Page 227 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

206 Chapter 8 Ceramics, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications

that of amorphous alloys (see metallic glasses, Section 6.14) and amorphous poly-
mers (Section 7.2).
Glass beads first were produced in about ZOO() B.C., and the art of glassblow-
ing started in about 200 B.C. Silica was used for all glass products until the late
1600s. Rapid developments in glasses began in the early 1900s. Currently, there
are some 750 different types of commercially available glasses, with applications
ranging from window glass to glass for containers, cookware, lighting, and view
screens for television sets and mobile phones, and to glasses with special mechani-
cal, electrical, high-temperature, antichemical, corrosion, and optical characteris-
tics. Special glasses are used in fiber optics (for communication by light with little
loss in signal power) and in glass fibers with very high strength (for use in rein-
forced plastics).
All glasses contain at least 50% silica, which is known as a glass former.
The composition and properties of glasses can be modified greatly by the addition
of oxides of aluminum, sodium, calcium, barium, boron, magnesium, titanium,
lithium, lead, and potassium. Depending on their function, these oxides are known as
intermediates (or modifiers).

8.4.l Types of Glasses

Almost all commercial glasses are categorized by type (Table 8.3):
° Soda-lime glass (the most common type)
° Lead-alkali glass
° Borosilicate glass
° Aluminosilicate glass
° 96 %-silica glass
° Fused silica glass.

Glasses also are classified as colored, opaque (white and translucent), multiform
(a variety of shapes), optical, photochromatic (darkens when exposed to light, as in
sunglasses), photosensitive (changing from clear to opaque), fibrous (drawn into
long fibers, as in fiberglass), and foam or cellular (containing bubbles, thus a good
thermal insulator). Glasses also can be referred to as hard or soft, usually in the
sense of a thermal rather than mechanical property. (See also hardness of glasses,
Section 8.4.2.) Thus, a soft glass softens at a lower temperature than does a hard
glass. Soda-lime and lead-alkali glasses are considered soft, the rest hard.

TABLE 8.3
Properties of Various Glasses

Soda-lime Lead-alkali Borosilicate
glass glass glass 96% silica Fused silica
Density High Highest Medium Low Lowest
Strength Low Low Moderate High Highest
Resistance to thermal shock Low Low Good Better Best
Electrical resistivity Moderate Best Good Good Good
Hot workability Good Best Fair Poor Poorest
Heat treatability Good Good Poor None None
Chemical resistance Poor Fair Good Better Best
Impact-abrasion resistance Fair Poor Good Good Best
Ultraviolet-light transmission Poor Poor Fair Good Good
Relative cost Lowest Low Medium High Highest