Thermally Tempered Glass Characteristics

Thermally tempered glass, also known as toughened glass, is a type of safety glass processed by controlled thermal treatments to increase its strength compared with normal glass. The tempering process creates balanced internal stresses which give the glass increased resistance to mechanical stress and, to a lesser extent, thermal stress.

Tempering Process Overview

The process involves heating annealed glass to a high temperature (typically around 620-680 °C, depending on the glass composition) followed by rapid cooling. This rapid cooling of the surface creates compressive stress on the surface while the core remains in tension. This stress profile gives the glass its improved mechanical properties and altered fracture characteristics.

Stress Distribution and Mechanical Strength

The characteristic stress profile-compressive stress on the surface and tensile stress in the core- significantly enhances the glass's bending strength and resistance to impact. However, the internal tensile stress makes tempered glass prone to shattering into small, relatively harmless fragments if the surface compression layer is compromised.

Behavior Under Elevated Temperatures

While the tempering process imparts increased mechanical strength, it does not drastically improve the inherent properties of the glass material itself at elevated temperatures. The glass retains its fundamental softening point characteristics, which are dependent on its chemical composition (e.g., soda-lime, borosilicate). At temperatures approaching the softening point, the internal stresses begin to relax, and the mechanical advantages gained during tempering diminish.

Thermal Endurance and Stress Resistance

Tempered glass possesses a somewhat enhanced ability to withstand temperature differentials compared to annealed glass. This is primarily due to the surface compressive stress layer which resists the tensile stresses induced by non-uniform heating. However, this enhancement is limited, and significant temperature gradients can still induce cracking or failure, particularly if there are pre-existing surface flaws.

Factors Affecting Thermal Performance

• Glass Composition: Different glass compositions (e.g., borosilicate, soda-lime) exhibit varying softening points and thermal expansion coefficients, which influence their high-temperature performance. Borosilicate glass generally possesses superior thermal stability.
• Tempering Level: The degree of tempering influences the magnitude of the surface compressive stress. Higher tempering levels provide greater initial resistance to thermal stress, but may also result in increased susceptibility to shattering if the surface layer is breached.
• Glass Thickness: Thicker glass sections exhibit a lower surface temperature rise per unit of applied heat flux, which can contribute to improved thermal resistance.
• Heating Rate: Rapid heating can induce large temperature gradients within the glass, increasing the risk of thermal shock and fracture.

Comparison with Annealed Glass

Annealed glass, lacking the compressive surface stresses of tempered glass, is far more susceptible to thermal stress and thermal shock. It is more likely to crack or break under even modest temperature gradients.

Applications and Limitations

Tempered glass is widely used in applications requiring increased mechanical strength and safety, such as windows, doors, shower enclosures, and automotive glass. However, its elevated temperature performance is limited, and it is not suitable for applications involving extremely high temperatures or rapid temperature fluctuations. Special high-temperature glasses, such as fused silica, are required for such applications.