vests. The Army, at the same time, was still evaluating Doron and nylon vests on a very limited trial basis. As positive feedback from these trials started to come in, and as positive reports from the Marines reached Army HQ, the Army decided that they needed to hurry up and issue body armor to all of their frontline personnel as well, so they requested 13,020 Marine vests on August 11, 1952. By the end of the war, the Army had procured well over 25,000 Marine Corps vests. It should go without saying, therefore, that the Marine vest, the M-1951, was the Korean War’s predominant form of body armor. It weighed 7.75 pounds and was a sleeveless vest with a zipper closure in front. It was made of 1050 denier ballistic nylon fabric and hard Doron fiberglass plates. The shoulder girdle area was protected by 12 layers of ballistic nylon, which was flexible and reportedly comfortable to wear; the chest, abdomen, and back were covered in sixteen curved, 3.175mm thick, overlapping Doron plates.

The Army, following further evaluations, ultimately decided to adopt a different design. The Army personnel assigned to the development of armor projects believed that ballistic nylon was just as effective as Doron, but easier to wear on account of its flexibility and ability to drape. Thus the all-nylon M-1952 was issued to troops over the last months of the war, starting in February 1953. Like the M-1951, it was a sleeveless vest with a zipper in front. It had two front and back panels composed of ballistic nylon bonded with laminating resin, but this was still reasonably flexible, and the vest was reportedly comfortable to wear. It weighed 8.25 pounds, so was slightly heavier than the M-1951, but its flexibility made up the difference.

The M-1951 and M-1952 remained essentially unchanged into, and over the entire course of, the Vietnam war. There were minor, mostly cosmetic, upgrades — but the weights and materials did not differ to any meaningful extent. The vest of the Marine Corps in Vietnam was designated the M-1955, which was, like the M-1951 it replaced, made of ballistic nylon with Doron plates. The Army vest, the M-1969, was of all-nylon construction. It was identical to the M-1952, save for the addition of a high collar, which added roughly half a pound to the vest’s weight, and which soldiers generally disliked, as it interfered with the wearing of the M-1 helmet.

Enter Kevlar. Just as the Vietnam War was ramping up, in 1965, DuPont scientist Stephanie Kwolek was working on a research project tasked to find high-tensile-strength fibers for use in reinforcing automobile tires. To this end she was experimenting with the stiff polymer precursors poly-p-phenylene terephthalate and polybenzamide, when she noticed that the reaction product of the two precursors displayed some unusual properties. As she later explained:

“The solution was unusually (low viscosity), turbid, stir-opalescent and buttermilk in appearance. Conventional polymer solutions are usually clear or translucent and have the viscosity of molasses, more or less. The solution that I prepared looked like a dispersion but was totally filterable through a fine pore filter. This was a liquid crystalline solution, but I did not know it at the time.”

Unusual-looking chemical solutions of that sort were not often pursued; they were usually considered failed or abortive experiments. Kwolek went on to mention that “I think someone who wasn’t thinking very much or just wasn’t aware or took less interest in it, would have thrown it out.” But she persisted, so the solution was spun on a spinneret, and the mechanical properties of the resulting fibers were tested. It quickly became apparent that this new “aramid” fiber was three times as strong as nylon and possessed a far higher modulus and heat resistance. And, despite its high modulus, even these first fibers were so fine that woven fabrics derived from them were flexible, and were able to drape like nylon.

The military quickly realized the potential of this new material, though they didn’t rush to implement it in armor systems and issue it to troops in Vietnam. Military-grade aramid fabrics were first designated as PRD-29, and PRD-49 — and later as Kevlar® 29, which was previously PRD 49-IV, and Kevlar® 49, which was PRD 49-III. Kevlar 29 was judged the superior grade for ballistic protection, and prototype flak vests were made which offered significantly greater protective ability than those made of nylon and Doron, at the same weight.

In June 1978, vests comprised of 13 layers of Kevlar 29 ballistic fabric within an envelope of water-resistant ballistic nylon were issued as the “Body Armor Fragmentation Protective Vest, Ground Troops” for United States soldiers. In the early 80s, this type of vest was standardized as part of the PASGT armor system, which also included a helmet made of resin-bonded laminate Kevlar 29. The PASGT system was quite long-lived — it remained in use by the US military until the mid-2000s, and still remains extremely common worldwide, as it was extensively exported. The PASGT’s successor, the Interceptor Body Armor system, was also made of aramid — in this case, the improved KM2 fiber. The most recently fielded body armor system, the Improved Outer Tactical Vests, which are a partial replacement for the Interceptor, still incorporate aramid fibers in their soft armor component.

