[Image above] Illustration of how ion exchange in glasses leads to chemical strengthening. Credit: Varshneya, International Journal of Applied Glass Science
This month’s featured topic for the Glass: Then and Now series—Chemically strengthened glasses—is a natural follow-up to last month’s focus on Toughness and mechanical properties. The articles selected for the March collection discuss one of the most important techniques for improving the resilience of glass and enabling its use in life-changing applications.
Arun K. Varshneya, ACerS Distinguished Life Member and president of Saxon Glass Technologies, provided this month’s introductory essay. A generous man, Varshneya (who along with his wife sponsors the Frontiers of Glass Science and Frontiers of Glass Technology awards at the annual GOMD meeting), provided an in-depth discussion that was excerpted for this CTT post. The full text can be read here.
By some estimates, the theoretical strength of a flawless glass is as much as 35 GPa applied in a tensile mode. On the other hand, ordinary glass products break in a brittle mode at as little as 7–100 MPa. In contrast, ductile metals are easily able to withstand as much as 150–350 MPa, and plastic products do not break because of their high energy absorption capability (toughness).
Glasses fail primarily because of the interaction of the tensile stress with microscopic surface flaws. Flaws are produced during handling by the manufacturer as well as the user. Almost any material contact abrades even the most pristine glass surface to produce what could be a fatal flaw.
Of the various glass strengthening techniques, chemical strengthening (also called “ion exchange strengthening”) is relatively new technology (discovered in 1962). In chemical strengthening, an alkali-containing glass is immersed in molten alkali salt having ions larger than the host ions. The host alkali ions of glass exchange with larger ions (see Figure), leading to the development of high surface compressive forces. Because an applied tension must overcome the compression before crack growth can occur, the introduction of surface compression effectively strengthens the glass product.
The early applications include aircraft cockpit windshields, which must be designed to withstand impact of birds flying at 400 knots (740 km/hr) and higher. On a more personal level, you probably have some chemically strengthened glass in your pocket right now as the cover glass on your mobile phone.
The advantages of this process are: (i) introduction of relatively high surface compression; (ii) no measurable optical distortion; (iii) thin plates, even 50 mm thin, can be strengthened; and (iv) irregular geometry products can be readily strengthened so long as the surface can be contacted by the molten salt.
Glass chemical strengthening has helped save thousands of human lives each year. EachEpiPen® brand autoinjector contains a chemically strengthened borosilicate glass cartridge manufactured by Saxon Glass Technologies, Inc. As a result, the failure rate fell from nearly 10% down to virtually nothing, enabling the device to be carried and self-injected by those who suffer from severe allergic reactions to peanuts, seafood, and insect stings, for example.
Future chemically strengthened glass products might include hurricane-resistant architectural windows, solar energy conversion tubular and flat-plate collectors, transparent armor, and more.
There is reason to celebrate glass as the most transformative material. It has done so much for the comforts of human living.
– Arun K. Varshneya, president of Saxon Glass Technologies
Articles for Chemically strengthened glasses