15
$\begingroup$

From my understanding, "hardness" and "toughness" in the context of material sciences (or perhaps specifically gems?) refers to two separate properties: "hard" materials are able to resist scratching/drilling/abrasions/cuts, while "tough" ones resist fracturing.

Diamond is considered an extremely hard material, but is also a brittle one, whereas something like Jadeite is extremely tough, but not as hard.

Imagine a wizard were able to conjure up and levitate a pane of such a material in front of themselves to act as a shield (with say, the same thickness as a medieval shield): I am wondering how those properties would factor into its durability against various attacks.

Would diamond's hardness but brittleness make it effective against blades and arrows, but liable to shatter if struck with a warhammer? Would Jadeite be impervious to blunt weapons, while also being hard enough that there's no point in extra hardness in the context of a medieval battlefield?

Would the calculus change if we went from medieval weaponry to say, cannons or modern firearms?

I'm trying to get a general understanding of how important each of those properties would be in such a context, whether a shield made of or having the material properties of certain gems would be an exceptional defense (compared to one made of say, wood or steel), and what kind of scale would be required to actually pierce/break such a shield (regular arms? firearms? war machines? armour-piercing rounds?).

$\endgroup$
7
  • 7
    $\begingroup$ Hardness isn't really a particularly useful property in a shield at all. A hard shield won't get scratched up in combat, but even a scratched shield will block weapons just fine. I'd be worried about a gemstone shield shattering- especially against firearms, in which case she shards could easily become deadly shrapnel. $\endgroup$ Feb 25 at 5:15
  • 2
    $\begingroup$ What are you asking exactly? Are you looking for ideal materials or specific situation where each property outshines the other? Or you need us to explain the concept for each property using a shield as example? $\endgroup$
    – user6760
    Feb 25 at 5:39
  • 2
    $\begingroup$ @Alex I don't really know for sure (this is not my area of expertise), but my gut says "probably not". Hardness would help a shield stand up to drill bits and stonecutting saws... but those tools aren't likely to show up on a battlefield. A steel shield can absorb an impact by flexing, bending, and denting. As Shawn O'Neil notes in their answer, a gemstone shield won't be able to absorb nearly as much of the weapon's kinetic energy before breaking. $\endgroup$ Feb 25 at 5:56
  • 2
    $\begingroup$ @Alex Remember there are parameters other than hardness and toughness. Tensile strength is what matters when up against something like an arrow which cuts its way through the target. Scratching tends to be displacement of material rather than cutting through it so hardness would be relevant there. Also hardness is not the same as abrasion resistance. They often line up, but apparently not always. $\endgroup$
    – DKNguyen
    Feb 25 at 17:17
  • 1
    $\begingroup$ Toughness tends to be inversely proportional to stiffness so you basically want to trade off just the amount of stiffness required to get enough toughness for it to survive the forces involved. If it is no longer rigid enough to do its job when that happens (i.e. the material deforms so much that it transfers too much force to the body) then you need another material or more of the same material (which you usually can't do). $\endgroup$
    – DKNguyen
    Feb 25 at 17:21

3 Answers 3

19
$\begingroup$

Toughness is more important in a shield

Toughness refers to the amount of energy necessary to deform a material. A tougher material will be able to absorb harder blows without being damaged, and if it is damaged or destroyed, it will absorb more of the impact in the process of being destroyed.

The diagram below is a stress strain curve. Stress is the force applied to each unit area of a material, and strain is the proportional deformation of the material. The curve below shows the elastic region as blue and the plastic region as yellow. The area under the curve gives the amount of energy per unit volume necessary to damage or destroy the object. The blue colored area gives the energy per unit volume the material could absorb and bounce back without being damaged, the yellow area and blue area combined give the energy per unit volume the material would absorb in the process of being destroyed. Typical stress strain curve, taken from: https://byjus.com/physics/stress-and-strain/

Crystalline materials tend to have steeper stress strain curves, and tend to fail more abruptly, meaning the fracture point occurs with less strain. This means the impact energy these materials can absorb when struck is generally underwhelming. Stress/strain data does exist for the materials you're considering using, so if you wanted you could calculate real-world toughness values for each.

