Tempered Glass
Toughened glass or tempered glass is a type of safety glass that has increased
strength and will usually shatter in small, square pieces when broken. It is
used when strength, thermal resistance and safety are important considerations.
At home you are likely to find toughened glass in shower and sliding glass patio
doors. In commercial structures it is used in unframed assemblies such as
frameless doors, structurally loaded applications and any glass where these is a
danger of human impact.
Using toughened glass can pose a security risk in some situations due to the
tendency the glass has to shatter utterly upon hard impact.
Toughened glass is typically four to six times the strength of annealed glass.
However, this strength comes with a penalty. Due to the balanced stresses in the
glass, damage to the glass will eventually result in the glass shattering into
thumbnail sized pieces. Although toughened glass is most susceptible to breakage
via edge damage, breakage can also occur from impacts in the centre of the glass
pane.
Shattering may not happen when the damage originally occurs and can be triggered
by a minor stress like heat or small impact that would not normally affect the
toughened glass. If any toughened glass shows any damage it must be replaced.
Toughened glass must be cut to size or pressed to shape before toughening and
cannot be re-worked once toughened. Polishing the edges or drilling holes in the
glass is carried out before the toughening process starts. Also, ironically, the
toughened glass surface is not as hard as annealed glass and is slightly more
susceptible to scratching.
Toughened glass is made from annealed glass via a thermal tempering process. The
glass is placed onto a roller table, taking it through a furnace which heats it
to above its annealing point of 600 °C. The glass is then rapidly cooled with
forced draughts of air. This rapidly cools the glass surface below its annealing
point, causing it to harden and contract, while the inner portion of the glass
remains free to flow for a short time. The final contraction of the inner layer
induces compressive stresses in the surface of the glass balanced by tensile
stresses in the body of the glass. This compressive stress on the surface of the
glass is typically as high as 50 MPa.
It is this compressive stress that gives the toughened glass an increased
strength. This is because any surface flaws tend to be pressed closed by the
retained compressive forces, while the core layer remains relatively free of the
defects which could cause a crack to begin.
The pattern of cooling during the process can be revealed by observing the glass
with polarized light, which shows the strain pattern in the glass.
Though the underlying mechanism was not known at the time, the effects of
"tempering" glass have been known for centuries. In the 1640s, Prince Rupert of
Bavaria (1619–1682), who was grandson of James I of England, and nephew of
Charles I, brought the discovery of what are now known as "Prince Rupert's
Drops" to the attention of the King. These are remarkable teardrop shaped bits
of glass which are produced by allowing a molten drop of glass to fall into a
bucket of water, thereby rapidly cooling it. These were often used by the King
as a practical joke.
Chemically strengthened glass
Chemically strengthened glass is a type of glass that has increased strength.
When broken it still shatters in long pointed splinters similar to float
(annealed) glass. For this reason, it is not considered a safety glass and must
be laminated if a safety glass is required.
Chemically strengthened glass is typically six to eight times the strength of
annealed glass.
The glass is chemically strengthened by submersing the glass in a bath
containing a potassium salt (typically potassium nitrate) at 450 °C. This causes
sodium ions in the glass surface to be replaced by potassium ions from the bath
solution.
These potassium ions are larger than the sodium ions and therefore wedge into
the gaps left by the smaller sodium ions when they migrate to the potassium
nitrate solution. This replacement of ions causes the surface of the glass to be
in a state of compression and the core in compensating tension. The surface
compression of chemically strengthened glass may reach up to 690 MPa.
There also exists a more advanced two-stage process for making chemically
strengthened glass, in which the glass article is first immersed in a sodium
nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves
more sodium ions on the glass for the immersion in potassium nitrate to replace
with potassium ions. In this way, the use of a sodium nitrate bath increases the
potential for surface compression in the finished article.
Chemical strengthening results in a strengthening similar to toughened glass,
however the process does not use extreme variations of temperature and therefore
chemically strengthened glass has little or no bow or warp, optical distortion
or strain pattern. This differs from toughened glass, in which slender pieces
can often be significantly bowed.
Also unlike toughened glass, chemically strengthened glass may be cut after
strengthening, but loses its added strength within the region of approximately
20 mm of the cut. Similarly, when the surface of chemically strengthened glass
is deeply scratched, this area loses its additional strength.
Chemically strengthened glass was used on some fighter aircraft canopies.
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