Yet this is not to say that aramid is the only option, nor should it even imply that aramid is the best option.

Scientists at DSM in the Netherlands discovered ultra-high molecular weight polyethylene (UHMWPE) fibers by accident in 1963, during experiments in fractionating out different polyethylene lengths from a solution. It was immediately noted that these very long-chained polyethylene molecules were incredibly strong, but, initially, only small amounts were produced, and it seemed impossible to produce meaningful quantities. This is on account of the fact that the UHMWPE fibers were nearly impossible to orient in solution, so they’d typically form intractable aggregates and clumps, and only a small fraction of each production batch would result in usable UHMWPE fibers.

(Spaghetti is the usual analogy. On a molecular level, strong UHMWPE fibers resemble bundles of stiff, uncooked spaghetti. UHMWPE in solution resembles tangled clumps of soft, overcooked spaghetti. The art of drawing strong Dyneema fibers — and it is an art — resembles turning cooked and tangled spaghetti into uncooked spaghetti!)

In 1978, 15 years after the original discovery, DSM engineers discovered the ingenious gel-spinning method, which enabled the production of commercial quantities. Dr. Piet Lemstra, who worked on the project, described as follows:

 “It’s actually very simple. The more you dissolve the long chain molecules in a solvent, the more they become separated. Then you cool it down to a gel state and the molecules are more or less disentangled, so it is easy to stretch them. You can then remove the solvent, or later, during the stretching/drawing process – either way you end up with nearly perfectly aligned molecules.”

Although the development of this clever gel-spinning process enabled the industrial production of UHMWPE fibers, DSM was not an industrial fiber company. It was — and to a large extent remains — a food and agricultural supply company that discovered UHMWPE fiber by accident. The UHMWPE team at DSM was essentially a “skunkworks” crew that did their experiments in spare time and on weekends, with little corporate oversight. In the years since its discovery and the development of a commercial production process, the decision-makers at DSM showed no interest in commercializing UHMWPE fiber, and this was not helped by the fact that the consulting companies they asked to evaluate their potential product did not think very highly of it.

“This [consulting] company said the fiber was just candlewax unable to resist high temperatures and it lacked any good properties. They saw no future in it,” says Lemstra. “And the management at DSM believed them.”

Around that time, in the very early 80s, Allied Signal (now Honeywell) obtained a patent on a very similar UHMWPE fiber product, though they claimed that the melting point of their material was 20-30 degrees higher than that of DSM’s fiber. As it turned out, the two UHMWPE fibers were largely identical, the melting point differences were due to an methodological error on Allied Signal’s part, and, as the DSM patent had priority, Allied was compelled to purchase a license for the original DSM patent.

With the license in hand, and no potential intellectual property conflicts on the horizon, Allied started marketing their own fiber, Spectra, as a competitor to Dupont’s Kevlar. Dupont countered with effective marketing campaigns which showed how Spectra would weaken at moderately high temperatures — from 60-80°C. The military, aided by institutional inertia, essentially said “if it ain’t broke, don’t fix it,” and stuck with aramid armors.

It could be argued, in hindsight, that Allied and DSM didn’t do enough to promote their material. Yet, at the same time, the first grades of UHMWPE fiber were quite poor. We have seen continual improvement since those early days, and today’s grades of UHMWPE fiber are more than 44% superior to the best grades of Kevlar on an equal-weight basis.

Dyneema and Spectra are presently used in high-end body armors, the very newest models of military helmet, and lightweight ballistic shields. Unlike aramid, moderately thick panels of UHMWPE (2cm+) are capable of stopping lead-cored rifle rounds at high velocity, but invariably fail against steel-cored rifle rounds. In fact, they can fail against steel-core pistol rounds; that the SS190 AP 5.7x28mm pistol round can punch through a UHMWPE Level III plate has been documented.

In any case, there’s significant room for improvement in UHMWPE fiber materials, and it’s likely that further gains will be made within the next 10-15 years.

Mechanical properties of selected woven and polymer armor materials

Fiberglass

E-glass fibres, manufactured from alumino-borosilicate, are the oldest armor fibers in modern use, as their use dates back to WWII’s Doron. This grade of fiberglass is infrequently used in modern body armor systems, but, on account of its very low cost, is often encountered as a spall mitigation liner in armored vehicles.

S-glass, of magnesium-aluminosilicate, is a superior grade of fiberglass with much-improved mechanical properties: lower density, higher strength, greater stiffness, and a 20% improved strain-to-failure. However, even the best-performing glass fibers perform only roughly half as well as aramid for ballistic applications.

