Ultrahard materials, doing some hypothetical thinking.

[William Gaatjes]

I was traversing around some old articles.

http://arstechnica.com/science/news...arder-than-diamond---but-only-momentarily.ars

Boron Nitride harder than diamond, but only momentarily

Boron nitride makes great nanotubes, too.

Diamond has long been regarded as the practical and theoretical hardness champion and has found uses ranging from industrial abrasives to a girl's best friend. Cubic boron nitride, a synthetic material, comes in at second place. That is, until researchers got their hands on an alternate crystal structure of boron nitride and observed a unique atomic bond rearrangement under high stresses. These vaulted it above diamond for raw hardness, but it didn't wind up keeping the crown long.

Alternate crystal structures sharing the same chemical composition, called allotropes, are common in nature. The different atomic bond orientations and planes of symmetry within the crystal give rise to different physical properties. Researchers had investigated the Wurtzite structure of boron nitride (w-BN, a repeating array of hexagons in 3-dimensional space) in the past, but it had inferior properties to the cubic structure of boron nitride mentioned above.

But using a standard hardness test, which indents the material with a small pyramid and measures the force required, researchers recently observed that the w-BN in fact easily exceeded diamond’s hardness value, 114 Gpa (16.5 million pounds/square inch), versus diamond’s 97 Gpa. There also turned out to be surprises in the mechanisms at work to generate that awesome value.

The geometry of the hardness indenter dictates that the stress it puts on the material will not be limited only to one direction relative to a crystal plane in the material. This results in more complicated stresses that allow the bonds to reorder themselves into a slightly different structure; in w-BN's case, this reordering massively increased the material strength. The hardness value increased a whopping 78 percent in the w-BN after the bond-flipping occurred.

Unfortunately for our friend w-BN, the good times did not last. A challenger to its newly claimed hardness title quickly emerged: hexagonal diamond (called lonsdaelite), an allotrope of diamond, shares a similar structure to w-BN. The same bond-flipping process occurred when hardness indentions were performed, and the all-carbon structure outperformed both diamond and w-BN with a strength of 152 GPa, which is 58 percent higher than regular diamond.

The samples tested were small, and the researchers pointed out that the material synthesis techniques are not well-developed. Lonsdaelite is a rare but naturally occurring mineral—if you consider massive meteorites impacting the Earth’s surface to generate the conditions needed for synthesis "natural." The potential for ultra-hard nanocomposites remains within reach, however.

With Valentine’s Day quickly approaching, why not consider the gift that says "This unattractive grey mineral, like my love for you, crushes puny diamonds”? We’re sure she’ll understand.

And i was thinking, we have the law of energy conservation. Energy is passed on, it never just fades away. IMHO :

Ultra hard materials, are materials that are able to turn mechanical stress into other kinds of stress. For example a rearrangement of the atoms in the atom lattice as seen above. But this will not hold because the atoms prefer a certain equilibrium. That is why the universe is as it is.

If you want to make a really tough material, it must be able to dissipate it's energy not into thermal vibration but into EM radiation or free electrons.
If i understand correctly, a material brakes apart because the atomic bonds cannot with stand (or guide away) the energy of mechanical impact.
When you mechanically impact something, this energy is converted into thermal vibrations. With exception of piezo electric materials. At least one would think that. I have to read up on piezo electric materials... :hmm:


Bit of hypothetical thinking.
Electron way :
Suppose we could make a material that is capable of storing a large amount of electrons. This means we ionize it. When we apply mechanical pressure to it, it must convert the energy of the mechanical stress in a lattice reconfiguration thereby releasing the electrons instead of generating thermal vibrations. The atoms then do not have to jiggle that hard, meaning that the atomic bond does not fall apart. With free electrons, it means that a source of free electrons must be added. Applying power. A material that is ionized but not falling apart at the lattice level. When it experiences impact, it transfers the energy not into thermal vibrations but into releasing electrical energy. This is nothing special, piezo electric materials do this all the time. But these materials are very brittle. And can only withstand so much mechanical impact.

Then i was thinking about this :
I do wonder what would happen if a piezo electric material is mechanically impacted, then it generates free electrons. But this release of free electrons must be compensated for. And i assume that when the electron return back into the piezo materials, the piezo material experiences mechanical deformation. This can cause resonance i would think that would destroy the material if resonating to hard. Our piezo electric crystals in our smart phones, watches and our computers, function this way. By resonating. Just not with to much energy.

Hypothetically thinking :
If you would measure the release of electrons and apply at the right time a counter voltage to compensate for the release of free electrons, it should be possible to dampen the piezo material in such a way that it does not experience the mechanical "recoil". Then it should be able to withstand much harder mechanical impact without falling apart. If this really is the case it is possible to make materials that can withstand impacts far harder then any material.

When it comes to generating EM waves. I have no idea yet.
Braking it down into steps :
The mechanical stress should be converted into thermal vibrations, then into electrons gaining energy and releasing it again as for example infrared. This already happens but only when the atomic bonds fail and the material becomes ionized. Thus electrons must gain energy without the atomic bonds failing. Then really though materials can be made that can withstand impact of any kind.

Practically, i was thinking of armor.

 

[jimhsu]

How wuold it being piezoelectric (or equivalent) make the material stronger? If I still remember my physics, applying a deformation pressure P to a material would result in a certain change to the mechanical structure of the material, independent of how the energy is actually liberated. How would "countering" dissipation energy result in strengthening the material? It would only (might) make sense for thermal energy, where chemical bonds are directly weakened by increased translation energy in the atoms, but I'm not sure for piezoelectric. Even so, I'm not sure it would make that big of a difference vs. a passive material.

 

[Lemon law]

I think, in the real world its rather shallow to think of shocks to any solid crystalline material, be it hard or soft, as only its ability to transfer shocks into mechanical vibration.

Few material very big exists as a perfect single crystal, there are spaces in the crystal lattice that are empty, or filled with a different material, and it does not explain why certain ceramics are notch sensitive and others are not.

 

[jimhsu]

Also note that shock resistance is not the same as elasticity, or hardness, or any number of other parameters that one can measure the "durability" of a material by. In other words, diamond is both brittle AND hard - it resists a large amount of permanent shape change due to application of an external force F (hard), but can fracture with very little noticeable shape deformation (brittle). Toughness refers to the amount of ENERGY, not FORCE that a material can withstand without fracturing - again, a material could withstand initial shocks very well, but be insufficiently capable of dissipating enough energy to withstand continuous pressure. The ideal material for armor would be something that is both hard, tough, and non-brittle.

 

[William Gaatjes]

Both of you have good points. I agree.

Time to dig up some information. I remembered about Sheer waves, Rayleigh waves. S waves and R waves.

Deformation on impact, i thought off. I remembered that a material does not break because of the deformation from the impact itself but because of the reaction force of the material it self. This force makes the material in effect oscillate. If resonance occurs, amplification can take place. But for amplification, energy is needed. If you could guide away or absorb that energy... :hmm:


I did some digging on actively countering forces, and there are already techniques ( not for absorbing impacts but to reduce sway), where they attached piezo crystals on a beam of another material. When the beam starts to sway, they counter the sway movement by energizing the piezo crystals in just the right way to dampen the movement of the beam. This has nothing to do with withstanding impacts of course, but the idea is just.

 

[William Gaatjes]

To come back about the brittleness of diamonds. IIRC that has to do with how the atoms in the lattice guides away force. There are some restrictions.