Biomedical Engineering Reference
In-Depth Information
properties of glass are probably its weakest link, although as we will see
later there are ways of making it strong.
The theoretical strength of most commercial glasses is very high owing
to the fact that they typically exhibit a mixture of strong covalent and
ionic bonding. However, most glasses exhibit brittle behavior in which
fracture occurs via the propagation of cracks that typically originate
from a flawed surface. Therefore, glasses seldom exhibit anything close
to their theoretical strength. The reason is that the sharp crack tips
concentrate the stress applied to the glass, raising the stress locally
way above the stress required to break bonds. Glasses without surface
flaws are very strong. However, it takes very little to cause a surface
flaw and weakening of the glass. A crack tip could result just from
touching the surface. In addition, since the mechanical properties are so
flaw-dependent, the ''strength'' of a glass product is statistically varying,
and thus the engineering strength is determined by the weakest samples
rather than by an average. Strengthening of glass involves the inhibition
of crack growth either by blocking the path of the crack by an interface
or by forcing the crack to be under compression rather than in tension.
The latter can be achieved by using the property of glass mentioned
earlier, namely that the structure of the glass is determined by its cooling
rate. Tempered glass is purposely rapidly cooled on the outside to freeze-
in a high-volume structure. As the glass on the inside cools slowly, it
will try to relax to a lower-volume structure. However, the glass on
the surface is already solid and is therefore pulled into compression as
the inside glass tries to contract. The compressive surface layer serves
to close the surface flaws, preventing them from propagating. It is not
impossible to break tempered glass, but the compressive external layer
has to be breached first, usually by a sharp object. Once through to
the inner layer, which is in tension, immediate widespread catastrophic
failure occurs. Tempered glass, because of the temperature gradients
required, typically needs to be thick. Strong thin glass can be achieved
by putting the surface into compression chemically. If solid commercial
glass containing sodium ions is put in a bath of molten potassium salt,
the sodium ions will come out into the salt and the potassium ions will
diffuse into the glass. If this is done at a temperature above which the
glass will not relax, then the large potassium ions forcing their way
into the smaller sodium sites puts the surface into compression, similar
to tempering. The rate at which this occurs and the ability to have a
glass that does not relax or devitrify at the temperature of a molten salt
bath depends upon the composition. Nevertheless, strengths have been
improved almost a factor of 10 using these methods.
