Chemically strengthened glass

Last updated

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 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.

Contents

History

Glass 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. [1] Throughout the ages, humans have explored early ion exchange in glass to decorate and color glass artefacts with silver or copper powders. [2]

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. [3] In 1913 Günther Schulze was the first to study the diffusion of silver ions into the glass using silver nitrate 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 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 of the layer of the glass involved in the diffusive process. [4]

After the development of Pyroceram in the 1950s Corning Inc. started an R&D program Project Muscle to improve the hardness of glass. [5] By that time the ion exchange technique became a well-understood industrial process. S. Donald Stookey started the research into using it for the purpose by June 1960, [6] and the topic was discussed at a symposium in Florence in September 1961, [7] but it was Steven Kistler [8] and independently Paul Henri Acloque and Jean Paul Tochon of Saint-Gobain who managed to improve the compressive strength threefold in 1962. [6] Replacement of smaller sodium ions (Na+) with larger potassium ones (K+) in the pristine glass matrix was able to prevent or heal over the possible formation of micro/nano-cracks on the specimen surface, increasing its mechanical strength. [8] Soon Corning researchers found that addition of aluminium and zirconium oxides improved the qualities even further. [9] [6] Since then, many efforts have been carried out until our days in this field both at research and industrial levels.

Process principle

The glass is chemically strengthened by a surface finishing process. Glass is submersed in a bath of a molten potassium salt (typically potassium nitrate) at temperatures of 334 °C (630 °F) or greater. [10] This causes sodium ions in the glass surface to be replaced by potassium ions from the bath.

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 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 690 megapascals (100,000 psi).

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 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 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. [11]

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

See also

Related Research Articles

<span class="mw-page-title-main">Nitrate</span> Polyatomic ion (NO₃, charge –1) found in explosives and fertilisers

Nitrate is a polyatomic ion with the chemical formula NO−
3. Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble nitrate is bismuth oxynitrate.

<span class="mw-page-title-main">Potassium nitrate</span> Chemical compound

Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula KNO3. It is a potassium salt of nitric acid. This salt consists of potassium cations K+ and nitrate anions NO−3, and is therefore an alkali metal nitrate. It occurs in nature as a mineral, niter. It is a source of nitrogen, and nitrogen was named after niter. Potassium nitrate is one of several nitrogen-containing compounds collectively referred to as saltpetre.

<span class="mw-page-title-main">Glass fiber</span> Material consisting of numerous extremely fine fibers of glass

Glass fiber is a material consisting of numerous extremely fine fibers of glass.

<span class="mw-page-title-main">Ion exchange</span> Exchange of ions between an electrolyte solution and a solid

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 chemicals, and separation of substances.

<span class="mw-page-title-main">Residual stress</span> Stresses which remain in a solid material after the original cause is removed

In materials science and solid mechanics, residual stresses are stresses that remain in a solid material after the original cause of the stresses has been removed. Residual stress may be desirable or undesirable. For example, laser peening imparts deep beneficial compressive residual stresses into metal components such as turbine engine fan blades, and it is used in toughened glass to allow for large, thin, crack- and scratch-resistant glass displays on smartphones. However, unintended residual stress in a designed structure may cause it to fail prematurely.

Samuel Stephens Kistler was an American scientist and chemical engineer, best known as the inventor of aerogels, one of the lightest known solid materials.

<span class="mw-page-title-main">Architectural glass</span> Building material

Architectural glass is glass that is used as a building material. It is most typically used as transparent glazing material in the building envelope, including windows in the external walls. Glass is also used for internal partitions and as an architectural feature. When used in buildings, glass is often of a safety type, which include reinforced, toughened and laminated glasses.

<span class="mw-page-title-main">Chromate conversion coating</span> Chemical treatment of metals

Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack on soft metals.

<span class="mw-page-title-main">Ceramic engineering</span> Science and technology of creating objects from inorganic, non-metallic materials

Ceramic engineering is the science and technology of creating objects from inorganic, non-metallic materials. This is done either by the action of heat, or at lower temperatures using precipitation reactions from high-purity chemical solutions. The term includes the purification of raw materials, the study and production of the chemical compounds concerned, their formation into components and the study of their structure, composition and properties.

<span class="mw-page-title-main">Tempered glass</span> Type of safety glass processed to increase its strength

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. Such stresses cause the glass, when broken, to shatter into small granular chunks instead of splintering into large jagged shards as ordinary annealed glass does. These smaller, granular chunks are less likely to cause deep penetration when forced into the surface of an object compared to larger, jagged shards because the reduction in both the mass and the maximum dimension of a glass fragment corresponds with a reduction in both the momentum and the penetration depth of the glass fragment.

<span class="mw-page-title-main">Sociedad Química y Minera</span> Chilean chemical company and lithium producer

Sociedad Química y Minera de Chile (SQM) is a Chilean chemical company and a supplier of plant nutrients, iodine, lithium and industrial chemicals. It is the world's biggest lithium producer.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

Chlorine gas can be produced by extracting from natural materials, including the electrolysis of a sodium chloride solution (brine) and other ways.

