"Lew Hartswick" wrote in message
...
DoN. Nichols wrote:

On 2008-01-17, Joe AutoDrill wrote:

Some metal content on the following site:

http://encyclopedia.thefreedictionar...neral+hardness

I was surprised to see hardened steel is rated as being harder than
glass. I presume this is hardened steel that has almost no structural use
whatsoever due to being so brittle.


Hmm ... did you ever take a chemistry lab course in either high
school or college?

Do you remember how you cut glass tubing to length?

For that matter -- how about the wheel type glass cutters sold
in the hardware store?

Enjoy,
DoN.

Good one Don. That aught to but the argument to rest. :-)
...lew...

Not really. Glass is very weak in brittle fracture. Steel is not. That's why
steel wheels scratch glass, not because the wheels are harder. In fact, the
wheels in glass cutters don't scratch glass, either: they compress it
locally until they exceed the glass's ability to resist brittle fracture in
compression.

The hardness of the wheels determines their service life but not the
relative hardness of the material.

You really can't compare the "hardness" of steel and glass directly. In
metals, "hardness" refers to the material's ability to resist plastic
deformation in compression. In brittle materials, such as glass, the primary
use of the term is for its ability to resist scratching. But you can get
into deep engineering discussions of microhardness and nanohardness of
brittle materials, as ceramics engineers sometimes do. They don't really
apply to the way we use the term.

--
Ed Huntress