Boron Carbide Amorphization and Strengthening Boron Carbider
In the 1960s and early 1970s, it looked like boron carbide was the perfect ceramic armor material. It was lighter than almost every other ceramic. It was harder than just about all of them. It was cheaper than most, as it was already produced on multi-ton industrial scales as an abrasive powder. It was also relatively easy to turn into dense ceramic parts; it could readily be hot-pressed to nearly full density, which wasn’t even possible with SiC back in those days.
In the LLNL’s “Light Armor Program” experiments, boron carbide was the standard which all other ceramics were compared to, and almost none of them surpassed it. But there it was tested against 7.62mm steel projectiles at 2700 feet per second (822 meters per second). When, in later experiments, boron carbide armor was tested against tungsten-cored projectiles at similar velocities – or steel projectiles at the much higher velocities typical of saboted 120mm kinetic energy penetrators, >4500 feet per second – it always seemed to underperform. And not by just a little bit; in some cases, it underperformed very badly.
The magnitude of this problem is illustrated by this account in “Body Armour – New Materials, New Systems,” a 2019 paper from Ian G. Crouch:
“In previously unreported work by Crouch, boron carbide targets, using 10mm thick VPP B4C tiles with an UHMWPE backing, in combination with a standard Soft Armour Insert, exhibited the following V50 values against three different 7.62mm AP rounds: 949 meters per second against the APM2; 1,002 meters per second against the B32; but only 598 meters per second against the FFV [tungsten carbide-cored M993] round.”
Suffice it to say that Adept has performed similar experiments and can corroborate this account.
In the 1970s and through the 1990s, this gradually became a well-known problem. Some simply assumed that boron carbide was somehow intrinsically weaker and more brittle upon impact than other ceramics. In other words, that anomalously poor dynamic material properties are the culprit. Others, hoping for a more mechanistic understanding of the phenomenon, presumed that boron carbide – much like single-crystal sapphire, another famous under-performer – is unable to deform plastically at any reasonable pressure, and that this is sufficient to explain its poor performance. In yet another theory, a 1989 report noted that mathematical modeling experiments could not “capture [boron carbide’s] apparent loss of strength just beyond initial yield, nor the chaotic behavior of the material in general.” (Emphasis added.) They went on to attribute the effects to heterogeneous failure and material defects.
D.J. Steinberg at the LLNL was most prophetic and deserves more credit than he ever got. He wrote in a 1991 paper, “Computer Studies of the Dynamic Strengths of Ceramics,” that boron carbide is a “special case,” that its performance upon impact is anomalously poor, and that:
“The crystal structure of B4C is highly unusual. The basic structure is rhombohedral, with an icosahedral structure unit occupying each vertex of the rhombohedron. According to Emin, the space inside each icosahedron is large enough to hold a magnesium ion. I believe that these icosahedra collapse under the influence of a strong shock, which is the cause of the sudden drop in the material strength.”
This is indeed more or less what happens, though it took more than a decade for it to be verified experimentally.
Adept Armor – Own work
In 2001, the US Army Research Labs released a report titled “Shock Response of Boron Carbide,” which summarized the state of knowledge at the time. It noted that while boron carbide maintained excellent performance under certain conditions, e.g. lead or steel-core small arms impacts, it exhibited a significant loss of shear strength when subjected to shock stresses exceeding its Hugoniot Elastic Limit (HEL), typically between 15 and 20GPa. This loss of strength led to a marked decrease in the material’s ability to withstand high-velocity impacts and impacts from tungsten-cored rounds, which would typically generate dynamic pressures above 20GPa. The report also highlighted boron carbide’s relatively low tensile (spall) strength, which contributed to its brittle failure under dynamic conditions. However, the exact mechanisms behind this loss of strength remained unclear, and the report called for further research into the causes of boron carbide’s underperformance, especially in comparison to other ceramics like silicon carbide, which actually saw improved performance at high pressures.
It wasn’t long before the cause was pinpointed experimentally. In 2003, Chen, McCauley, and Hemker, in their paper “Shock-Induced Localized Amorphization in Boron Carbide,” provided experimental verification for what D.J. Steinberg had hypothesized a decade earlier: That the icosahedral crystal structure of boron carbide collapses under extreme pressure. Using high-resolution electron microscopy (HREM) on fragments produced by ballistic tests, they discovered nanoscale intragranular amorphous bands within the boron carbide. These bands form along specific crystallographic planes and act as weak points, leading to material failure under high-velocity impacts, particularly when the material is subjected to impact pressures of 20GPa and above. The amorphous bands were found to be only 1 to 3nm wide, but their presence explains the abrupt drop in strength observed in boron carbide under extreme stress, particularly when impacted by tungsten-cored or extreme high-velocity projectiles.