E-glass
Density: 2.6g/cm³
Ultimate Tensile Strength: 3450 MPa
Modulus of elasticity: 72.4GPa
Elongation at break: 4.8%

S-glass
Density: 2.48g/cm³
Ultimate Tensile Strength: 4750 MPa
Modulus of elasticity: 90GPa
Elongation at break: 5.7%

Aramid

Aramid is, without doubt, the most ubiquitous ballistic fiber in the world at the time of this writing. It is extensively used in standalone body armor systems, spall liners and backings, ballistic helmets and shields, and virtually every other conceivable application — including “armoring” fiber optic lines for extended stays in harsh environments, and armoring satellites from micrometeorite impact. It has excellent resistance to fire and heat, but is sensitive to UV light (photooxidative degradation) and humidity (hydrolysis), so it is typically sealed or coated prior to use, and its characteristic bright yellow color is infrequently seen.

Aramid – Kevlar 29
Density: 1.44g/cm³
Ultimate Tensile Strength: 3620 MPa
Modulus of elasticity: 70.3 GPa
Elongation at break: 3.6%

Aramid – Kevlar KM2
Density: 1.44g/cm³
Ultimate Tensile Strength: 3880 MPa
Modulus of elasticity: 84 GPa
Elongation at break: 4.52%

Ultrahigh Molecular Weight Polyethylene

Broadly speaking, this class of polymer composite offers the best performance-to-weight ratio. More specifically, there are a number of grades available, ranging from low-cost grades with a weight efficacy roughly equal to aramid, to very highly-optimized grades, such as Dyneema SB-117 and Spectra Shield 5143, that are much more effective than aramid on a per-weight basis.

UHMWPE is stable in the presence of UV light, but has a low melting-point that leads to performance degradation at high temperatures. This becomes a serious consideration at temperatures over 70°C.

UHMWPE – Spectra 1000
Density: 0.97g/cm³
Ultimate Tensile Strength: 2570 MPa
Modulus of elasticity: 120 GPa
Elongation at break: 3.5%

Note: The antiballistic properties of Dyneema HB212 are significantly better than Spectra 1000, but its mechanical properties have not been made public in a comprehensive way.

Carbon Fiber

Carbon fiber is a structural composite that’s rarely used as a ballistic material on account of its brittleness, relatively high density, and lack of ductility. It boasts an extremely high yield strength and flexural modulus, however, and its strength and stiffness make it attractive for certain niche applications in armor. It is also highly resistant to UV light, chemical exposure, and temperature extremes. The very best grades of carbon fiber — Toray’s T1100G (ultra-high strength) and M60J (ultra-high stiffness) — represent the current state of the art, and are uncommon, whereas the grades T300 and T800 represent the fibers most frequently used in aerospace and automotive applications. T300, in particular, dominates the carbon fiber industry — 95% of sales are of the “standard modulus” T300 grade.

Carbon Fiber – Toray T300 (Standard modulus)
Density: 1.55g/cm³
Ultimate Tensile Strength: 3800 MPa
Modulus of elasticity: 230 GPa
Elongation at break: 1.5 – 1.76%

Carbon Fiber – Toray M60J (Ultra-high modulus)
Density: 1.93g/cm³
Ultimate Tensile Strength: 3950 MPa
Modulus of elasticity: 588 GPa
Elongation at break: 0.7%

As the previous post noted at length, steel has been used in armor from ancient times. It is still the world’s most ubiquitous armor material, on account of its excellent mechanical properties, coupled with the fact that it is an extremely low-cost solution.

Steel body armor, as a means of defeating high velocity aimed projectiles, was extensively used in WWI. Its use was generally experimental, and the experiments generally received poor-to-middling feedback, thus at no point in the war was steel body armor common. Soldiers generally found it burdensome, and not worth the effort of wearing, which is little surprise, as the steel body armor of those days was both thick — regularly weighing in excess of 30lbs — and it offered imperfect protection, being roughly equivalent to the flak jacket of later eras. For these reasons, taken together, the use of steel armor was generally relegated to soldiers in more-or-less stationary positions: Sentries, machine gun crews, and sappers. Even so, the use of steel body armor was judged a failure, and its use all but disappeared in the days following WWI. [1]

The poor feedback had much to do with the fact that 81% of WWI’s combat casualties were due to rifle and machine gun fire, against which steel armor offered no meaningful protection. Yet it must be noted that this is anomalous, for in WWII and subsequent wars, casualty statistics indicate that 70-80% of casualties were due to fragmentation, explosions, and indirect fire, and only 20-30% of casualties were due to direct rifle or machine gun fire. It stands to reason that the widespread use of steel body armor — or any armor — could have saved many lives in WWII. [2]

It is unfortunate, then, that steel armor saw only very limited use in that war. British and American field tests, in 1941, with manganese steel vests were judged unfavorably; all reports indicated that the advantages they might bring their wearers would be negligible compared to the loss of combat efficacy and the increased carrying load — despite the fact that the body armor system in question was around 1.5mm thick, on average, and weighed less than 3 pounds in total.

