Glasses and amorphous materials
Which correctly describes chemical strengthening (strengthening by ion exchange), used for smartphone cover glass and similar products?
- ABelow the strain point, Na⁺ in the surface layer is exchanged for larger K⁺, creating compressive stress in the surface layer
- BNa⁺ in the surface layer is exchanged for the smaller Li⁺, which shrinks the volume of the surface layer and so creates compressive stress
- CCarrying out the ion exchange above the softening point makes it proceed faster and so gives a larger compressive stress to a greater depth
Show answer and explanation
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.