Sapphire (crystalline aluminum oxide, Al₂O₃) is one of the strongest transparent materials available.
When used as sapphire windows, sapphire provides outstanding durability against mechanical stress, scratching, and impact.
What These Numbers Mean:
Hardness: Sapphire resists scratching by almost everything except diamond and a few synthetic super-hard materials.
Compressive Strength: It can withstand extremely high pressures without breaking — great for high-pressure optical applications (like deep-sea cameras or aerospace windows).
Tensile Strength: It resists being pulled apart but not as high as metals; however, still excellent among ceramics.
Fracture Toughness: Better than glass, but still more brittle than metals — sudden impacts at the wrong angle can cause cracks.
Important Notes:
Orientation matters: Mechanical strength can vary depending on the crystal orientation (c-plane, a-plane, r-plane, m-plane sapphire).
For maximum strength, manufacturers usually use c-plane sapphire windows.
Thickness matters: Thicker sapphire windows naturally handle more mechanical load.
Edge preparation: Polished or beveled edges reduce stress concentration and improve mechanical strength dramatically.
Practical Applications Where Strength Matters:
Protective covers for high-speed cameras
Aerospace sensor windows
High-pressure viewport windows
Medical and defense optics
Watch crystals and scanner windows