Sapphire is a single crystal aluminum oxide (Al2O3). It is one of the hardest materials

Sapphire is a single crystal aluminum oxide (Al2O3). It is one of the hardest materials. Sapphire has good transmission characteristics over the visible, and near IR spectrum. It exhibits high mechanical strength, chemical resistance, thermal conductivity and thermal stability. It is often used as window materials in specific field such as space technology where scratch or high temperature resistance is required

Sapphire glass are ideal for applications where high pressure, high temperature, high thermal loads, vacuum, scratch resistance, corrosion resistance, small coefficient of friction as well as optical and insulating properties are a consideration.

Sapphire windows normally need to work under extreme operating conditions, so hard AR coating is required to withstand harsh environmental conditions and improve resistance to scratching.
Sapphire substrate has a high refractive index, thus usually requires an antireflection coating on sapphire windows for sufficient transmission in the region of interest.

In some applications where sapphire windows need to be soldered or bonded to metal.