Secret below the surface for toughened glasses
15 Jan 2000
Toughened glass is a slight misnomer. Although it can stand more stress than normal glass, when it does break, it shatters spectacularly. This is because the glass - used for spectacle lenses and windscreens - is toughened by compressing the outer layer. Once a crack starts, the stresses in the tough layer accelerate its propogation leading to shattering.
Researchers from Penn State University have devised a new method of toughening glass which dampens cracks as they form. The idea itself is simple, says the team, writing in Science; instead of compressing the outside of the glass, the toughened layer is inside the pane.
Chemical toughening of glass is normally achieved by replacing sodium atoms in the outermost 1mm-thick layer of the glass by slightly-larger potassium atoms. This crowds the lattice and compresses the material. In the Penn process, the researchers took normal toughened glass and replaced some of the surface potassium with more sodium. This returned the outermost layer to an untempered state, but retained the compression just below the surface.
This change has a dramatic effect on the behaviour of cracks in the glass. Microscopic cracks in the surface layer travel until they reach the toughened layer; and then they stop, rather than shooting through the pane and shattering it as they would in normal tempered glass. Many cracks can form before the one which finally breaks through the toughened later and breaks the glass; this creates a `crazing' effect which could be used as a warning that the glass is approaching its breaking point.
