Formation Process of the Compressive Stress Layer in Physically Tempered Glass
Heating Pre-treatment:Ordinary annealed glass is uniformly heated to 50–60°C above its glass transition temperature (Tg)-approximately 620–700°C-bringing the glass into a viscoelastic, deformable state and relieving any pre-existing internal residual stresses.
Rapid Quenching/Cooling:High-pressure cold air is blown uniformly onto both surfaces of the glass simultaneously. The surface temperature drops sharply, causing the surface layer to cool, contract, and solidify first. At this stage, the inner layer remains in a high-temperature, viscous state, allowing stresses generated by surface contraction to fully relax through the viscous flow of the inner layer.
Constrained Contraction Creating Stress: As the glass continues to cool to room temperature, the inner layer cools and begins to contract in volume. However, since the surface layer has already fully solidified and cannot contract synchronously, the contraction of the inner layer is constrained by the rigidity of the hardened surface. This ultimately results in the formation of a permanent, uniformly distributed compressive stress layer on the surface, with a corresponding tensile stress layer formed in the interior to maintain equilibrium.
Formation Process of the Compressive Stress Layer in Chemically Tempered Glass
Low-Temperature Ion Exchange Environment:The glass is immersed in a molten alkali salt solution at approximately 400°C. Since this temperature is below the glass transition temperature, the glass undergoes no macroscopic deformation.
Large-Ion Substitution and Crowding: Smaller sodium ions (or lithium ions) in the glass surface layer are exchanged for larger potassium ions from the salt solution; the larger potassium ions embed themselves into the existing ionic vacancies within the glass surface.
Crowding Effect Generating Compressive Stress: The large volume of potassium ions creates a "crowding" or "stuffing" effect within the surface layer. This volume difference between the alkali ions generates compressive stress; the extent of ion exchange and the depth of the exchange layer directly determine the thickness of the compressive stress layer and the magnitude of the compressive stress.