Chemically strengthened glass is a type of

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...

that has increased strength as a result of a post-production

chemical process In a scientific sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by an outside force, and involves a chemical reaction of some ...

. When broken, it still shatters in long pointed splinters similar to

float glass Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and other various low- melting-point alloys were used in the past. This method gives the sheet uniform thickness and very flat ...

. For this reason, it is not considered a

safety glass Safety glass is glass with additional safety features that make it less likely to break, or less likely to pose a threat when broken. Common designs include toughened glass (also known as tempered glass), laminated glass, and wire mesh glass (al ...

and must be

laminated Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materia ...

if a

is required. However, chemically strengthened glass is typically six to eight times the strength of

. The most common trademark for this kind of glass is Gorilla glass. The glass is chemically strengthened by a

surface finishing Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, soldering, solderability, c ...

process. Glass is submersed in a bath containing a

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantiti ...

(typically

potassium nitrate Potassium nitrate is a chemical compound with the chemical formula . This alkali metal nitrate salt is also known as Indian saltpetre (large deposits of which were historically mined in India). It is an ionic salt of potassium ions K+ and nitr ...

) at about . This causes sodium

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conv ...

s in the glass

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

to be replaced by

ions from the bath solution. These

s are larger than the

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

ions and therefore wedge into the gaps left by the smaller sodium

s when they migrate to the

solution. This replacement of

s 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 . The strengthening mechanism depends on the fact that the compressive strength of glass is significantly higher than its tensile strength. With both surfaces of the glass already in compression, it takes a certain amount of bending before one of the surfaces can even go into tension. More bending is required to reach the tensile strength. The other surface simply experiences more and more compressive stress. But since the compressive strength is so much larger, no compressive failure is experienced. 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 Sodium nitrate is the chemical compound with the formula . This alkali metal nitrate salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordinary saltpeter, potassium 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 Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension ...

. 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. Another negative of chemically strengthened glass is the added cost. While

tempered glass Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension ...

can be made cheaply through the fabrication process, chemically strengthened glass has a much more expensive route to the market. These costs make the product prohibitive for use in many applications. Chemically strengthened glass was used for the

aircraft canopy An aircraft canopy is the transparent enclosure over the cockpit of some types of aircraft. An aircraft canopy provides a controlled and sometimes pressurized environment for the aircraft's occupants, and allows for a greater field of view ove ...

of some

fighter aircraft Fighter aircraft are fixed-wing aircraft, fixed-wing military aircraft designed primarily for air-to-air combat. In military conflict, the role of fighter aircraft is to establish air supremacy, air superiority of the battlespace. Domination o ...

References

{{Glass science Glass types Glass coating and surface modification

See also

References