The new nanocrystalline spinel is made of the same materials, magnesium aluminate (MgAl2O4), but the grain size has been reduced to 28 nm.Apparently the Army has also been experimenting with Spinel, stating that they're getting good results with the material and there are many advantages to it. (pdf warning)
Light weight transparent armor in sizes up to 16” x 40” Weight savings and thickness reductions of 50-60% over current systems Reduce cost per square foot by 20-50% for Spinel ceramic plates for armor Superior resistance to scratching, sand erosion and fracture due to rock strikes will provide major replacement cost avoidance payoffs Operations and Support (O&S) cost savings will be achieved as a result of reduced vehicle maintenance and increased window service life Reduce Spinel manufacturing costs by up to 50%
*On the geologist's, or Mho's hardness scale, Spinel has a hardness of 8, while sapphire is hardness 9 - second only to diamond at 10. The watch crystal on my everyday watch is a clear synthetic sapphire and two years of wearing it hasn't left a single scratch on it. Hardness is a poor indicator of how tough a mineral is; it's only what stone can scratch another. Some relatively soft stones, like jade, are phenomenally tough to break.
And don't very hard materials often tend to be brittle, as happens with some metals? Or am I misconstruing that?
ReplyDeleteIt does happen in metals, but I'd be cautious about comparing metals and nonmetals, especially crystals. Diamonds are insanely hard, due to the covalent carbon-carbon bonds. But they cleave perfectly along bond planes, so they crush easily.
ReplyDeleteUndergraduate mechanical engineers are required to take a class often called strength of materials. It's a complex subject and a tough class. Take glass, which is great in compression - I'm sure you've been in building with architectural glass blocks in the walls. Glass is brittle and doesn't bend, it just snaps. Strong in one direction, but not in another.
I woud guess they chose spinel for reasons related to how tough it is and how hard it might be to break, not its scratch resistance.
From what little I remember about metals, grain structure was significant, but not analogous to crystalline lattice structure, amorphous crystal structure, etc. Too deep for me, but it is interesting how different materials respond in the various ways they are strong and weak, vulnerable to outside forces - physical, chemical, etc.. Like the compressive strength of an egg shell along one axis, but not another.
ReplyDeleteThese new materials are fascinating. Thanks for the post and the response.