In 1942, following reports from aircrews of extremely dense flak that would punch through airframes and injure crewmen, the UK Royal Air Force started issuing steel armor to its aircrewmen. The armor they developed was initially comprised of 1mm thick manganese steel plates, 2 inches square, sewn into pockets, over a backing of flax canvas. It was offered in different cuts, from full-cuts which included leg and groin protection, generally intended for gunners who had to operate from a standing position, to vests which covered the front of the torso only, generally for pilots. Depending on the extent of coverage they provided, the weights of these early flak armor suits were from 4.5 to 16 pounds.

The US quickly adopted the design, and issued flak jackets to US Air Corps aircrewmen and US Navy personnel on aircraft carriers. The composition of the steel remained identical from a metallurgical perspective, and the thickness and size of the plates was not changed, but the backing was upgraded from canvas to ballistic nylon, a fact which increased performance considerably. Late in World War II, when flak jackets were ubiquitous among airmen, there was a reduction of 60% in the total number of combat-related wounds.

While these nylon-steel flak jackets were in use, the United States Army and Navy were busy trying to find a more efficient armor. More efficient, in this context, meaning lighter weight and enhanced protection. The development of Doron, a laminate of fiberglass and polyester resin, achieved both of these objectives. At an equivalent weight, the new fiberglass composite was significantly more protective than steel or aluminum; at a meaningfully lighter weight, it still offered slightly more protection. Following the adoption of Doron, which will be further discussed in a future post, steel was supposedly rendered obsolete as a body armor material, and gradually passed out of active military service. [3]

As it turns out, the reports of steel body armor’s death were greatly exaggerated, for it has enjoyed a marked resurgence in recent years. AR500 — an industrial grade of abrasion resistant steel with a Brinell hardness of approximately 500, thus “A R 500” — had initially been marketed as a steel target material, made possible by its ability to resist penetration from rifle fire in thin sections, and popular due to the satisfying sound and sparks it makes upon impact, which are audible and visible from great distances. Around 2011-2012, private US companies began selling AR500 armor plates to the civilian and law-enforcement markets in earnest. These plates were approximately 6.3-6.5mm thick, weighed 7.5lbs (3.4kg) in the popular 10×12 inch “shooter’s cut,” and were generally, but not always, coated with a layer of polyurea to mitigate spalling and help prevent ricochets. Ballistic testing showed that such plates generally had little problem resisting multiple shots from the 7.62×51mm M80 Ball and similar rounds, and could stop M855 at muzzle velocity, but were likely to fail against extreme-high-velocity rounds, such as 5.56mm M193 rounds traveling at 3100+ feet per second — velocities typical of shots fired at close range from a 16″+ barrel. AR500 armor plates are wholly incapable of stopping AP rounds at typical engagement distances, even such lowly AP rounds as the 7.62x39mm API-BZ. Yet on account of their low thickness, durability, wide availability, and extremely low cost at less than $100 per plate at retail, AR500 armor plates very quickly became the most popular form of rifle-rated armor on the civilian market. Indeed, plates seemed to sell faster than the manufacturers could produce them!

As of 2016, major military contractors and armor OEMs, doubtless taking a cue from AR500 Armor, had begun to offer steel armor plates. The most noteworthy of these was Tencate’s 8075SA, which, at a thickness of just 5.08mm and a weight of 6.2lbs (2.83 kg), was claimed to stop the M193, M855, M80 Ball, and 7.62×39 PS Ball rounds — all at muzzle velocities from full-length barrels. 2.83 kilograms is still, however, markedly heavier than the ceramic composite plates currently available, and all steel plates are, again, incapable of stopping steel-cored AP rounds. The 8075SA was also noteworthy for its unusually high price — it retailed for several hundred dollars per plate, and as of January 2018 seems to have been discontinued, presumably due to low demand.

The militaries of the world have, as yet, shown little-to-no interest in steel armor, though the civilian market for such plates continues to thrive. I believe that this is unlikely to change in the near future.

I’ll mention in passing that steel, and often very good grades of steel, remains the premier material for use in vehicular armor. In light of this, it’s also worth mentioning that the vehicular armor market is roughly ten times larger than the body armor market. Steel remains, therefore, the world’s premier armor material.

