Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 137
Chemical toughening
If the hot glass article is immersed in a molten salt such as potassium nitrate,
some of the Na + ions in the surface of the glass will be exchanged with K +
ions from the salt. The K + ions are about 35% larger than Na + ions and the
time of treatment is chosen so that the ions diffuse to a depth of about 0.1
mm into the surface of the glass, which therefore attempts to occupy a
greater volume. This is resisted by the material beneath the K + -enriched
surface which therefore exerts a compressive stress on the surface layers.
Maximum stresses of –400 MPa can be achieved by this method, although
the depth of the compressed layer is considerably less than with thermal
toughening. Chemical toughening tends to be more costly than thermal
toughening, but it can be used on thinner sections.
After either treatment, before a surface crack can be propagated the tensile
stresses applied have to overcome these stresses of opposite sign, which
result in a four- to ten-fold increase in strength. In other words, it is found
that a sheet of toughened glass may be bent further before it will break and,
furthermore, the glass breaks into very small fragments, which are much less
dangerous that the sharp shards produced when annealed glass is broken.
The toughened glass produces more cracks and therefore smaller fragments
because it contains more stored elastic energy to propagate the cracks than
in the case of annealed glass.
4.1.4 Self-cleaning glass
The glass industry has addressed the problem which affects almost every
building, namely to maintain the optical clarity and external aesthetic appeal
of glass without constant regular cleaning. On modern buildings, the use of
glass in atria and overhead glazing can sometimes make maintenance more
difficult. When glass is exposed to the environment, dirt builds up on the
surface and reduces its visual appeal: droplets and rivulets form on the
surface when it rains, resulting in a loss of clarity.
In recent years, the industry has responded by introducing glass panels
with a range of extremely thin coatings produced by chemical vapour deposition
(CVD), which are designed to reduce the amount of maintenance that glass
requires without impairing its optical properties in any way. These coatings
are based on titanium dioxide (titania) and have a dual-action cleaning process.
Firstly, they function as semiconductors by absorbing the sub-320 nm light
to promote oxidation and reduction chemistry of organic materials
(photocatalysis), which has the effect of loosening any dirt particles adhering
to the glass surface. Secondly, titania coatings give contact angle measurements
for water below 20° after exposure to natural sunlight and, thus, a sheet of
water forms during rainfall (rather than droplets) which tends to wash the
dirt away.