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Kevlar is an aramid fiber that is used in ballistics protection vests. From the structure (structure shown from Wikipedia article), you can see why it would work to dissipate the energy from a bullet since there is an extensive network of interactions between the linear chains. The wearer ends up bruised from the force, but unpunctured and avoids serious injury from handguns bullets.

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More modern ballistics protection devices use ceramic plates (along with Kevlar) to give greater stab resistance and protection from higher velocity rifle rounds. My question is, what ceramics do they use? Do they have a "flexible" ceramic that will deform to absorb the energy and not break? Or do the plates work by breaking to absorb the energy and, thus, have to be replaced after each event?

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As far as I can tell there are two material types used for this - pure ceramics or ceramic mixtures that are designed to break and be replaced, and composite metal + ceramic materials that are designed to be able to handle a couple of rounds without losing too much effectiveness. There are also techniques involving purely metal layers in overlapping structures with fibers of materials like Kevlar or ultra-high-molecular-weight polyethene (UHMWPE) (of which Spectra is a brand).

Shopping around, it seems the best materials are boron and silicon carbides (extremely hard materials, though not quite as hard as diamond), which can be formed into plates with multiple levels of structure by starting with hexagonal plates and forming larger plates from those. In addition to personal armour, the same materials and techniques are used for military vehicle (including aircraft) armour. In the ceramic/metal composite materials, you get ceramic plates formed up within a metal sheet, where a ceramic element will shatter to take most of the impact, while the rest will be absorbed by the metal matrix. The metal matrix then remains intact (but may be deformed) so that the armour is still useful but weakened around the area of impact.

Good places to start from are the Wikipedia pages for Chobham armour (the common name for the tank armour that includes composite metal/ceramic matrix materials), and Small Arms Protective Insert.

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  • $\begingroup$ Thanks! That makes more sense than the mental picture I had of the ceramic plates and how they would function. $\endgroup$ – Janice DelMar Jun 8 '12 at 4:07
  • $\begingroup$ There are a lot of downsides to ceramic armor; among them, they are relatively fragile when dropped, stored in the trunk of a vehicle, or submerged in water. Furthermore, as you point out, they are fail quickly when hit multiple times; steel armor of similar mass can withstand many thousands of hits while the ceramic is finished after a dozen. $\endgroup$ – user209 Jun 12 '12 at 23:32

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