A ceramic knife might be sharper and harder than an steel knife, but if you drop it, it will shatter. Steel is “tough” - and Gorilla Glass is tougher than Sapphire. Here’s a good video showing the difference.
Until actual engineers or physicists show up and tell me I’m confused…
Stronger can refer to material properties such as yield strength and tensile strength, as well as toughness, which requires both high yield strength and futility, per the wiki. Toughness - Wikipedia Yield strength is how much stress a material can withstand before it deforms irreversibly; tensile strength is how much stress the material can withstand before failure. Mohs strength hardness has a decent positive correlation with tensile strength. For high toughness, it’s also necessary for the material to elastically deform. Or plastically, if we’re defining failure as rupturing or cracking the material, but not merely bending it.
Ceramics like alumina/ruby/sapphire, aren’t very ductile. Neither is glass usually, but it’s better than those ceramics. So, drop the GG covered phone, the GG deforms elastically a minute fraction, dissipates energy and increases the time allowed for deceleration, which lowers the required acceleration to stop the falling body. Compare with sapphire, which probably has a much higher yield strength, won’t deform under the impact of the grain of sand between your texting fingers and the face of the phone, but also won’t deform at all under shock, until failure. No deformation, no increase in deceleration time, much larger acceleration required to stop the falling body, equals a greater force on the face plate. Often this greater force is enough to break the plate, despite the sapphire’s higher yield strength.
For the phone plate’s ability to resist impact and shock, higher toughness is what’s needed. EDIT: and I see Beowulf mentioned toughness already.
My understanding is that “alumina” is synonymous with Al[sub]2[/sub]O[sub]3[/sub], a.k.a. “aluminum oxide”. It is just the name of a chemical compound and says nothing about the crystal structure or lack thereof. For example, you may use the term “amorphous alumina” to refer to non-crystalline aluminum oxide.
Thus, “alumina” is a general term that applies to all the other names that Chronos mentions, as well as, for example, the white smoke made by the space shuttle’s solid rocket boosters.
“Well, it does have a high yield strength, but just forget about it, guys, it’ll never work.”
I just love autocorrect. That is a pretty funny typo though.
FWIW, The wiki used ductility, not futility.
The most common occurence of amorphous aluminium oxide most people would encounter is as an anodizing layer (the often-brightly-coloured covering on many aluminium parts) - anodizing layers can be converted to tougher crystalline forms but most of it it amorphous.
It’s fun to tell people that aluminum reacts energetically on contact with water, and wait a few minutes for them to ask “Wait, what about pop cans?”.
Since we are on a tangent—
aluminum and mercury
Do you explain about an oxide layer that forms on any bare aluminum surface, or do you explain that every soda can has a plastic coating to protect it from the acidity?
Well, you start with a rusty aluminum can …
On LNG (Liquefied Natural Gas) plants, the cold box (cryogenic heat exchanger) is made out of Aluminum because steel gets brittle at those temperatures. There is some mercury (ppm to ppb levels depending on source) in Natural gas that we remove in activated Carbon/ catalyst beds before it comes in contact with Aluminum.
There is a thread for that
https://boards.straightdope.com/sdmb/showthread.php?t=857932
Oh, and I think they ended up going with a different design, but at one point, the LIGO gravitational wave detectors were to be upgraded with mirrors made of flawless sapphire crystals, 40 kg each.