Processing and properties

10.4.2 Processing and properties

The characteristic network structure imparts isotropic properties to the glass which change gradually with change in temperature. They can change shape without fracture, which affords them a number

Non-metallics I – Ceramics, glass, glass-ceramics 531

Tension (⫹)

Compression (⫺) 400 200 0 200 400

(a) Thermally toughened glass plate 400 200 0 200 400 600 800

(b) Chemically toughened glass plate

Figure 10.15 Distribution of residual stress in toughened glass plate (stress values in MN m −2 ).

of different forming methods, i.e. blowing, drawing, pressing, fiber forming and rolling, to produce a variety of products, e.g. tubes, sheets, filaments, coatings, etc. Glass blowing by ‘individuals’ is still fascinating to watch but most blowing is done by machine to produce bottles, jars and light bulbs, usually by pressing a hot glass ‘blob’ into a mold before blowing with compressed air to the finished shape. Continuous glass fibers are formed by drawing the molten glass through orifices in the base of a heating chamber under closely controlled temperature–viscosity conditions. After spray cooling and chemical sizing to improve the abrasion resistance, the filaments can be made into strands and yarns.

Glass sheets can be made by a novel float glass process. In this process, the molten glass is poured onto a bath of molten tin when the glass spreads uniformly across the surface to a controlled thickness as it cools from 10 to 700 ◦

C to increase the viscosity and remove imperfections. The glass produced is perfectly parallel-sided and clear for plate glass without additional polishing, etc. Final treatment involves annealing to remove thermal stresses, followed by slow cooling to room temperature.

C. Subsequently the glass is reheated to 850 ◦

While glass is weak in tension it is quite strong in compression. This property is used in the manufacture of toughened glass by producing compressive stresses in the surface layers. This is achieved by thermal tempering when the glass is heated above T f but below the softening temperature and cooled in a jet of air. The surface cools more rapidly than the interior, becoming rigid while the inside is still ‘plastic’. On continued cooling the contraction of the inner region is restrained by the rigid outer layers. The stress distribution over a cross-section of a toughened glass plate is shown in Figure 10.15a. Surface stresses of 200 MN m −2 may be produced by this method, which has to be overcome to cause fracture of a toughened glass sheet with an additional tensile stress to initiate a surface crack. Toughening can also be produced by altering the surface composition by high-temperature diffusion to reduce its coefficient of thermal expansion. Lower temperature treatment involves the replacement of Na by large ions, such as K, by immersing the glass in a molten salt. On cooling from a relatively low temperature the core is rigid and the presence of large ions in the surface layers leads to compressive stresses, as shown in Figure 10.15b. The glass bottle manufacturing industry relies on this method of toughening. Sheet glass in a sandwich composite with polyvinyl butyrate (PVB) plastic interlayer is also used as a toughened material. On impact the PVB which is bonded to the glass by autoclaving at pressure at about 140 ◦

C deforms to absorb energy and tends to prevent the shattered glass from causing lacerations. Laminated glass can also withstand extreme changes of conditions as experienced in aircraft windows.

532 Physical Metallurgy and Advanced Materials Table 10.5 Composition and applications of some commercial glasses.

Glass type SiO 2 Na 2 O CaO Al 2 O 3 B 2 O 3 MgO PbO Vitreous silica

Low coefficient of expansion; optical components, fiber- optics, lamps

Soda lime

Convenient, easy working range 700–1000 ◦ C

Borosilicate

Chemically durable, (Pyrex)

low coefficient of expansion, tolerant to thermal shock; used for laboratory glassware/chemical glassware

Lead glass

34 1 65 X-ray protection glass Lead glass

23 Color TV protection glass

Lead/crystal glass 20–30 Relatively constant viscosity over large working temperature range; used for decorative glassware

Lead/optical glass 54 1 37 8% K 2 O added as modifier; Pb increases refractive index and dispersion of

SiO 2 -based glass Aluminosilicate

High softening glass

temperature (>900 ◦ C) used for combustion tubes, cookware

Glass products, particularly sheets, must be free from flaws not only to withstand servicing stresses, e.g. toughening, but also quality control. The removal of heterogeneities during glass manufacture is termed fining. Bubbles are removed by flotation, which depends on Stokes’s law, v

= constant × gρ r 2 /η , where ρ is the glass density, g the gravitational constant and r the radius of the bubble. The larger bubbles float out of the glass quicker than small ones. A number of bubble

enlargement methods are available to aid the bubble removal. The small bubbles often disappear from molten glass by dissolution when the internal pressure increases according to p = 2γ/r. Controlling the composition is important for fining. Alumina, for example, is frequently added (1–2%) to silica- based glasses to improve the resistance to water and moisture. Unfortunately, it increases the melt viscosity and makes bubble removal more difficult.

Table 10.5 gives the approximate composition, characteristics and application of some common glasses.

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