Hardness may mean a few different things, but in this context it tends to mean resistance to fracture or deformation due to friction. Higher hardness may improve the shield's protection against slashing attacks, but it provides diminishing returns. As long as the shield is harder than the metal of the blade striking it, the blade will dull rather than abrading the shield. Generally speaking, nobody is going to cut through a shield like they would saw through a tree unless you just sit there and let them, so impact resistance, or toughness, is more important.

$\endgroup$
6
  • 1
    $\begingroup$ To clarify, does this logic hold true for the toughest gems (some of which are supposedly tougher than steel, according to google), or is it just a general trend amongst crystalline materials? IE, would even Jadeite/Nephrite fracture at a lower level of strain than the same volume of steel, and absorb less energy while doing so? $\endgroup$
    – Alex
    Feb 25 at 6:55
  • 5
    $\begingroup$ I'm not aware of a gem tougher than steel. Many are harder and stronger than steel, but crystalline materials in general cannot survive significant deformation. The rigidity of a crystal lattice only allows minimal deformation before bonds irreversibly break. Metallic bonds are unique in that the valence electrons are shared across the entire substance, allowing the material to deform without breaking those bonds. Plastics, which are also fairly tough, are usually made up of chains of monomers cross-linked to each other. This allows the material to stretch considerably without breaking. $\endgroup$ Feb 25 at 7:40
  • 5
    $\begingroup$ If you wanted to maximize toughness of a gemstone shield (without doing any work on it beyond conjuring it) the best recommendation I could make would be to conjure it as a laminate of many thin layers of gemstone. The discontinuity between the layers would prevent a crack from propagating through the thickness of the material, forcing the attacker to break each layer. $\endgroup$ Feb 25 at 7:54
  • 4
    $\begingroup$ If your wizards can conjure gemstone in any form and are willing to do some work on it, they could conjure extremely thin gemstone filament and use it to weave a fabric, which can then be used as fiber reinforcement for an adhesive or resin, much like modern composites. $\endgroup$ Feb 25 at 7:59
  • 5
    $\begingroup$ I looked up a technical article on the fracture toughness of jadeite and it reported 71E7 dynes cm^-1.5 (cgs) which is 71 MPa m^-.5 (SI) - unless I made a mistake. While this is tougher than some steels, other steels are much tougher. Maraging steels being about 175 (SI). The article noted that Jadeites toughness is apparently not due to atomic bonding, but are apparently related to texture and the restrictions which it imposes on crack propagation. $\endgroup$ Feb 25 at 16:30
10
$\begingroup$

There are several issues when designing a shield.

What is a shield? It is something that protects you against some incoming weapon. It can do that by deflecting the weapon, by absorbing it, or by reflecting it, or some combination of all three. What is the weapon? This is not specified, but presumably the weapon you have to fear is the one that is best at getting through the shield. So the shield and the weapon evolve together. This is particularly true of things like tank armour and armour piercing shells. A good shield ought to be light. I don't know if this extends to magic but I think it ought to.

The incoming weapon has momentum and energy. The best thing to do is to deflect it. That way the shield will only have to take some of the momentum and a small amount of the energy. If you can, have your shield at a small angle to the incoming weapon. If you can't anticipate where it is coming from, angle your armour so a weapon from most angles will glance off. That is the first job of plate armour.

This isn't easy with a flight of arrows. If you put the shield at a small angle to the incoming arrow, then it is hard to hide behind. On to the second option: absorb the energy. A hide shield is light and good against arrows - the arrow can penetrate the hide, but unless you are holding it against your body, you are still protected, and the shield with an arrow in it is still a shield, but your arm does not have to take the sudden shock when the arrow hits.

This is also true with tanks and armour-piercing shells. A HEAT shell can generate supersonic needle of molten metal, which can get through feet of metal plate. The defence is to have an irregular barrier out front that breaks the point of the needle, and turns it into a spray. There is then more material or void that allows the spray to cover an area, followed by something like ordinary armour plate with a high work to fracture that can absorb the energy.

