Page 228 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 8.5 Glass Ceramics 20

8.4.2 Mechanical Properties
The behavior of glass, like that of most ceramics, generally is regarded as perfectly
elastic and brittle. The modulus of elasticity for commercial glasses ranges from 55
to 90 GPa, and their Poisson’s ratios from 0.16 to 0.28. The hardness of glasses, as
a measure of resistance to scratching, ranges from 5 to 7 on the Mohs scale, equiva-
lent to a range from around 350 to 500 HK. (See Fig. 2.15.)
Glass in bulk form generally has a strength lower than 140 MPa. The relative-
ly low strength of bulk glass is attributed to the presence of small flaws and microc-
racks on its surface, some or all of which may be introduced during normal handling
of the glass by inadvertent abrading. These defects reduce the strength of glass by
two to three orders of magnitude, compared to its ideal (defect-free) strength.
Glasses can be strengthened by thermal or chemical treatments to obtain high
strength and toughness (Section 18.4). The strength of glass theoretically can reach
35 GPa. When molten glass is freshly drawn into fibers (fiberglass), its tensile
strength ranges from 0.2 to 7 GPa, with an average value of about 2 GPa. These
glass fibers are stronger than steel; they are used to reinforce plastics in such appli-
cations as boats, automobile bodies, furniture, and sports equipment (Tables 2.2
and 9.1).
The strength of glass usually is measured by bending it. The surface of the
glass is thoroughly abraded (roughened) to ensure that the test gives a strength level
that is reliable for actual service under adverse conditions. The phenomenon of
static fatigue, observed in ceramics, also is exhibited by glasses. As a guide, if a glass
item must withstand a load for 1000 hours or longer, the maximum stress that can
be applied to it is approximately one-third the maximum stress that the same item
can withstand during the first second of loading.

8.4.3 Physical Properties
Glasses are characterized by low thermal conductivity and high electrical resistivity
and dielectric strength. Their thermal expansion coefficients are lower than those for
metals and plastics and may even approach zero. For example, titanium silicate glass
(a clear, synthetic high-silica glass) has a near-zero coefficient of thermal expansion.
Fused silica (a clear, synthetic amorphous silicon dioxide of very high purity) also
has a near-zero coefficient of expansion. The optical properties of glasses (such as
reflection, absorption, transmission, and refraction) can be modified by varying
their composition and treatment. Glasses generally are resistant to chemical attack
and are ranked by their resistance to corrosion by acids, alkalis, or water.

8.5 Glass Ceramics

Although glasses are amorphous, glass ceramics (such as Pyroceram, a trade name)
have a high crystalline component to their microstructure. Glass ceramics contain
large proportions of several oxides; thus, their properties are a combination of those
for glass and those for ceramics. Most glass ceramics are stronger than glass. These
products first are shaped and then heat treated, with devitrification (recrystalliza-
tion) of the glass occurs. Unlike most glasses, which are clear, glass ceramics are
generally white or gray in color.
The hardness of glass ceramics ranges approximately from 520 to 650 HK.
Because glass ceramics have a near-zero coefficient of thermal expansion, they have
high thermal-shock resistance. They are strong, because of the absence of the poros-
ity usually found in conventional ceramics. The properties of glass ceramics can be