The hardest material that isn't brittle?

[Researcher X]

How hard can something be without being easily breakable?

 

[Mapes]

"Brittle" means hard enough that the material fails not by plastically deforming but by crack propagation. Harder materials tend to be more brittle. So you have to define what you mean by "easily breakable." Perhaps you're looking to maximize http://en.wikipedia.org/wiki/Fracture_toughness" ?

 

[Researcher X]

I mean brittle in the scientific sense, but also that the material doesn't easily fracture, which is the more colloquial, non-scientific sense of "brittle".

So, I'm looking for the hardest (most difficult to pierce/cut) material that also is either not brittle at all, or if it is, requires loads of force to flex enough for the brittle failure to occur.

Fracture toughness is another good quality to add to the balance. What I'm trying to get at here is: if you wanted to make something as unbreakable as possible what would you make it out of with today's materials? Tungsten is one of the hardest metals there is, yet breaks easily, so that's out.

Essentially, what's the best material to make something from if you are going on being hardest to damage in any fashion. It would have to balance difficulty to cut or pierce, with difficulty in breaking, and difficulty in bending. So it would have to be hard, tough, and stiff. The hardest materials tend not to be tough (like Tungsten), so it would seem to have to balance somewhere in the middle. Possibly a high grade of steel? What about something utilizing carbon nanotubes (though I hear they are weak in compression).

 

[Wikisteff]

Well, silicon carbide is decently tough, with a fracture toughness of about 4 MPa/m^.5 (http://en.wikipedia.org/wiki/Fracture_toughness) ... that's about 3 to 20 times as tough as concrete. SiC (moissanite) is also one of the harder substances known: at a Mohs hardness of 9.5, it'll scratch tungsten carbide or sapphire. Perfect diamond is probably very tough, but natural defects and radiation-induced defects are likely to make diamond pretty brittle in any event. A real nanotech record-holder would be some of the ultratough materials using nanotubes and a plastic or epoxy that can bend and deform in response to shocks and loads... but it's hard to quantify what the limits would be without experiments. I once found a quote in nature of such a material that could be bent and rebent a million times that was three times as strong as steel in extension. That's probably in the neighbourhood, but it might suck in compressive strength.

If you find something better, let me know! :)

 

[Mapes]

Engineering ceramics are still likely to fracture before they yield, however. For a material that can be made in bulk form today, I don't think you're going to beat steel (~1 GPa strength, 100 MPa m^1/2 fracture toughness).

 

[Wikisteff]

Thoroughly agree with Mapes; it's hard to beat steel for the title, particularly in terms of the size of the process zone (think: how big a rip can I make before the whole thing tears apart). I'd be very interested in a refinement of Mapes' fracture toughness-strength chart that shows the various processed forms of alloys. For instance, where's iron carbide on the map? Where's tungsten carbide? As I understand it, work-hardening and other process techniques push you down and to the right on this chart; that may matter as you look for the "best" choice, since new places on the graph may present themselves - at least in the space of alloys.

In terms of absolute possible performance, cleverly fashioned weaves of diamond manufactured along atomic plane-parallel edges might be the best that you can do. Can't even hope to manufacture it today, but it would be difficult to propagate shears across nonbonded surfaces at the molecular level, especially if you built your nanostructure with some gaps to relieve the stress. In tension, woven graphene or single-walled nanotubes might behave similarly.

 

[Researcher X]

Little bit of a bump, but I was reading about kinetic energy impactors on Wikipedia:

"The armor-piercing concept calls for more penetration capability than the target's armour thickness. Generally, the penetration capability of an armor piercing round is proportional to the projectile's kinetic energy. Thus an efficient means of achieving increased penetrating power is increased velocity for the projectile. However, projectile impact against armour at higher velocity causes greater levels of shock. Materials have characteristic maximum levels of shock capacity, beyond which they may shatter on impact. At relatively high impact velocities, steel is no longer an adequate material for armor piercing rounds due to shatter. Tungsten and tungsten alloys are suitable for use in even higher velocity armour piercing rounds due to their very high shock tolerance and shatter resistance."

I always thought Tungsten had very low shatter resistance, but this claims Tungsten as being superior for surviving high speed impacts than equivalent high quality steel.

 

[DaveC426913]

Engineering ceramics are still likely to fracture before they yield, however. For a material that can be made in bulk form today, I don't think you're going to beat steel (~1 GPa strength, 100 MPa m^1/2 fracture toughness).

What a cool diagram.

Where do carbon nanotubes fall?