Feb. 26, 1999 University Park, Pa. --- Few things are as fragile as glass, and if a Penn State researcher has his way, some types of glass will be less fragile.
"Chemical and heat tempered glasses have been around for a long time," says Dr. David J. Green, professor of ceramic science and engineering. "These glasses can withstand more stress before breaking than untreated glass, but when they break, they usually break catastrophically."
Another problem with chemical and heat tempered glass is that while each individual piece of glass becomes stronger, the variability of strength between pieces of glass increases dramatically. Engineers choosing glass for specific purposes must account for this wider range of strengths.
Working with Dr. R. Tandon of Caterpillar Inc., in Peoria, Illinois, and V.M. Sglavo of the University of Trento, Italy, Green developed a theoretical approach to designing strengthened glass. The team reported their work in today's (Feb. 26) issue of the journal Science.
Conventional tempering of glass alters the outer surface of the glass so that it is under compression. Glass under compression can withstand higher levels of stress before reaching the failure point.
"Rather than simply altering the outside layer of glass, we would like to engineer the glass so that it has a specific compression profile making the final product stronger and less variable," says Green, a faculty member in the College of Earth and Mineral Sciences.
The researchers tested their theory using the chemical tempering process on sodium aluminosilicate glass, but believe that they could adapt the process to other tempering processes and other materials.
In chemical tempering, potassium atoms are often used to replace some of the sodium atoms near the surface. These potassium atoms are slightly larger than the sodium atoms and they compress the layer in which they are substituted by crowding the other atoms. Chemical tempering usually occurs in the outer millimeter of the pane of glass.
"If we place the maximum compression layer beneath the surface, when cracks propagate from the flaws on the surface, they reach the layer and stop," says Green.
The researchers created these internal compressed layers by subjecting the glass to chemical processing where potassium substituted for sodium, but then exchanged some of the potassium near the surface back to sodium. This created glass with an untempered surface, but with a tempered, compressed layer below.
"Unexpectedly, glass made in this way exhibits multiple cracking," says Green. "Unlike untreated glass or conventionally tempered glass where a crack that begins progresses rapidly to catastrophic failure, small cracks begin to form in the untempered layer and then the cracks are arrested by the compressed layer."
Many cracks may form before the ultimate crack that propagates through the compressed layer and shatters the glass. This surface crazing can be used as a warning that the glass is approaching its breaking point and needs to be replaced. Creating glass that will only break at a certain, predetermined stress level may also be possible.
"The strength range of a batch of conventionally tempered glass may be as broad as 25 percent on either side of the average strength," Green says. "However, the specially designed glass we are looking at has a range of only 6 percent on either side of the average." This smaller range provides more consistency when manufacturing the glass.
Chemically tempered glass is used in eyeglasses and sunglasses and thermally tempered glass is used in automobile windshields. This new tempering method could allow thinner glass to be used in such things as photocopying machines, scanners and electronic displays that would make them stronger and lighter. Eventually, glasses could be designed with specific strengths and a higher reliability.
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