Glass-Ceramics in Armor Plates
Glass-ceramics are an unusual and fairly exotic class of material, so they could do with an introduction. The recently updated textbook definition is:
“Glass-ceramics are inorganic, non-metallic materials prepared by controlled crystallization of glasses via different processing methods. They contain at least one type of functional crystalline phase and a residual glass. The volume fraction crystallized may vary from ppm to almost 100%”
What this means is that a glass-ceramic is what you get when you have an amorphous (glassy) non-metal, and then you process it so that it partially recrystallizes. Most common glasses – like window glass – are predominantly silicates, so the production of a typical glass-ceramic involves taking an amorphous silicate and treating it so that SiO2 crystals emerge from the amorphous matrix. This is usually performed by simple heating, which is given the technical term “ceramming” when applied to glass-ceramics. The production process for the first commercialized glass-ceramic, Pyroceram, goes something like this:
- A silicate glass composition containing nucleating agents (such as TiO2 or metallic particles) is melted at high temperatures to ensure homogeneity.
- The molten glass is then shaped into the desired form using conventional glass-forming techniques like casting, rolling, or pressing. At this stage, we could have a perfectly serviceable glass bowl or sheet.
- Now the glass article is heated to a temperature where nucleation occurs. (Typically just above the glass transition temperature, Tg.) This step encourages the formation of tiny crystalline nuclei within the glass.
- The temperature is then raised, to promote the growth of these nuclei into crystals, allowing them to develop throughout the glass matrix.
- After the heat treatment, the material is slowly cooled to room temperature to relieve internal stresses and prevent cracking.
So a silicate glass is made, formed into its final shape, and then heat treated in a process that nucleates crystals inside the glass part. These crystals act as a reinforcing phase which strengthens and toughens the part.
Yet most glasses, however strengthened and toughened, are still weaker and much softer than conventional ceramic armor materials. Even the hardest glass-ceramics are substantially softer than low-purity aluminum oxide. For this reason, many people – including professional armor engineers – are wary of their use in armor plates and similar systems. But glass-ceramics have three advantages that they don’t share with ceramic parts, and these advantages make them very useful in specific light armor system types.
The Third Advantage: Anomalously Good Dynamic Material Properties
A previous article noted how boron carbide, though an outstanding material in static and low-rate experiments, performs in a “chaotic” and unusually poor manner at high pressures and strain rates. To simplify things a bit, this is because its molecular structure is dynamically unstable and breaks down at high pressure – going from crystalline to amorphous – which rapidly weakens and embrittles the material.
Silicate glasses and glass-ceramics behave in a rather opposite manner: There’s quite a lot of evidence to suggest that they densify and strengthen at moderately high pressures, and transform from amorphous to crystalline. That, rather than breaking down, they undergo pressure-induced densification, where their constituent atoms are compacted and rearranged. In silicates, this can result in the formation of stishovite.
Stishovite is to SiO2 as diamond is to carbon: It’s the densest, hardest, and strongest silicon-oxygen compound, and it only forms at very high pressures. For stishovite, this means pressures above 9-12 GPa – higher pressures than are required for the formation of diamond. It’s therefore very rare in the Earth’s crust, and its presence is often used as a marker for meteorite impacts; if there’s stishovite on the surface, you might be standing near an impact crater.
But small impacts, over small areas, can also result in very high pressures. And it has been shown that glasses and lightly-recrystallized glass-ceramics form stishovite upon high-pressure ballistic impact.
In any case, the dynamic properties of glasses and glass-ceramics – stiffness, compressive strength, and tensile strength – are remarkably strain-rate dependent. At very high strain rates, those properties rise dramatically. See, for instance:
The First Advantage: Ease of Manufacture
Our silicate glass, prior to recrystallization, is formable and castable, which allows complex shapes to be easily manufactured, even at very low or variable thicknesses. If you want a monolithic part with a 10mm central thickness which tapers to a 4mm edge thickness, that’s easily possible. If you want a convex lens, it can be done. And if you want a large 2mm-thick part, of (mostly) uniform thickness throughout, that is also possible. Effectively none of these things are readily possible, or even possible at all, with conventional ceramic materials.
Ceramics based on silicon carbide or boron carbide, which must be sintered at very high temperatures, will warp severely if anything along such lines is attempted. Monolithic sintered strike-faces can hardly be made at any thickness under 5mm. Hot-pressing in SiC or B4C can make slightly more complex geometries at lower thicknesses, but that process is very expensive, low-throughput, and still highly limited in what it can produce without stress gradients breaking the parts before they leave the press.
