Sapphire is famous for being the second hardest mineral after diamond, and is used as a protective cover and bearing for watches. Sapphire is also used for windows that require reliability because it is resistant to scratching even in harsh environments where it is exposed to sand. However, because of its high hardness, sapphire is not easy to process, and because it is machined from a large ingot, it is expensive to process, making it a high-grade alternative to glass. Sapphire windows are used in familiar applications such as watch windows and camera windows of smartphones.
The mechanical properties of sapphire also have an orientation dependence, and the a-plane tends to be higher in both bending strength and Vickers hardness. In many cases, a-plane materials are used for watch windows.
Bending strength varies depending on the surface finish. In order to ensure the strength, it is desirable to use final polished one.
