Glasses crack when we stretch them as the hidden tension between the interfaces is extended or increased. This problem can be solved by creating a compression region on the surface. But i heard Global Glass leaders like Corning and others do some chemical and structural changes to the glass too...like they did in Gorilla Glass.
The answer by Vadim Verlotski is essentially correct. The exchange must be carried out at a high temperature to increase the rate of ion exchange, but the temperature must be low enough that relaxation of the network does not occur. Relaxation would reduce the induced compressive stress. See Varshneya's textbook for details.
A less degree of strengthening can be obtained by chemically etching the glass surface to remove flaws at the micrometer level. Unfortunately, any abrasion of the surface will reintroduce these flaws and eliminate the strengthening effect.
In some cases, weathering of the surface will actually strengthen a glass by converting the surface to a gel-like material which is no longer brittle. If this layer is thicker than the depth of any flaws, the glass will be strengthened. Unfortunately, this requires a less than unusually desirable chemical durability of the glass. An example is found in binary potassium silicate glasses.
Discussions of these methods can be found in a number of text books.
Chemical ion exchange reinforcement of glasses has, in principle, the same effect as the well-known thermal reinforcement: building of compressive stresses in the surface. In contrast to the thermal reinforcement is formed by the chemical strengthening, a very thin layer having compressive stresses.
The compressive stresses arise because a part of small Na+ ions is replaced by larger K+ ions. This happens in a thin (
The answer by Vadim Verlotski is essentially correct. The exchange must be carried out at a high temperature to increase the rate of ion exchange, but the temperature must be low enough that relaxation of the network does not occur. Relaxation would reduce the induced compressive stress. See Varshneya's textbook for details.
A less degree of strengthening can be obtained by chemically etching the glass surface to remove flaws at the micrometer level. Unfortunately, any abrasion of the surface will reintroduce these flaws and eliminate the strengthening effect.
In some cases, weathering of the surface will actually strengthen a glass by converting the surface to a gel-like material which is no longer brittle. If this layer is thicker than the depth of any flaws, the glass will be strengthened. Unfortunately, this requires a less than unusually desirable chemical durability of the glass. An example is found in binary potassium silicate glasses.
Discussions of these methods can be found in a number of text books.
A method that is rarely discussed is to prevent water or water vapor from reaching the surface of the glass, and in particular, The crack tip. Slow track grows under stress reduces the usable Strength of a soda lime silicate glass by up to 50 or even 60%. A thin hydrophobic layer might therefore help for fatigue, but would not help impact strength.
forgive the typos in previous - - - I edited the first sentence to be correct
A method that is rarely discussed is to PREVENT water......
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