As for the mechanical properties of selected steel alloys used in modern armor systems:

AR500 is a low alloy steel, of medium carbon content (0.31%), quenched and tempered to a hardness of approximately 500HB. It is weldable. Yield strength is typically 1515MPa, tensile strength 1620MPa, charpy impact at 24 J, elongation at 14% in 2″. It costs roughly $2/kg in bulk, and is among the cheapest armor-grade steels available, likely due to its lean metallurgical profile, along with the fact that it’s produced in bulk for industrial use. (Nickel-rich and cobalt-rich alloys are much more expensive, as are high-strength specialty steels that are only produced in small batches.) The US military specification MIL-DTL-46100E “High Hardness Armor,” is closely analogous to AR500, but is intended for vehicular use.

ARMOX Advance is a ultra-high hardness steel used primarily for vehicular armor, but also of interest for body armor. It is a low alloy steel, with nickel as the primary alloying element. It is quenched and tempered to a hardness of approximately 670HB. Yield strength is 1600MPa, tensile strength 2250MPa, charpy impact at 14 J, elongation at 9%. Armox claims that just 4.5mm of Armox Advance can defeat the 5.56x45mm M193 at 937m/s or 3075fps — just a couple hundred feet per second below muzzle velocity.

Deutsche Edelstahlwerke Megafort-Z (previously Thyrodur Z) is a Ni-Cr-Mo-alloyed steel marketed as a premium vehicular armor material. It can be quenched and tempered to a hardness of approximately 575HB. At this hardness, tensile strength is 2000MPa, and elongation is over 14%. A 3.8mm thick panel of Megafort-Z can defeat the 7.62×39 PS Ball round at 700 m/s, which is muzzle velocity. This is a very impressive feat.

Armox advance and Megafort-Z represent, each in their own way, the high-end of the steel armor market, whereas AR500 is without question the most common alloy used for body armor today.

For comparison’s sake, the armor steel used in the flak jackets of WWII, Hadfield steel, was an austenitic manganese steel with a carbon content of 1.2% and a manganese content of 12%. Its tensile strength was approximately 500MPa and its yield strength was 860MPa. Hadfield steel is not a very hard steel, at generally just 240HB — however it shows extensive strain-hardening effects, and can reach hardnesses of over 400HB (along with correspondingly higher strength) upon impact or if pre-treated via compression or shot peening.

It may be interesting to note that there are maraging steel grades which exhibit tensile strengths higher than 3100MPa, combined with great elongation and fracture toughness, and hardness values which exceed 700 HB. (!!) These maraging steels are, however, chemically complex, and orders of magnitude more expensive and difficult to produce than low alloy steels. (I’ve seen maraging steels cost as much as $345/kg, and there are surely even more expensive grades.) Their production and export are also tightly controlled, as steels of this extremely high grade are often used to manufacture centrifuges for the separation of nuclear materials. For these reasons, maraging steels with tensile strengths over 2300MPa have never been utilized in military armor. Their military uses are presently restricted to missile skins and other applications where strength is prioritized at any and all costs.

It is frequently said that steel metallurgy has been exhausted — that it’s unlikely that future steel armor systems will be able to improve significantly upon the current state of the art. This is an erroneous belief, but that’s a matter for a future post.

[1] An interesting aside is that the design of the modern military helmet dates back to WWI. Dr. Bashford Dean, the author of the previous post, contributed a number of helmet designs to the US Army, one of which rated very highly, but was judged to be too similar in appearance to the standard-issue German helmet of the time, so it wasn’t approved for use! The German helmet was itself based on a medieval design, and was judged to be the most effective helmet of its time.

[2] There are numerous personal accounts of this, but the very best is the famous “Company Commander: The Classic Infantry Memoir of World War II” by Captain Charles B. MacDonald. The use of artillery in Captain MacDonald’s account was seemingly ubiquitous — it featured in every action and every operation — and the vast majority of the casualties mentioned in the book were hit by shrapnel from shells or mortars.

[3] Although steel body armor passed out of military use a very long time ago, the steel M1 helmet remained in use until the early 80s, at which time it was replaced by the Kevlar PASGT helmet. Interestingly, the M1 and the PASGT are very similar in weight, are quite similar in ballistic protective ability, the M1 is significantly cheaper to produce, the M1’s liner made for a one-size-fits-all solution, and soldiers in the field have always found the M1 useful for cooking and shaving, whereas the PASGT was useless for those purposes… so one could certainly call into question the wisdom and necessity of the M1’s replacement! The Kevlar PASGT, although of similar performance vs. bullets, does, however, exhibit a much higher V50 versus fragments and fragment-simulating projectiles.