Glass strengthened by exchanging Na⁺ in its surface layer for larger K⁺ in a molten salt, introducing large compressive stress in the surface layer. It can be used even for thin sheets.
Japanese name: 化学強化ガラス
Chemically strengthened glass appears in 1 practice question across 1 topic, including Glasses and amorphous materials.
In chemical strengthening, glass is immersed in molten potassium nitrate at a temperature below the strain point, and Na⁺ in the surface layer is exchanged for K⁺, which has a larger ionic radius. At this temperature the network hardly flows, so the state in which large ions have been forced into sites meant for small ions remains; the surface layer tries to expand, is constrained by the interior, and a large compressive stress develops in it. Whereas thermal tempering uses a temperature difference, chemical strengthening uses a difference in composition, so thin sheets and complex shapes can be strengthened and a large stress can be left. The technique was developed and put into practical use in the 1960s.
The performance of chemically strengthened glass is expressed by the surface compressive stress (CS) and the depth of the compressive layer (DOL). In soda-lime glass K⁺ diffuses slowly and DOL is limited to about 10 µm, but in aluminosilicate glasses diffusion is fast, and a few hours of treatment can create a layer with CS of several hundred to about 1000 MPa and DOL of a few tens of µm. Because DOL is set by diffusion, it deepens roughly in proportion to the square root of the treatment time. However, raising the temperature or extending the time too much lowers CS through stress relaxation, so the conditions are chosen by balancing the two.
A tensile stress (CT) develops inside to balance the compression of the surface layers. Assuming a triangular profile in which the compressive stress falls linearly with depth, force balance gives CT ≈ CS × DOL/(t − 2DOL), where t is the sheet thickness. For example, with CS = 800 MPa, DOL = 40 µm and t = 0.70 mm, CT is about 52 MPa. The thinner the sheet and the deeper the DOL, the larger CT becomes, and if it is too large the glass shatters into fine pieces when broken. In glasses containing lithium, two-step exchanges of Li⁺ for Na⁺ and Na⁺ for K⁺ combine a deep layer with a high surface stress.
Chemically strengthened glass is used for the cover glass of smartphones and tablets, watch covers, and windows of aircraft and railway vehicles. Because the compressive layer is thin, a flaw deeper than the DOL has its tip in the internal tensile layer, and the strengthening effect is lost. To make the glass resist flaws from sharp, hard particles, deepening the DOL is therefore effective. Also, if the glass is cut after strengthening, the cut edge has no compressive layer, so normally the shape is finished before strengthening. The surface stress is measured without breaking the glass, for example with surface stress meters based on optical waveguiding.
Answer: A. Below the strain point, Na⁺ in the surface layer is exchanged for larger K⁺, creating compressive stress in the surface layer
In chemical strengthening, a glass containing Na⁺ is immersed in molten KNO₃ at about 400 °C. When Na⁺ in the surface layer is replaced by larger K⁺, the larger ions are in effect squeezed into sites sized for Na⁺; the surface layer tries to expand but is constrained by the interior, so a large compressive stress of several hundred MPa or more develops in it. This temperature is below the strain point, where the network hardly flows, so the stress remains without relaxing. Above the softening point, the stress would relax by viscous flow. If the surface layer tried to shrink through exchange for smaller Li⁺, tensile stress would arise in it instead. Thermal tempering is difficult for thin sheets, in which it is hard to set up a temperature difference, whereas chemical strengthening can be used even on sheets 1 mm thick or less.
The surface compressive stress is much larger in chemically strengthened glass, several hundred MPa or more. However, its compressive layer is thin, a few tens of µm, so it is vulnerable to deep flaws. Thermally tempered glass has a smaller stress of around 100 MPa, but its compressive layer is thick, about one fifth of the thickness, so it resists deep flaws better.
Because Na⁺, compensating the charge of AlO₄ tetrahedra, is only weakly bound to the network and moves easily, ion exchange proceeds quickly and a deep layer can be formed. Also, the well-connected network makes stress relaxation slow, so a high compressive stress can remain. Soda-lime glass can also be chemically strengthened, but the layer is shallow.
If it is held for long at a high temperature close to the treatment temperature, the strengthening stress is gradually lost by ion diffusion and network relaxation. It does not change at ordinary service temperatures, but it is unsuitable for use at temperatures of several hundred °C. Where resistance to sudden temperature changes is needed, a glass with small thermal expansion is chosen.
In chemical strengthening, a glass containing Na⁺ is immersed in molten KNO₃ at about 400 °C.
Which correctly describes chemical strengthening (strengthe…Tempered glass Amorphous silicon Metallic glass Network former Network modifier Non-bridging oxygen Reduced glass transition temperature