Body Armor Basics: Hard Armor Plates
What is body armor?
Modern body armor is an insert, made of a strong material, that fits inside a garment such as a vest or plate carrier. Body armor comes in two varieties: “Hard” and “soft.” Hard armor plates generally consists of rigid panels which are built to offer protection from common rifle rounds. Soft armor generally consists of flexible, layered composites or textiles, and provides protection from handgun and fragmentation threats.
Hard armor types:
Hard armor plates come in three basic varieties:
Ceramic composite plates
These plates are made in two parts. The primary component is a layer of a technical ceramic such as alumina, silicon carbide, or boron carbide. These ceramics are brittle, and must be bonded to a secondary component — a backup layer — which is made of a ductile and tough material. This backup layer is usually a fiber composite. If the ceramic is not paired with a backup layer, it will fail in tension — and, for all intents and purposes, simply shatter — as soon as it is impacted by any bullet. The backup layer prevents tensile failure, so the ceramic is forced to fail in compression, and this is the secret to the performance of ceramic armor: Although ceramics are brittle and have low tensile strengths, they have extremely high compressive strengths; if an armor plate is built appropriately, it will bring the compressive strength of its ceramic layer into play.
Ceramic composite plates offer good protection against all small arms threat types, though the degree of protection they offer depends largely on the composition and, especially, the thickness of their ceramic layer. Ceramic composite plates can be damaged if dropped or handled excessively roughly, but most well-designed armor plates incorporate foam layers to minimize this risk.
These are thick plates made of ultra-high molecular weight polyethylene (UHMWPE) fiber composites. These plates exhibit an extremely good performance-to-weight ratio against lead and mild-steel-cored ball rounds, but perform poorly against rounds with harder steel cores, such as the M855. UHMWPE plates are extremely tough and chemically stable, but can degrade if exposed to very high temperatures for extended periods of time. 
These are thin plates made of low-alloy martensitic steel alloys. For several reasons, steel body armor plates are not favored: (1) Because they are particularly vulnerable to fast-moving lead-cored ball rounds, (2) because their performance-to-weight ratio against AP rounds is very poor, (3) because there are known risks associated with redirected bullet fragments, and (4) because they are typically quite heavy, at anywhere from seven to nearly ten pounds for a 10×12” SAPI-style armor plate.
What are the relative characteristics of ceramic composite armor plates, in detail?
– Good performance-to-weight ratio.
– Unrivaled performance against steel-cored and AP threats.
– Generally lightweight.
– Multi-hit performance can be, at times, unpredictable.
– As mentioned above, ceramic armor systems generally consist of a ceramic layer over a fiber composite layer. The function of the ceramic component — which always exhibits high hardness and compressive strength — is to shatter, deform, or erode the incoming projectile. The fiber-composite backup layer has three functions: First to support the ceramic layer so it doesn’t fail in tension and instead is forced to fail in compression. Second, to catch the shattered ceramic and bullet debris. Third, to absorbs whatever residual kinetic energy remains, which is done primarily via the plastic deformation of the backup layer.
– There are a few different types of ceramic in common use. With respect to their selection and utilization, there are two general rules: First, the ceramic layer must be harder than the projectile’s core.  This rule eliminates silicates and many oxides from contention as armor materials. Second, against AP threats in particular, it is axiomatic that the ceramic layer needs to be thicker than the projectile core’s diameter.  This rule places hard limits on the theoretical mass of hard armor systems.
– In high-end armor systems, the composite backer generally makes up one-third of the armor plate’s total mass, with the ceramic strike-face making up the remaining of two-thirds of that same plate’s mass. In lower-end systems that utilize heavier ceramic materials, the ceramic layer can make up three quarters of the plate’s total mass, or more.
– “Level III” ceramic armor plates are rated to stop six rounds of 7.62x51mm M80 Ball. Without any known exceptions, these same plates will also stop 7.62x39mm MSC, 5.56x45mm M193, and 5.56x45mm M855. As such, currently-available Level III ceramic armor plates are compliant with the NIJ’s forthcoming RF2 specification, and are capable of stopping all common domestic rifle threats.
– “Level IV” ceramic armor plates are rated to stop one round of .30-06 AP M2. With very few exceptions, however, these plates will exceed their rated capability and stop more than one round. A Level IV plate will also stop all of the aforementioned “RF2” threats, and will also stop standard-issue military EPR rounds such as the M80A1 and M855A1.
