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Under normal circumstances, monolithic diamond is pretty terrible at resisting impact, due to it being weak in certain planes, also known as cleavage planes.

To solve this, why don't we just make the diamonds smaller? They'd be much harder to hit on the cleavage plane that way (wink). What I had in mind was 10-20 micrometer diamond plates, arranged into a brickwork-like structure with an elastic polymer connecting them together. So, nacre but made from nanodiamonds instead of calcium carbonate.

Now, cleavage is present on the microscopic level (it arises from the "ideal" molecular structure of the crystal, not the deviations from it), so I don't know if it would work or if it would be better than boron carbide armor.

Would this diamond armor be able to resist firearms more effectively than either silicon carbide or boron carbide?

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  • $\begingroup$ I feel like Diamond armour has a strong fantasy vibe (King Arthur had a diamond shield in Faerie Queen). Are you going for a "the legends were true, and based on science" feel? $\endgroup$ May 25, 2021 at 2:22
  • $\begingroup$ @OwenReynolds Well, I'm now more of an Orlando Furioso guy, and I'm more about trying to make something really firearm resistant here. Though maybe if we were to make a shield and put it on a robotic arm, that could take the brunt of the impact from a .50 cal... $\endgroup$ May 25, 2021 at 10:02

4 Answers 4

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You have just designed an armor which is an excellent abrasive: do you maybe want to polish your opponent?

Just quoting from one of the many suppliers:

[brand] flexible diamond coated discs combine the durability and aggressiveness of diamond, on a flexible backing.

The resistance of your armor with this material will be dictated by the resistance of the matrix, in this case the elastic polymer, which is not exactly the top notch.

For a body armor to be effective against firearms you want something which can dissipate the kinetic energy of the bullet preventing it to be done by the body. Diamond won't help there, even in microsize.

Generally speaking, when you make a composite material, when you add a material to a matrix, that material should provide some property which the matrix lacks. If you add diamond which is though but not resilient, you are adding toughness, that's why a polymer with added diamond makes a good abrasive but not a good armor.

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  • $\begingroup$ Nacre is mostly chalk, but I don't see many people writing with them. Also, those discs are diamond-coated with no intricate microstructure. $\endgroup$ May 24, 2021 at 16:53
  • $\begingroup$ As for the matrix, can't we reinforce it with CNTs? I mean, looking at the arrangement, they would have to resist tensile stress, which is where CNTs excel. $\endgroup$ May 24, 2021 at 17:25
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We already have cheap bulletproof armors, you can buy a plate for less than than 100 dollars and a full suit of mixed carbon fiber and plates won't cost too much.

The reason bullet proof armors can't get better than level 4 is because there's a point where it doesn't even matter how heavy and thick the armor you are wearing gets. A bullet fast enough will knock you off your feet and you'll get most of the damage from the impact on your liver,lungs, stomach, heart and bones.

You can hide in a literal tank and use it as a shield, if the bullet is strong enough, it doesn't need to penetrate, the impact on the tank will throw you around and you will hit your head on the other walls of the tank.

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  • $\begingroup$ Bullets from man-portable weapons only knock people off their feet in movies, otherwise they would knock the people firing them off their feet. Yes, bullets that fail to penetrate body armour do bruise people, but serious injuries are from penetration. As for tanks - tank crews wear helmets to prevent injuries from bouncing around while driving. Please provide details of ANY incident EVER where a round failed to penetrate a tank but bounced it around enough to injure the crew. $\endgroup$ May 24, 2021 at 23:28
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    $\begingroup$ @KerrAvon2055 You're missing Furty's point. You can still scramble an egg without breaking the shell. No matter how hard the shell is, enough kinetic energy will scramble the soft gooey centre. $\endgroup$
    – Thorne
    May 25, 2021 at 3:25
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    $\begingroup$ @KerrAvon2055 en.wikipedia.org/wiki/Spall When your tank gets hit and the round fails to penetrate, it's not getting knocked around the inside of the tank that'll kill you, it's the inside of your tank's frame getting turned into shrapnel that will kill you. $\endgroup$
    – nick012000
    May 25, 2021 at 3:43
  • $\begingroup$ @KerrAvon2055 I literally broke 4 bones from a 3 meters fall... not a jump... just a fall... 4 bones... not one but 4 fully fractured bones... from only 3 meters and I'm a really light person... not underweight but not heavy and I'm quit fit... You'r gonna tell me a simple fall can break my bones but a bullet will only bruise me? $\endgroup$
    – user85816
    May 25, 2021 at 8:51
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Bullet proof vest are effective because they dissipate energy, not because they are tough.

Toughness is the material's ability to resist being fractured.

Lets say you have armor that is very tough but doesn't dissipate energy, like plate mail. That will stop most attacks, but eventually the attacks will break the toughness of the material and go straight through. Most bullets can do this to human sized armor.

If the energy dissipates then the bullet needs more energy to move the vest more to have enough energy to break through. Because of this lighter weight polymers with good dispersion are preferred to tougher materials that don't disperse energy as well.

However, you don't need every part of the vest to be strong enough to stop the bullet on its own, since the force is pushed over a larger area. If an average bullet exerts 1 kilonewtons and Kevlar is tough enough to withstand 2 kilonewtons of force exerted over it, making armor that can withstand 5 kilonewtons out of materials that don't dissipate as well isn't worth it. Both vests will survive, but one will dissipate worse and be more expensive of heavy.

Weight and safety concerns

Furthermore, Kevlar is half the density of diamond, with more tensile strength. Replacing Kevlar with diamond will increase the weight of the bullet proof jacket to potentially more that double its current weight. The armor could be cut down on weight reducing the dispersion even more, which would lead to other problems. Furthermore, with less dissipation the jacket will still cause more damage to the wearer as the same force is applied to the user over a smaller area.

All in all, making armor tougher doesn't always make it better.

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  • $\begingroup$ Composite armor works differently. The top layer is very hard (and thus brittle) with a tough layer below that. The hard layer causes the bullet to break up after which the fragments are caught by the more ductile tough layer where their remaining energy is dispersed into moving the material. The cracking of the top layer also absorbs energy from the bullet and helps spread the force. Infantry bodyarmor plates can be so brittle that dropping them from a height more than once can already degrade them. $\endgroup$
    – Demigan
    May 25, 2021 at 12:42
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You need to dissipate kinetic energy to "stop" bullet. Diamonds, per sé, would only conserve and pass this energy to the armor user. (from its incompresible nature)

I suggest to go to non-newtonian fluid, the very same that you could "walk on". Fist layer in diamond matrix, then several layer of this fluid.

There are a lot of videos of this kind of bulletproof jacket on the web

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  • $\begingroup$ Would you rather be hit by a car with a crumple zone or without a crumple zone? Since the fluid is thin you have problems with it shattering and then rip into the body as shrapnel. Then afterwards the remaining fluid can leak out reducing the capabilities of the rest of your armor. This is why modern composite armor has a tough and ductile material below it's hardened material to catch shrapnel from the bullet and armor that is accelerated by the bullet. $\endgroup$
    – Demigan
    May 25, 2021 at 12:47
  • $\begingroup$ Good point, just adding some diamond-layer to absord the schrapnel between the kineticproof layers should do the trick. $\endgroup$ May 25, 2021 at 15:11

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