 |
 |
Glass and Malleability
Name: Aaron
Status: student
Grade: 9-12
Location: MN
Country: USA
Date: April 2009
Question:
Does glass have any malleability?
Replies:
The general concept you are talking about is the ability of a material
to reshape in response to a stress. Stress is a force applied to an
object. In the case of malleability, the stress is compressive (think
'pushing'), often rolling or pounding. A similar concept is ductility,
where the stress is tensile (think 'pulling'), such as pulling a wire.
If I can restate your question, you are asking if glass can deform
based on a compressive load. The answer is that glass has very, very
little malleability or ductility. Rather than deforming/bending, the
glass is more likely to break or shatter.
Some materials are highly malleable and ductile - such as Play-doh and
gold. Other materials are more one than the other (lead, for
example is more malleable than ductile). Materials like ceramics are
neither very ductile nor malleable.
Your question says 'any' malleability. If you look at the atomic
level, then all materials can deform slightly based on loads. Crystal
matrices can deform/bend slightly, and defects in the crystal can
cause both increased and decreased deformation (a defect could make
something more brittle, or it could make it more malleable). Also,
temperature plays a big role in how attracted atoms are to each other,
which affects malleability. You can take a material like Play-doh
(malleable at room temperature) and freeze it, which would make it
more likely to shatter at a given load, or take lead and heat it,
and find its malleability is much lower at elevated temperature. So
the answer all depends on the conditions you choose.
So strictly speaking, yes, all materials have some slight malleability
or ductility. In practical terms, though, glass is not malleable.
Generally speaking, metals and some polymers are malleable, while
ceramics are not.
This question was already partially answered earlier by
Ask-A-Scientist as well -- please see:
http://www.newton.dep.anl.gov/askasci/mats05/mats05072.htm
Hope this helps,
Burr Zimmerman
Definition: A substance is "malleable" if it deforms under a stress and does NOT
return to its original shape when the stress is removed. Soft metals, like gold,
are malleable. A non-crystaline substance has four zones of deformation properties
as a function of increasing temperature: the "glassy" zone, the "leathery" zone, the
"rubbery" zone, and the "flow" zone. If you graph the "strain" that is the movement
or distortion on the "Y" axis and the "stress" that is the applied force along the
"X" axis. The general shape of the curve will be high, but constant, values of
strain with increasing stress. This is the "glassy" region, which tends to be
relatively hard and brittle (glass shatters). There is a relatively narrow region of
stress where the strain starts to decrease. This is the "leathery" region of
stress/strain. The substance in this zone is tough. It can absorb the applied stress,
but tends not to distort very much. Leather is a good example of this region.
Next is the rubbery zone in which the substance will deform when a stress is applied,
but tends to return to its original shape when the stress is removed. At still higher
temperature is the "flow" zone. In this zone the substance is irreversibly distorted
when the stress is applied.
The temperature range of the various zones and the numerical values of both strain and
stress depends upon the material, the stress, the strain, and also the rate at which
the stress is applied.
To answer your question directly, the glassy zone "tends" to be hard and brittle, but
just how hard and how brittle, depends upon the substance and various factors
mentioned in the previous sentence.
Vince Calder
Hi Aaron,
Glass, like many (but not all) amorphous substances, is sometimes
referred to as a supercooled liquid, and the claim has been made that
centuries-old windows have slowly flowed slightly under the force of
gravity, so the bottoms of the windows are slightly thicker. Careful
analysis has shown this to be false. There remains some argument
whether in fact glass is a solid or a supercooled liquid, but normally
it is considered a rigid unmalleable solid (at room temperature of
course), simply because under any normal exposure to stress, it cannot
be made to flow or otherwise change permanent shape in any way.
Many other substances (such as some plastics) are entirely amorphous
and relatively rigid below their Glass Transition Temperature, but
still can be considered malleable to a small degree, since application
of stress can cause (over time) a permanent deflection.
Regards,
Bob Wilson
Click here to return to the Material Science Archives
| |
Update: June 2012
|
|
