Dick jokes aside, popular culture loves things being hard. This is the hardest sword around, this shield is so hard that it can stand any damage and so on. This is largely bullshit, to be honest. Popular culture just has the habit of buying the idea of diamonds being the hardest generally known substance to man and going with it, because it seems it is too hard to teach that hardness alone doesn’t add to anything worthwhile. You need toughness to go along.
Hardness in Mohs is how well a material can resist penetration of other material, i.e. scratched. In all fairness, this is rather weak scale and is mostly useful with minerals Mohs scale is intended for. For geologists and craftsmen, the Mohs scale is still relevant. The higher the item is on Mohs scale, the better polish it can attain, with some exceptions.
The hardest naturally occurring material known to man is lonsdaleite, or hexagonal diamond. It was first identified in the late 1960’s from a Canyon Diablo meteorite, where specimens were found in microscopic size. If you checked the provided link there, you may notice that when actual stuff is talked, the Mohs scale was kicked to the curb. Mohs scale is essentially useless for industrial use.
In a more industrial meaning, hardness relates how much material resits compression and changing its shape. When we go up to superhard materials like diamond, their modulus of rigidity are very high as is their bulk modulus, the resistance to uniform compression. They do not deform plastically either.
Think it like this; when you strike wood with a hammer, it just dents. It deforms to form a spot where hammer was struck while leaving the rest of the wood intact. Strike glass and it’ll shatter without deformation, sending shards flying about.
There are numerous tests in which material hardness is tested with. Vickers, Rockwell and Brinell hardness tests are the most often used, followed by Mohs. Oustide the Mohs one, all the aforementioned use different methods to attain the scale of hardness and the results can be converted between each other. If you’re interested in reading further into this topic, I recommend giving this site a go.
But as said, hardness alone is of no real use when it comes to how popular culture wants to showcase it. Toughness is needed. As a general rule of thumb; The harder something is, the more fragile it is. For example,. despite diamonds being the hardest general substance, you can pick up your hammer and smash them to bits. They’re also common as hell, and nobody should be willing to pay the insane prices jewellery shops are selling them. You can put that on DeBeers.
Toughness is the ability of any said material to absorb energy and plastically deform without fracturing. This is the opposite of hard materials that have low toughness, as they do not absorb energy or deform. They fracture, sometimes in an explosive ways. Toughness is after all a combination of both material strength and ductility.
Just like how hardness has its tests, the Charpy V-notch test and Izod impact strength tests are the most commonly used measured. While Charpy is more about the fracturing the tested material, Izod tests impact power.
Ultimate tensile strength needs to be mentioned in this context, as it is effectively how much a material or structure can withstand elongation as opposed of compressive strength. Materials like diamond would have a very sharp breakage, called brittle failure. Other that are more ductile in nature would malform in plastic deformation before point of fracture. Something like glass has Mega Pascal of 33, depending on the glass variety and so on, while something like diamond having 2800 MPa, which still loses to multiple other materials, like graphene at 130 000 MPa.
How does all this come together with our topic? Let’s take the Destroyer-Class BETA as an example. It has a shield on top of it that is said to be Mohs 15 in hardness. That is to say, it is harder than diamond. To follow the rule of thumb, this material then should be relatively easy to shatter, especially with the large surface area it has. However, as the BETA are biological mining machines, and the fact that their shield’s can withstand considerable stress before penetration, saying that it is Mohs 15 does not actually mean anything. It’s the same with any other fantasy sword or the like that gets called harder than anything else. It it to give an idea of a tough, unbreakable object, which is rather far from reality, all things considered. But Mohs scale is simple and easy and doesn’t require studying. Saying that something is harder than diamond is enough to give a certain mental image.
What the Destroyer-Class BETA has, and all those other fantasy things, is high toughness and resilience to deformation. A sword good sword should be able to bend itself and conform to stress without breaking, something that Japanese sword don’t actually do that well because they were made hard. They were probably the hardest sword made, and thus far more brittle than swords that conformed and bent. The hardness contributes to the sharpness without a doubt, but sharpness alone can’t win a fight. Skill aside, sword’s shape, material, balance and toughness are all factors. Katana being so hard, materials like bone could dull them fast. The idea of a sword being samurai’s soul is gross exaggeration, as the katana was the least used weapon during wars over spears and bows, and were discarded if a better sword was in vicinity. Later on you got guns that made close combat weapons largely obsolete. Japanese swords are most likely the most romanticized swords there are, mostly thanks to movies and comics, but there are sources that put things right.
With shields or general tools, you do not want it to be hardest. You do not want a hammer shattering into your eyes or shield breaking down when an enemy hits it. With shields, you want it to malform and take the impact’s force instead of breaking it and allowing the opponent to advance. The worst idea you can have is shattering armouring or shield.
In giant robot series, it’s not uncommon to see armouring shattering like it was made of bricks or glass. It’s much easier to understand and is more dramatic, but a good armour doesn’t shatter. At best, it’s ripped apart into shreds under massive power. After all, most metals can be chipped like wood with proper tools, unlike something like diamond.
All in all, this was a very long and convoluted way to that hardness is almost a fetish in popular culture and is far too often depicted in a very uneducated manner, especially when Mohs scale is mentioned. But hey, all of that is fiction. Let’s just say this sword is Mohs 25 and has intrinsic tensile strength 500 Giga Pascals and tell it’s made of bullshittium and all would be well.
The thing is, in the end, that you can’t have a material that’s soft and hard at the same time, in general sense. A diamond like material can’t elongate itself and will experience fracturing instead. As such, in a case of Destroyer-Class BETA example, without any further information we can assume that the Mohs 15 hardness of indicative of other material strengths, making its shield very brittle. You don’t want your shuffle to be shatter the first time you hit a stone, you want it to be able to take the force from the impact. Of course, in-fiction it’s hard to penetrate with any weapon, where the whole deal with bullshittium steps in again.
Hard fiction often tends to step around these issues most of the time, as there are things like flexural strength and multiple others when it comes to material sciences, and simply relies on general terms and points of comparisons. General fiction on the other hand will simply play it safe and pull a diamond from its pocket and tell that this shit is hard, and that’s all you need to know.
Now let’s end this post with some great machining in slow motions.
“Almost sexual, isn’t it Smithers?”