A good shield is a composite. The front layer will deflect or blunt if it can, the middle region should spread the attack over a larger area, and the the final layer should hold together, and stop bits of flying shield hurting you. Each layer has separate material requirements.

Crystals typically have a very small work to fracture. Diamonds can be cleaved by a sharp edge. Nephrite is not a crystal in the ordinary sense; it has a fibrous structure like mineral asbestos, so it is effectively a composite like carbon fibre. This makes it less hopeless than other crystal materials. It can make a passable axe-head but it is not better than steel if you are making a shield. I do not know the origin of that factoid, but the quote I found has no figures or science. Wood would be better for that.

'Angels with shields of pure crystal' may look good in visionary writing, but like 'harps strung with gold' it will not work. Materials science replaces our vague word 'tough' with lots of separate properties, each of which may be critical at some point within a composite device. This was my subject. You can't just search for 'what is the toughest material?' and make a shield of that. But you asked, and that's the main thing.

$\endgroup$
4
  • 1
    $\begingroup$ (To clarify, the linked article claims Nephrite is tougher than steel, not that it is "better than steel if you are making a shield"). That said, it does seem to lack a citation. Unfortunately search engines seem adamant in replacing searches for "toughness" with "hardness" in this context, and I've got no idea how to get a straight answer on how tough it actually is compared to eg wood or steel. Though, one reddit page I found suggests that just "toughness" doesn't really make sense as a question, and there are apparently a few different kinds thereof that I'd need to look up specifically $\endgroup$
    – Alex
    Feb 25 at 12:08
  • $\begingroup$ There is no easy answer. If there was, my reply would have been much shorter. Nephrite is a sort of jade. Imagine making a shield out of that. If it was thin, it would shatter. If it was thick you could not lift it. That is pretty much what all those different definitions of 'toughness' might tell you if you had the figures. Now, imagine a single bone shaped like a shield. That might be rather good. Trust your experience. $\endgroup$ Feb 25 at 14:42
  • $\begingroup$ A special (and quite effective) case of deflecting an attack would be rebounding, where the incoming force is redirected mostly in reverse rather than aside. This would be accomplished through elasticity, with the material typically deforming under initial impact and then reshaping back to the original form. I don't think gems in particular are well-known for elasticity; crystals can be elastic but I assume typically less so than other materials. $\endgroup$
    – Miral
    Feb 27 at 3:59
  • $\begingroup$ @Miral Imagine your shield is a trampoline. You might be able to send back a thrown stone roughly in the direction it came from, but you will have twice the shock to your arm, because you are not just absorbing the momentum but reflecting it. There is also a risk that it will fall on your own side from an unexpected direction. I'm not gonna fight in your army. $\endgroup$ Feb 27 at 9:11
7
$\begingroup$

Would the calculus change if we went from medieval weaponry to say, cannons or modern firearms?

Yes.

Allow me to introduce Composite Armor - Paraphrasing - but my understanding is that you have layers of different materials with different properties.

So for example - you have a face plate of steel (which is your general tough armor) - then a layer of a Ceramic plate (extremely hard, not very tough), then other materials as needed.

The reason used is that (in the context of Tank Ammunition) Kinetic Penetrators (Equivalent to big bullets) will shatter the ceramic layer, then as they try to traverse the ceramic layer, the sharp edges are highly abrasive and grind down the penetrator.

Personal Body armor I believe also has Ceramic plates, for the same reason, either the Ceramic plate shatters the projectile or the plate is shattered and it deforms/debrides the projectile, which is then caught by the nylon liner. This type of Armor is rated up to level 3 - which is proof against .308 Rifle round.

If you could use Diamond instead of a Ceramic plate - it would be great.

$\endgroup$
1
  • 1
    $\begingroup$ Composite armor is usually multiple layers of hard and tough. The hard layers deform the projectile to get it out of its optimal penetration profile, and the tough layers hold everything together. Diamond isn't generally used for the hard layer, but sapphire sometimes is. Sapphire is cheaper, more durable, and easier to form. Plus, if you add a little nitride to it you can make it transparent for windows. $\endgroup$
    – Perkins
    Feb 26 at 16:41

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .