Chemically strengthened glass

Chemically strengthened glass is a type of glass that has increased strength as a result of a post-production chemical process. When broken, it still shatters in long pointed splinters similar to float glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. However, chemically strengthened glass is typically six to eight times the strength of float glass.

The glass is chemically strengthened by a surface finishing process. Glass is submersed in a bath containing a potassium salt (typically potassium nitrate) at 300°C (572°F). This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution.

These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. The surface compression of chemically strengthened glass may reach up to 690 MPa.

There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450°C (842°F), which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article.

Chemical strengthening results in a strengthening similar to toughened glass. However, the process does not use extreme variations of temperature and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can be significantly bowed.

Also unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength.

Chemically strengthened glass was used for the aircraft canopy of some fighter aircraft.

Glass science topics

Basics	Glass Glass transition Supercooling

Formulation	AgInSbTe Bioglass Borophosphosilicate glass Borosilicate glass Ceramic glaze Chalcogenide glass Cobalt glass Cranberry glass Crown glass Flint glass Fluorosilicate glass Fused quartz GeSbTe Gold ruby glass Lead glass Milk glass Phosphosilicate glass Photochromic lens glass Silicate glass Soda-lime glass Sodium hexametaphosphate Soluble glass Tellurite glass Ultra low expansion glass Uranium glass Vitreous enamel Wood's glass ZBLAN

Glass-ceramics	Bioactive glass CorningWare Glass-ceramic-to-metal seals Macor Zerodur

Preparation	Annealing Chemical vapor deposition Glass batch calculation Glass forming Glass melting Glass modeling Ion implantation Liquidus temperature Sol-gel technique Viscosity Vitrification

Optics	Achromat Dispersion Gradient-index optics Hydrogen darkening Optical amplifier Optical fiber Optical lens design Photochromic lens Photosensitive glass Refraction Transparent materials

Surface modification	Anti-reflective coating Chemically strengthened glass Corrosion Dealkalization DNA microarray Hydrogen darkening Insulated glazing Porous glass Self-cleaning glass Sol-gel technique Toughened glass

Diverse topics	Glass-coated wire Safety glass Glass databases Glass electrode Glass fiber reinforced concrete Glass ionomer cement Glass microspheres Glass-reinforced plastic Glass-to-metal seal Porous glass Prince Rupert's Drops Radioactive waste vitrification Windshield

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Chemically strengthened glass

See also