– NIJ-certified plates are required to withstand two drop tests, where the plate is slammed into a hard surface from a height, prior to ballistic experiments. For this reason, most ceramic plates include a thick foam layer on their strike-faces. Properly made ceramic plates can certainly fracture and degrade if they’re mistreated, and any ballistic impact will assuredly degrade their performance, but they’re not exactly fragile. They’re built to stand up to rough handling and hard use.
– Of all armor types, ceramic-faced armor offers the poorest and least predictable multiple hit performance characteristics. Some manufacturers attempt to remedy this with “mosaic-style” ceramic strike faces, where the strike face is comprised of multiple smaller ceramic tiles, e.g. in a grid of hexagonal or rectangular tiles. Done properly, this can dramatically improve a ceramic armor plate’s multi-hit performance, and its damage tolerance, at a significant penalty to its thickness and performance-to-weight ratio. 
What are the relative characteristics of UHMPE armor plates?
UHMWPE armor pros:
– Exceptional performance against lead-cored and mild steel cored rifle threats.
– Good multi-hit performance.
– Impact resistant.
– Can be extremely lightweight.
UHMWPE armor cons:
– Weak to threats with hardened steel cores or penetrators.
– Can degrade if exposed to temperatures over 158°F (70°C) for extended periods of time. [1, 5]
– Hard armor plates made of UHMWPE, without a hard strike face, were introduced commercially around the year 2001, more than twenty years ago now. Such plates generally exhibit extremely good performance against soft-cored ball rounds. Indeed, as far back as 1996, a few years before UHWMPE materials really started to catch on, UHMWPE hard plates were identified as the lightest weight armor options for defeating certain rifle ball round threats, such as the 7.62x51mm M80 Ball and 5.56x45mm M193. 
– UHMWPE-based hard armor plates are thick, at 0.9” to over 1.1” for NIJ Level III protection. In these plates, against soft-cored ball round threats, the top 1/3rd of material functions in much the same way as the ceramic layer in a ceramic-composite plate: It disrupts and deforms the projectile. The underlying layers of material then serve to catch metal fragments and absorb residual kinetic energy by converting it to plastic deformation, fiber strain energy, heat, etc.
– Because UHMWPE-based hard armor plates can’t disrupt or deform projectile cores that are made of hard steel, to say nothing of still harder materials, these plates are weak to threats with steel cores or penetrators, such as M855. The Level IV (RF3) and RF2 ratings are effectively impossible for UHMWPE-based plates without hard strike faces to attain.
– UHMWPE plates are remarkably impact resistant and resilient, but care must be taken due to the fact that they can degrade if regularly or continually exposed to very high temperatures.
What are the relative characteristics of steel body armor?
Steel body armor pros:
– Good multi-hit performance.
– Can be thin.
Steel body armor cons:
– Very weak to high-velocity threats, such as the extremely common 5.56x45mm M193.
– Incapable of stopping most AP threats.
– Very heavy.
– Bullet fragments may pose a threat to the armor wearer and other people in close proximity.
– Level III steel body armor is generally 0.2 to 0.275” thick, and is made of a martensitic low-alloy steel hardened to roughly 500-600 on the Brinell scale, which comes out to 52-57 Rockwell C. 10×12” SAPI-style plates of such construction typically range from 7 to over 9.5 pounds.
– Steel body armor is typically sold with a “Level III” or “Level III+” rating. What this means, in practice, is that it will stop 7.62x51mm M80 Ball, all common 7.62x39mm ball rounds, and will usually stop 5.56x45mm M855. Yet all steel plates perform unreliably at best against fast-moving lead cored ball rounds, such as 5.56x45mm M193 — to say nothing of faster or harder hitting ball round threats, such as .270 Winchester, .243 Winchester, or .22-250. Because the 5.56x45mm M193 is an RF1 threat, steel body armor may be unable to attain a rifle rating from the NIJ when 0101.07 is implemented.
– There are, less commonly, “Level IIIA” or “IIIA+” rated steel plates. These are rated to stop handgun threats, and will not stop rifle rounds anywhere near muzzle velocity.