Aluminum oxide – and various other oxide ceramics – can be made in very complex geometries, at low thicknesses, but only via complicated processes that are expensive to run. Practically speaking, it’s just not done, outside of various specialty industrial and electronic parts.
The glass-ceramic advantage is that complex parts can be made cheaply and easily.
The Second Advantage: Low Density
As was exhaustively reviewed in A Facile Method for the Estimation of Ceramic Performance in Light Armor Systems, there’s no material property more important than thickness. Hardness, density, and compressive strength contribute to a ceramic strike-face’s performance, but thickness has an outsized effect – to such an extent that it takes a substantial hardness and compressive strength advantage to overcome even a small thickness disadvantage.
As such, there’s no material property more important than density, for body armor and other light armor systems are weight-limited – there are very low weight ceilings – and density determines the viable thickness of the strike-face element.
And here silicate glass-ceramics have a tremendous advantage, with many viable chemistries in the 2.2 to 2.5 gm/cc range. Amorphoid, for instance, is at 2.2 gm/cc. This is ~10% lighter than boron carbide – the lightest and by far the most expensive conventional armor ceramic. It’s also more than 30% less dense than silicon carbide, and roughly 45% lighter than an equal thickness of aluminum oxide. A 4mm thickness of Amorphoid is practically the same weight as a 2mm thickness of alumina – and, unsurprisingly, its performance is substantially superior.
At the time of this writing, Oct 2024, glass-ceramics are the lightest viable strike-face materials for body armor systems.
(From Tan, Shengzhi et al. (2021). An Improved Material Model for Loading-path and Strain-rate Dependent Strength of Impacted Soda-lime Glass Plate. (Open Access.) Journal of Materials Research and Technology. 15. 10.1016/j.jmrt.2021.09.010.)
With strain strengthening and possibly even conversion to stishovite, an already strong glass-ceramic can become a surprisingly effective material.
Under conditions that take advantage of strain strengthening, even regular glass can be an extremely effective ballistic material. In the LLNL paper, “Impact Studies of Five Ceramic Materials and Pyrex,” as the name describes, five ceramic materials were compared with Pyrex in heavy vehicular armor experiments. This graph tells the tale:
At ~1750 m/s – about 5741 feet per second – Pyrex glass outperformed alumina and wasn’t far from AlN and SiC. At ~2600 m/s – about 8530 feet per second – it outperformed SiC, TiB2, and alumina, and was roughly on par with boron carbide. Only AlN – a ceramic material with similarly strange dynamic material properties at high strain rates – performed markedly better than the rest. The authors concluded their paper with this note: “At velocities above about 2.0 km/s the aluminum nitride outperforms all the others. Moreover, Pyrex [glass] ranks with boron carbide, titanium diboride and silicon carbide at about 2.6 km/s and may outperform them at higher velocities.”
Glass-Ceramics in Light Armor Plates
The above advantages clarify why glass-ceramics are valuable in light armor plates, especially against light steel-cored rounds like M855 and 7N6. Those rounds aren’t especially difficult to defeat if you’ve got an armor plate with a hard strike-face, but it can be very difficult to make a non-oxide strike-face at the “correct” (optimal, low) thickness. Thin Al2O3 plates are possible, but Al2O3 is heavy and, without question, the worst-performing conventional armor ceramic. For now – until future advances in ceramic processing technologies make it easier to produce non-oxide ceramics in complex shapes at low thicknesses – relatively thin, very lightweight, high-performance glass-ceramics are the best solution for armor plates at the RF2 level, and for plates at similar levels in military service, e.g. the USMC LITE plate.
It’s more challenging to make a glass-ceramic plate that’s effective at the RF3 level. Not impossible by any stretch of the imagination, but much more difficult to take advantage of the strengths of the material; in the 8-15mm thickness range, even the best glass-ceramic is outperformed by SiC-TiB2 and conventional boron carbide, and in that thickness range those ceramics are not so difficult to produce.
Amorphoid
Amorphoid is a lightly recrystallized glass-ceramic designed specifically for RF2 plates that works well in the 3.75 to 4.5mm thickness range. We developed it for use in our ultra-light Thunder plate, and now it’s also beginning to see use in lightweight armor made by other companies. Contact us if you’d like to discuss glass-ceramics or amorphoid in more detail.
