Thermal tempering of glass

Because of the transparency and chemical inertness of inorganic glasses, their uses in everyday life are ubiquitous. However, for many applications, especially where safety is concerned, as manufactured, glass is deemed to be too weak and brittle. Fortunately, glass can be significantly strengthened by a process referred to as thermal tempering, which introduces a state of compressive residual stresses on the surface (see Sec. 11.3.3). 

During the second quenching step, the entire glass sample is cooled to room temperature. Given that on average the glass interior will have cooled at a slower rate than its exterior, its final specific volume will be smaller than that of the exterior. The situation is shown in Fig. 13.5c and leads directly to the desired final state of stress (Fig. n.S/ ) in which the external surfaces are in compression and the interior is in tension. 

This effect was discussed briefly in Sec. 9.4.1 and illustrated in Fig. 9.8a. Simply put. the more time the atoms have to arrange themselves during the cooling process (slow cooling rate), the denser the glass that results. 

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H. Ohkubo and S. Nishio, Mist Cooling for Thermal Tempering of Glass, in P. J. Marto and I. Tanasawa (eds.), Proceedings of the 1987 ASME-JSME Thermal Engineering Joint Conference, 5, pp. 71-78, ASME, New York, 1987. 

R. Gardon, Thermal Tempering of Glass, Glass Science and Technology V. Academic Press, New York. (1980)... 

Incorporation of an outer layer having a lower coefficient of thermal expansion, as in glazing or tempering of glass. These will be discussed in greater detail in Chap. 13. 

Residual stresses are generally deleterious to the mechanical properties and should be avoided. This is especially true if a part is to be subjected to thermal cycling. In some situations, however, residual stresses can be used to advantage. A case in point is the transformation toughening of zirconia discussed in Chap. 11, and another excellent example is the tempering of glass discussed in the next section. 

Qualitatively explain how the following parameters would affect the final value of the residual stresses in a tempered glass pane (a) thickness of glass, (b) thermal conductivity of glass, (c) quench temperature, (d) quench rate. 

Surface defects are responsible for the limited glass resistance and its large scatter [6]. The creation of a compressive stress layer in the surface of the material can limit the formation or propagation of flaws and improve the mechanical properties thermal and chemical tempering of glass are two main methods for producing a compressive stress in the glass surface. The thermal method is widely used to make windows and other transparent flat structural components [2, 7],... 

Polycarbonate is available monolithically (in a single sheet) in thicknesses up to 1/2 inch. In this range of thickness, polycarbonate is twice as expensive as thermally tempered glass. In thicknesses over 1/2 inch where lamination is required, it is roughly three times as expensive as an equivalent thermally tempered laminated lite. 

Meyers, G.E. "Design Criteria and Acceptance Test Specification for Blast Resistant Thermally Tempered Glass Glazing," Department of Defense Explosive Safety Seminar, Anaheim, CA, Aug 1986. 

A third technique to produce tempered ware uses two glasses (or ceramics) formed together (laminated), each with different thermal coefficient of expansion. This is used to make Correll dishes by Coming. For this tableware, a pyrocer-amic material with one type of thermal coefficient of expansion is covered with another pyroceramic with a greater thermal coefficient of expansion and is then baked until the outer layer melts uniformly. Materials with greater thermal coefficients of expansion will expand more when heated and will contract more when cooled. The greater contraction (once cooled) of the outside material causes compression. 

Preparation of Glasses for Special Applications 17.3.2.4, Strong and Shatter-Resistant Glasses 17.3.2.4.1. Thermal Tempering. 

Thermal treatment of photochromic glasses is generally possible, but the tempering process must be very closely controlled. The thermal stability depends on the type of the glass. If photochromic glasses are thermally overheated they may become distorted and have a degraded photochromic performance. It is therefore recommended to keep the temperature below about 200°C and to use only short durations of tempering. 

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