– It is axiomatic that ceramic-composite armor only performs well when the ceramic layer is harder than the penetrator of the bullet it needs to defeat, and the same is true for steel: Steel body armor does not perform well when it’s hit with projectiles that are harder than it is. The penetrator of the M855 is fairly soft, at roughly 40 Rockwell C — but the M855A1 has a much harder penetrator, at 58 Rockwell C. This is substantially harder than most steel armor plates. Accordingly, steel body armor will under-perform against EPRs such as M80A1 or M855A1.
– There is some well-founded concern over the fact that steel armor plates stop high-energy bullets directly on their surface. The metal from the projectile’s core and jacket have got to go somewhere. Those metals typically shoot off the surface of the plate — either parallel to the plate, or at an up to 45° angle. This may pose certain obvious risks to the wearer of the armor plate.
– Shots that impact a steel armor plate at a high angle of obliquity can ricochet. At impact angles over 60°, ricochet is highly likely.
– Steel body armor can be given polymer coatings to somewhat mitigate the bullet fragment problem, if not the ricochet problem, but this can negate one of steel body armor’s advantages: That it can be very thin.
– Steel’s primary virtue is that it is usually extremely tough and multi-hit capable.
Other hard armor plate types:
Titanium composite armor: Exemplified by our patent-pending Mantis armor plate, titanium composite armor combines the core virtue of steel armor — toughness — with the performance characteristics of ceramic armor. Like ceramic armor — and unlike UHMWPE and steel body armor — the Mantis is tested in compliance with the RF2 threat rating. However, it must be noted that a titanium strike face thick enough to defeat the .30-06 APM2 would be excessively thick and heavy.
Flexible rifle armor: Exemplified by DragonSkin, Hexar, and LIBA, this type of armor generally consists of an array of discrete ceramic elements over a thick, integral, soft armor backer. Because the backer is soft rather than pressed, this variety of armor is flexible. The discrete ceramic elements can come in any number of shapes, sizes, and arrangements. DragonSkin utilized an array of overlapping discs. Hexar utilizes a tessellated array of hexagons. LIBA utilizes an array of cylindrical pellets, which may have fins which allow them to interlock. Flexible rifle armor is similar to mosaic-style armor in that, when compared to monolithic ceramic armor, it offers improved multiple-hit performance at a penalty to its thickness and performance-to-weight ratio. It may also be more comfortable, for it is not exactly “hard,” and can conform to the wearer’s body shape.
Hard Armor Plates: The Bottom Line
There are drawbacks associated with every armor type. The world’s top militaries have carefully considered the advantages and disadvantages of each, and have come to favor ceramic composite armor. UHMWPE-only plates are sometimes preferred for certain roles, e.g. for light infantry — and can be quite useful in certain situations, e.g. in littoral, riverine, or maritime operations — but their use is not widespread. Steel body armor plates are, generally, not in military service at all. Titanium-UHMWPE composite plates are something new, and we’re working on carving out a niche for them in the Mantis titanium armor plate. LIBA, a variety of flexible armor, has already carved out its own room-clearing niche — for SWAT in Europe and urban SOF in Israel — on account of its multi-hit capabilities.
If you have any questions, please do not hesitate to let us know.
See, e.g.: Manning, Russell J. E. (2006). Temperature in Cars Survey, Brisbane. RACQ Vehicle Technologies Department.
All common armor ceramics — from alumina to boron carbide — are harder than the tool steel cores of AP projectiles such as the .30-06 APM2. However, armor ceramics are not always harder than the cemented tungsten carbide cores of more advanced AP projectiles such as the 7.62x51mm M993, and this can result in poor performance.
This places a practical lower bound on ceramic armor tile thickness. But because armor plates are built with a margin of safety in mind, the thicknesses employed are almost always greater than that.
Because small tiles don’t form proper fracture conoids upon impact. See the glossary entry for “fracture conoid,” and elsewhere on the knowledgebase, for more on this.
See, e.g.: Liu, Xiaoyan & Yu, Weidong. (2005). Evaluation of the tensile properties and thermal stability of ultrahigh-molecular-weight polyethylene fibers. Journal of Applied Polymer Science. 97. 310 – 315. 10.1002/app.21720.
Verlinde, A.S., van Dingenen, J.L.J., Dyneema Inserts: Personal Armor Against Rifle Bullets, Proceedings of Personal Armor Systems Symposium 96, Colchester, UK, Sep 1996