2.4.3 Composition of Standard Glass
Ward‐Harvey (2009) dates the glass industry back to ancient Egypt – more than 3500 years ago. Yet, even today we are making advances in chemical composition and processing that are enabling the growth of technology, such as flexible glasses for displays.
As mentioned in Section 2.2.2, in oxide glasses, the oxygen atoms form bridges and network formers, such as silicon, boron, phosphorus, or germanium, form strong bonds with them in a randomly arranged network structure. Network modifiers, such as sodium and calcium, generally sit in ionic form within interstitial holes. Such ions and intermediates are added to optimize the properties of glass for specific functions and/or aesthetics. A striking example is the addition of small amounts of specific metal ions to add color to glass: manganese for purple, iron for green, copper for blue, etc. The glass constituents play a role not only in the final product, but also during the production process. Fluxes made from sodium or potassium carbonate can lower the melting point and glass transition temperature of the formers while stabilizers, such as calcium oxide, provide water/humidity resistance to intermediates, such as sodium silicate.
Silicon ions are the most common network formers, but pure silicate glasses can be hard to work due to quartz's high melting temperature (1996 K). Soda‐lime glasses (originally Na2O (sodium oxide) + CaO (lime) but also typically includes MgO (magnesia) and Al2O3 (alumina)) are thus the norm for multiple uses, such as windows and jars. Borosilicate glasses, which can include 5–13% boron trioxide, are less vulnerable to thermal shock and are used in cookware and labware (e.g. Pyrex). Aluminosilicate glasses (5–10% alumina) also have good thermal resistance, but are harder to shape and so are used for purposes such as fiberglass. Phosphate glasses (phosphorus pentoxide) have been found to be compatible with the organic mineral phase of bone and are finding increasing biomedical uses (Rahaman 2014).
Modern glasses can have quite a complex chemical composition and the final structure is made even more complex by the potential bonding arrangements. Figure 2.8 shows the composition of a typical container glass examined by X‐ray fluorescence by Hsich (1980).
Figure 2.8 Composition of a typical soda‐lime container glass given in weight percent.
Source: Modified from Hsich (1980).
The composition of glass can also be modified post initial hardening. For example, the surface of Corning's well‐known Gorilla glass – used in a variety of smart phones and devices – is toughened by a process called ion exchange (Corning n.d.). The material is immersed in a molten alkaline potassium salt causing smaller sodium ions in the glass to be replaced by larger potassium ions from the bath. The larger ions create a surface layer with high residual compressive stress and increase the surface's resistance to damage. However, the full magic to Gorilla glass comes from controlling the stresses at the surface and throughout the center of the glass through its forming processes (Bushwick 2013).
2.4.4 Thermal Tempering
The resistance to weathering, scratching, and breaking of glass plates is significantly improved by a single and simple process called thermal tempering (Lebedev 1912). When a tempered or toughened glass sheet breaks, it shatters into small fragments that are less harmful than the sharp‐edged large pieces of glass into which ordinary annealed glass generally breaks. Thus, the safety aspects of doors and windows are improved immensely. Highly compressed surface layers of toughened sheets of glass also improve the strength and durability. For a given thickness, the shape and size of fragment after fracturing depend on the tensile stress in the middle. The stress distribution is parabolic with the maximum tension in the middle plane and about half of the compressive stress at the surfaces. The change in the engineering properties of tempered glass is due to not only mechanical stresses, but also structural changes within the glass plate. These changes in the structure also affect optical properties, as illustrated in Figure 2.9.
Figure 2.9 Thermally tempered lath with (a) parabolic stress distribution in white light through a Babinet compensator; (b) laser‐beam scattered‐light tomography revealing the stress distribution in the middle plane across width and (c) thickness.
Source: Sinha (1971).
In Section 4.11 of Chapter 4, we briefly describe the tempering of structural and automotive glass and its strength properties. The thermal tempering process involves heating the glass sheet uniformly to temperatures of around 650–700 °C, or measurably higher than the transformation range, T g, and then subjecting it to rapid cooling, usually by jets of air. Since cooling is usually symmetrical about the midplane of the glass plate, this process results in an approximately parabolic stress distribution in the glass plate with compression at the surfaces and tension in the midplane. It is recognized that the midplane tension, called “degree of temper,” is generally represented by the corresponding birefringence (double refraction). The depth‐dependent structural changes that occur in the glass plate during a toughening process induce this birefringence due to changes in the density and refractive indices within the plate, and planes parallel and perpendicular to the surfaces. The process of inducing small, but desirable, changes in the structure of the same type of optical glass by suitable heat treatment can also be applied in the fabrication of optical glass components.
2.4.5 Material Characteristics
As temperature rises, most materials, including glass, expand and density decreases. The coefficient of linear thermal expansion (CLTE), ε, is therefore expected to increase with increase in temperature. Figure 2.10 illustrates this obtained from a slowly heated lath of window (soda‐lime–silica) glass. Since thermal history is important for glass, it is important to mention the heating rate(s) used in the experiment. The rate of increase of specimen temperature, T, was 2 °C/minute in the range 21–100 °C, 0.5 °C/minute in the range 100–400 °C, and about 0.25 °C/minute in the range of temperature above 400 °C. As expected, ε increased with increase in T, but the slope of the ε–T curve increased rapidly after about 500 °C. However, neither linearity below 500 °C nor an abrupt drop above this temperature was noticed. This contrasts with the observations of Lebedev (1926) on the temperature dependence of refractive index, presumably for a similar glass.
Figure 2.10 Temperature dependence of the CLTE for a lapped and polished lath (152.2 mm × 25.8 mm × 5.8 mm) taken from