Sapphire windows is one of the hardest materials after diamond. They are often used in extreme enviorments including space with radiation is an issue. For medical research/production sapphire is used in microfluidic devices.
Optical sapphire is a benefit for military hardware as it performs as its very wide optical transmission that is better than traditional glass. Sapphire's wide optical transmission band from UV to near-IR, (0.15-5.5µm) No Ultra-Violett radiation darkening protects hardware.
Sapphire wafers are used in teh optoelectronics and microelectronic industries. Unlike less expensive Silicon, Sapphire wafers are used as insulator. Sapphire's Advantages over other substrates include:
UniversityWafer, Inc have all diameters/dimensions from 1mm round to 200mm and thicknesses as thin as 100 micron sapphire available.