M5 Thermal Tempering and Chemical Tempering

Topic

The practical strength of this glass—which is otherwise limited by surface defects, as described in another model in this series—can be deliberately increased without eliminating those defects or altering the theoretical cohesive strength of the bonds themselves. It suffices to induce a surface compressive stress that a defect must overcome before a crack originating there can open under net tensile loading. Because this surface compressive stress must be overcome before a surface defect is effectively subjected to tension, the glass's apparent strength increases, even though neither the defects nor the theoretical cohesive strength have changed at all.

This surface compressive stress can be induced, firstly, through thermal tempering: a rapid, controlled cooling of the hot glass surface causes it to contract and stiffen before the interior—which remains hot and capable of flow—does so. Because the surface contracts and stiffens before the interior, the interior is constrained by the surface when it eventually cools and attempts to contract; this mismatch leaves the surface under a permanent compressive stress.

Alternatively, this same surface compressive stress can be induced through chemical tempering: an ion-exchange process in which smaller sodium ions near the glass surface are replaced by larger potassium ions from a salt bath in which the glass is immersed. As these larger potassium ions occupy the sites previously held by the smaller sodium ions within the network, the near-surface region attempts to expand; being constrained by the surrounding glass, it is placed under that same surface compressive stress. Sustaining this ion exchange for a longer period results in a deeper ion-exchange layer, extending the surface compressive stress further into the glass.

Regardless of the process—thermal or chemical—that induced it, this surface compressive stress cannot exist without a counterpart: since the glass as a whole must remain in stress equilibrium, the surface compressive stress is balanced by core tension—a zone of tensile stress occupying the glass's interior, beneath the compressed surface layer. A higher surface compressive stress in the outer layer necessitates a correspondingly higher core tension in the interior to maintain the stress balance between the two zones.

Thus, this model—along with the apparent strength and surface compressive stress mentioned at the outset—clarifies that this mechanism increases only the glass's apparent strength, not its theoretical cohesive strength (which is defined by a separate model within this set). Because this surface compressive stress and its associated core tension are "frozen" into the glass, another distinct model in the set allows them to be measured non-destructively via photoelastic birefringence, confirming whether the tempering process achieved the intended magnitude and distribution of residual stress; furthermore, this core-tension zone serves as the basis for yet another model in the set, which describes the spontaneous fracture that can occur within that zone due to the expansion of an inclusion trapped in the glass.

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