Corundum: Wonder Material

[SpyderEdgeForever]

http://www.minerals.net/mineral/corundum.aspx" onclick="window.open(this.href);return false;

http://geology.com/minerals/corundum.shtml" onclick="window.open(this.href);return false;

It really is very cool and very useful. It is used in the forms of alumina, sapphire, and rubies, and for everything from jewelry and ornaments, to knife sharpeners, abrasives, cutting materials, polishing and sanding, and even (much more rarely), for actual knife blades, both tactical, sporting, and scientific-medical.

It is almost as hard as diamond on the Mohs Scale, 9.5 with diamond being 10, and, I would like to see more nano-corundum/nano-sapphire based composites, and knives and armor. It is also used for armor plates.

[Cliff Stamp]

Alumina is the most common abrasive and ceramic material/base for cutting tools, I am not sure it could be used more considering its very dominate position. It is being eroded in the ceramic cutting department due to various ceramic composites which have very high resistance to fracture because of the ability to use phase transitions to absorb stress.

[kiwisailor]

Hi Cliff, never heard of "Phase Transitions".
Care to give a brief explanation?
Would be appreciated, as a retired Jeweller, corundum was a mineral that popped up in lots of forms.
Regards, and thank you for all your great articles..

[Cliff Stamp]

Phase refers to a way the structure is aligned. For example steel in general is a mix of :

-martensite
-austenite
-cementite + alloy carbides

There are other phases (bainite/pearlite) but they are usually minor in hardened steel.

When a knife is subjected to impact then the austenite can absorb the energy and transform into martensite. This reduces the energy and can prevent gross fracture.

In the same way additions to solid ceramic can undergo phase changes, absorbing energy, and reducing crack propagation.

[kiwisailor]

Whoosh, that went way over my head Cliff..

Thanks for the heads up. Fascinating.

There I was back in my little workshop thinking I was hot $h!t realigning the molecules in some Gold/Silver after rolling/beating it, therefore work hardening it.
Always got a thrill getting the gas out and annealing it so I could work it again without cracking/tearing it.
Something quite magical melting down some scrap silver, pouring it into ingot then pulling it through a draw plate to make wire.

Not even close to the same league as "Phase Transitions" of Steel.

Thanks again for sharing your in depth knowledge. Humbling..
K