What makes the glass in your smartphone so strong?

NPR Science Friday : Gorilla Glassâmdash;originally an internal product name for Corning’s superstrong glassâmdash;is used in more than 750 million smartphones around the world. What makes it so strong is a process called ion exchange. Dave Velasquez, director of marketing and commercial operations for Gorilla Glass, explains that the compound is created by dipping millimeter-thick alumina-silicate-based glass into molten potassium nitrate. While the glass is soaking, the potassium ions replace the sodium ions that are in the glass. Because potassium atoms have a larger diameter than sodium atoms, the surface of the glass becomes more crowded and, hence, compressed. It is this compression that gives the glass its strength and prevents flaws from forming and spreading. The compression is also why, when the glass on a smartphone does break, it breaks catastrophically.