There was a lot of scientific work done on boron carbide in the years which followed, and it branched down two paths.
Towards an atomic-level understanding of boron carbide amorphization
The first path focused on further proving that amorphization happens in boron carbide and is associated with instant reductions in performance. Some of that work went deep into the atomic mechanisms behind it, which by now appear to be fairly well understood.
To summarize: Boron carbide’s structure consists of 12-atom B12 or B11C icosahedra connected by three-atom chains, typically carbon-carbon-carbon (C-C-C) or carbon-boron-carbon (C-B-C). Under high-pressure shock loading, the C-C-C chains are particularly susceptible to deformation, as they are less stable than the C-B-C chains. Once the chains collapse, the surrounding icosahedral structure also begins to fail, resulting in the formation of narrow amorphous bands (1-10 nm wide), especially along specific crystallographic planes where the C-C-C chains are more likely to collapse. These amorphous bands become the weak points that lead to catastrophic failure during high-velocity impacts.
To make matters yet worse, as boron carbide’s icosahedral structure breaks down, rapid molecular motion and inefficient kinetic energy absorption lead to intense local temperature spikes. Temperatures in boron carbide under shock loading can rise well above its melting point, contributing significantly to the loss of shear strength and eventual failure. This phenomenon was identified and is more fully described in a 2020 paper titled “Shocked ceramics melt: An atomistic analysis of thermodynamic behavior of boron carbide” by DeVries et al.
Towards Amorphization-Resistant Boron Carbide
The second path, motivated by more practical concerns, took that contemporary understanding of boron carbide amorphization and attempted to devise variants that might be resistant to breaking down under pressure.
It became apparent, even as early as the mid ‘00s, that the C-C-C and C-B-C chains which bind icosahedra together are the weak links. Doping B4C with various elements has been proposed as a way to stabilize its structure and prevent amorphization; the idea is to either replace or modify the C-C-C chains surrounding boron-rich icosahedra, thereby preventing their collapse. There are a few key strategies:
- Strengthening the chains. When boron carbide is doped with small amounts of silicon, Si atoms tend to replace carbon atoms in the three-atom chains, particularly in the C-C-C configuration. This substitution strengthens the overall structure by forming more stable Si-C or Si-B bonds, which are less prone to collapse under pressure. By reinforcing these chains, silicon doping increases the material’s resistance to deformation and amorphization.
- Weakening the chains. When boron carbide is doped with small amounts of aluminum, Al atoms replace carbon in three-atom chains, resulting in B-Al-C and C-Al-C chains. These chains are substantially longer and weaker than B-C-C and C-C-C. This controlled weakening has the effect of promoting dislocation-mediated plasticity, which is typically absent in undoped boron carbide. By reducing the stiffness of the chains, aluminum doping allows the material to absorb and dissipate strain energy through plastic deformation, rather than through catastrophic failure. This activation of dislocation slip mechanisms helps to prevent the formation of brittle amorphous bands and allows the material to withstand higher stress levels without undergoing sudden collapse. In this way, aluminum doping provides a more ductile response to impact and improves the overall toughness and energy absorption capabilities of B4C while simultaneously reducing the risk of amorphization.
- More boron, less carbon. Boron carbide, nominally B4C, exists in a range of potential compositions from the carbon-rich B4.3C to the boron-rich B14C. Practically, largely on account of how it’s made, it’s far closer to the carbon-rich side of the spectrum – and, in fact, B4C is substantially accurate as commercial boron carbide often contains carbon impurities and carbon in solid solution. The boron-rich variants, such as B6.5C, contain fewer of the particularly deleterious C-C-C chains. As a result, boron-rich compositions form fewer amorphous shear bands and experience less severe degradation during high-pressure impact.
- Tough secondary phases. Another approach involves incorporating secondary ceramics such as SiC, TiB2, ZrB2, or HfB2 into the boron carbide matrix. These tougher components slow crack propagation and alter the way cracks travel through the material. However, since this method does not directly address the root causes of amorphization, its effectiveness is limited unless the ceramic composition contains less than roughly 70% boron carbide by weight.
The great problem with all of these methods is that none of them are entirely effective. B-enrichment reduces amorphization intensity by roughly 30%, Si-doping by roughly 31%, Al-doping by 44%, and secondary phase reinforcement by substantially less than 30% unless there’s a very large volume of that second phase.
The optimal method probably involves Al-doping plus reinforcement with a ductile phase. Al-doping + AlN incorporation at 20% by weight might mitigate amorphization, and improve ductility under impact, as to enable boron carbide’s use in heavy threat armor. Adept is working on this. Contact us to learn more.