<span class="mw-page-title-main">Gorilla Glass</span> Chemically strengthened glass made by Corning

Gorilla Glass, developed and manufactured by Corning, is a brand of chemically strengthened glass now in its ninth generation. Designed to be thin, light, and damage-resistant, its surface strength and crack-resistance are achieved through immersion in a hot potassium-salt ion-exchange bath.

Calcium nitrite is an inorganic compound with the chemical formula Ca(NO
2)
2. In this compound, as in all nitrites, nitrogen is in a +3 oxidation state. It has many applications such as antifreeze, rust inhibitor of steel and wash heavy oil.

In materials science, Nabarro–Herring creep is a mechanism of deformation of crystalline materials that occurs at low stresses and held at elevated temperatures in fine-grained materials. In Nabarro–Herring creep, atoms diffuse through the crystals, and the rate of creep varies inversely with the square of the grain size so fine-grained materials creep faster than coarser-grained ones. NH creep is solely controlled by diffusional mass transport.

In metallurgy, peening is the process of working a metal's surface to improve its material properties, usually by mechanical means, such as hammer blows, by blasting with shot, focusing light, or in recent years, with water column impacts and cavitation jets. With the notable exception of laser peening, peening is normally a cold work process tending to expand the surface of the cold metal, thus inducing compressive stresses or relieving tensile stresses already present. It can also encourage strain hardening of the surface metal.

In materials science, toughening refers to the process of making a material more resistant to the propagation of cracks. When a crack propagates, the associated irreversible work in different materials classes is different. Thus, the most effective toughening mechanisms differ among different materials classes. The crack tip plasticity is important in toughening of metals and long-chain polymers. Ceramics have limited crack tip plasticity and primarily rely on different toughening mechanisms.

Glass typically has a tensile strength of 7 megapascals (1,000 psi). However, the theoretical upper bound on its strength is orders of magnitude higher: 17 gigapascals (2,500,000 psi). This high value is due to the strong chemical Si–O bonds of silicon dioxide. Imperfections of the glass, such as bubbles, and in particular surface flaws, such as scratches, have a great effect on the strength of glass and decrease it even more than for other brittle materials. The chemical composition of the glass also impacts its tensile strength. The processes of thermal and chemical toughening can increase the tensile strength of glass.

<span class="mw-page-title-main">Superfest</span> East German glass

Superfest, also called CV-Glas or Ceverit until 1980, was a brand of drinking glasses in the GDR. Due to being made of chemically strengthened glass, they were notably strong. The Superfest glasses were produced between 1980 and 1990 in what was then state-owned Sachsenglas Schwepnitz.

References

  1. ↑ "Origins of Glassmaking | Corning Museum of Glass".
  2. ↑ Mazzoldi, P.; Sada, C. (2008). "A trip in the history and evolution of ion-exchange process". Mat. Sci. Eng. B. 149 (2): 112–117. doi:10.1016/j.mseb.2007.11.038.
  3. ↑ Berneschi, Simone; Righini, Giancarlo C.; Pelli, Stefano (2021). "Towards a Glass New World: The Role of Ion-Exchange in Modern Technology". Applied Sciences. 11 (10): 4610. doi: 10.3390/app11104610 . ISSN   2076-3417.
  4. ↑ Schulze, Günther (1913). "Versuche über die Diffusion von Silber in Glas". Annalen der Physik. 345 (2): 335–367. Bibcode:1913AnP...345..335S. doi:10.1002/andp.19133450208. ISSN   0003-3804.
  5. ↑ Starkman, Dean; Hamilton, Martha M.; Chittum, Ryan; Salmon, Felix (2013-06-18). The Best Business Writing 2013. Columbia University Press. ISBN   978-0-231-53517-5.
  6. 1 2 3 Madsen, Lynnette D.; Svedberg, Erik B. (2016-01-14). Materials Research for Manufacturing: An Industrial Perspective of Turning Materials into New Products. Springer. p. 8. ISBN   978-3-319-23419-9.
  7. ↑ Eitel, Wilhelm (1976). Silicate Science, Vol. VII : Glass Science. Elsevier. p. 365. ISBN   978-0-323-14428-5.
  8. 1 2 Kistler, S. S. (February 1962). "Stresses in Glass Produced by Nonuniform Exchange of Monovalent Ions". Journal of the American Ceramic Society. 45 (2): 59–68. doi:10.1111/j.1151-2916.1962.tb11081.x. ISSN   0002-7820.
  9. ↑ Nordberg, Martin E.; Mochel, Ellen L.; Garfinkel, Harmon M.; Olcott, Joseph S. (May 1964). "Strengthening by Ion Exchange". Journal of the American Ceramic Society. 47 (5): 215–219. doi:10.1111/j.1151-2916.1964.tb14399.x. ISSN   0002-7820.
  10. ↑ PubChem. "Potassium Nitrate". pubchem.ncbi.nlm.nih.gov. Retrieved 2024-04-23.
  11. ↑ Munden, Jacob (23 June 2018). "What is Chemical Tempering?". Barrett Limited.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.