Chemically Strengthened Glass
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Chemically strengthened glass is a type of
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
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 som ...
. 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 of a low melting point, typically tin, although lead was used for the process in the past. This method gives the sheet uniform thickness and a very flat surfa ...
. 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 (als ...
and must be laminated if safety glass is required. However, chemically strengthened glass is typically six to eight times the strength of float glass. The most common trademark for this kind of glass is Gorilla glass.


History

Glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
is one of the oldest materials created by humans, dating back to about 4,000 years ago, when craftsmen working in Mesopotamia, the land between the Tigris and Euphrates Rivers, discovered the art of mixing sand, soda, and lime to make glass. Throughout the ages, humans have explored early ion exchange in
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
to decorate and color glass artefacts with
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
or
copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...
powders. It was only around the beginning of the 20th century that the foundations for a possible application of the ion-exchange process in the technical-industrial field were laid. In 1913 Günther Schulze was the first to study the
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of
silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...
ions 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 convent ...
into the
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
using
silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...
salt (AgNO3) as ion source, starting a whole series of studies aimed to understand the chemical and physical nature of the phenomenon and its effects on some physical properties of the
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
so treated. In particular, a few years later, in 1918 at the Schott Glass Laboratory, it was demonstrated that ion-exchange produces an increase of the
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of the layer of the
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
involved in the diffusive process. After the development of
Pyroceram Pyroceram is the original glass-ceramic material developed and trademarked by Corning Glass in the 1950s. Pyroceram is an opaque, white, glass material, commonly used in kitchenware, glass stove tops, wood stove doors, etc.. It has high heat tole ...
in the 1950s,
Corning Inc. Corning Incorporated is an American multinational technology company specializing in glass, ceramics, and related materials and technologies including advanced optics, primarily for industrial and scientific applications. The company was name ...
began a research and development program called ''Project Muscle'' to improve the hardness of glass. By that time, the
ion exchange Ion exchange is a reversible interchange of one species of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid. Ion exchange is used in softening or demineralizing of water, purification of ch ...
technique had become a well-understood industrial process. S. Donald Stookey started research into using it for strengthening by June 1960, and the topic was discussed at a symposium in
Florence Florence ( ; ) is the capital city of the Italy, Italian region of Tuscany. It is also the most populated city in Tuscany, with 362,353 inhabitants, and 989,460 in Metropolitan City of Florence, its metropolitan province as of 2025. Florence ...
in September 1961, but it was Steven Kistler and, independently, Paul Henri Acloque and Jean Paul Tochon of Saint-Gobain who managed to improve the compressive strength threefold in 1962. Replacement of smaller
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
ions (Na+) with larger
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
ones (K+) in the pristine
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
matrix was able to prevent or heal over the possible formation of micro/nano-cracks on the specimen surface, increasing its
mechanical strength Mechanical may refer to: Machine * Machine (mechanical), a system of mechanisms that shape the actuator input to achieve a specific application of output forces and movement * Mechanical calculator, a device used to perform the basic operations of ...
. Soon Corning researchers found that addition of aluminium and zirconium oxides improved the qualities even further. Since then, many efforts have been carried out in this field both at the research and industrial levels.


Process principle

The actual strength of the glass is not significantly altered by the ion exchange process. Instead, a state of beneficial residual stress is introduce 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 product appearance, adhesion or wettability, solderability, ...
process. Glass is submersed in a bath of a molten
potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...
salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
(typically
potassium nitrate Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula . It is a potassium salt of nitric acid. This salt consists of potassium cations and nitrate anions , and is therefore an alkali metal nit ...
) at temperatures of or greater. This causes sodium ions 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 potassium ions from the bath. These potassium ions are larger than the
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the molten potassium nitrate. 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 . 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. Because the surface of chemically strengthened glass is in compression, it is also significantly more scratch resistant than untreated glass. This is why cell phone screens are typically made this way. Since phones are commonly carried in a pocket or purse with items such as keys, scratch resistance is important. 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 chemical formula, formula . This alkali metal nitrate salt (chemistry), salt is also known as Chile saltpeter (large deposits of which were historically mined in Chile) to distinguish it from ordi ...
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 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 heat treatment, thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into Compression (physics), comp ...
can be made cheaply through the fabrication process, chemically strengthened glass has a 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 of some
fighter aircraft Fighter aircraft (early on also ''pursuit aircraft'') are 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. Domina ...
.


See also

* Architectural glass * Superfest


References

{{Glass science Glass types Glass coating and surface modification