Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 63

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.