When asked to name the most difficult thing in the universe, some might suggest failing your second year or getting Flat Arthur to admit reality. When asked about the hardest substance, most people would probably say diamond. But like many questions in this series, the answer isn’t as simple as your memory might tell you.
For centuries, diamond has been the epitome, even the definition, of hard materials. We can now say that we have surpassed them, but the story is not that simple. In 1812, Friedrich Mohs wanted to create a hardness scale for materials. Since Morse had no way to measure how much material deforms under pressure, he used talc as a starting point and defined it as talc. He placed diamond, the hardest mineral he knew, at 10, and everything else he left to be somewhere between the two. Hardness is a measure of how hard one material is compared to the other, such as the famous (but unreliable) test that determines whether a claimed gemstone is authentic by testing whether it can scratch glass.
It was judged based on its ability to cause visible damage to the material. There are problems with Moh’s scale, including the fact that the values he assigned to intermediates are not linear. However, it is still used for several purposes today. Geologists in the field find it useful to be able to identify unknown rocks without the use of testing equipment.
stance can resist without deforming.
Initially, however, irrespective of the scale used, diamonds remained the hardest substance. Indeed, the Vickers test is conducted by applying great force to a pyramid-shaped diamond pressed against the substance being tested. The four carbon bonds, uninterrupted by the impurities of other gemstones, were tough to beat, and it was widely assumed nothing could be harder.
Even this was recognized to be something of an oversimplification, however, because diamonds come in different types, some harder than others. The hardest diamonds are classified as type IIa, which means they have very few impurities. Although IIa diamonds only account for 1-2% of natural diamonds, they contain many of the world’s most famous stones. Most synthetic diamonds, which are intended to be as hard as possible, are Type IIa.
Studies of wurtzite boron nitride (wBN) crystals, which are produced in tiny volcanic particles, suggest that it is possible to exceed the hardness of diamond. Simulations do not always match reality, and natural wBN pieces are so small that they cannot be tested directly.
A debate ensued as to whether this combination of atoms on either side of carbon on the periodic table could really outperform the original combination. When chemists succeeded in mass producing wBN in 2009, they discovered that it was less hard than diamond on the Vickers scale, but not everyone agrees.
Another natural substance claimed to be harder than diamond is lonsdellite, but this is also debatable. Like diamond, lonsdellite is made of carbon, but it is arranged in a hexagonal pattern rather than a cube. There’s a reason bees build their hives in hexagonal shapes. A hexagon is a very stable shape. In theory, lonsdellite should be 58% harder than diamond.
The oid compressed into lonsdellite is not completely pure, and trace amounts of other elements weaken the product. As a result, even though natural lonsdellite, which is harder than diamond, theoretically exists, no one has discovered it.
Let’s take a look at the lab that led to the creation of graphene in 2004. (Graphene had actually been made before, but scientists didn’t understand what was in it.) Graphene is made of carbon in a hexagonal structure. It is similar to Lonsdellite in that it is synthetic, but because it is man-made, purity issues are avoided. However, the atoms exist as a single layer.
Graphene is such an amazing material that its inventors won his 2010 Nobel Prize in Physics for their discovery, but its hardness is rarely mentioned among other properties. However, it is theoretically harder than diamond. On the other hand, a natural question arises as to what hardness means for something that is essentially a two-dimensional material.
Claims have also been made for several other synthetic materials made from layers of carbon nanotubes, such as Dyneema and buckypaper. Unfortunately, all of these currently exist in such small quantities that it is not practical to simply apply the Vickers test and call them the most stringent.
Carbide materials are sought after for practical reasons, not just for show. Diamonds are used in many industrial applications that require cutting or polishing hard materials. Harder materials produced in large quantities could have tremendous value if they could accomplish the same thing more quickly. However, so far this requirement has not been met, so diamonds remain the machinist’s best friend.