Material
Sapphire Al2O3 (single crystal)
Single-crystal alumina, grown rather than sintered. The standard infrared window and dome material: hard enough to survive rain and sand at speed while staying optically transparent.
Overview
Sapphire is chemically the same as alumina but is a single crystal grown from a melt rather than a sintered powder compact, and it behaves like a different material entirely. Because there are no grain boundaries to scatter light, it is transparent from the ultraviolet through the mid infrared. Because it is very hard, it survives rain, sand and particulate erosion at flight speed where softer infrared window materials would be destroyed. That combination makes it the default for seeker domes and sensor windows on anything that flies fast through weather. It is expensive, is limited in size by what can be grown, and its optical properties vary with crystal orientation, so parts must be cut on the right axis.
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Defense applications
Infrared seeker domes and windows
The window a missile or targeting pod looks through. It must transmit in the infrared, survive erosion at speed, and not distort the image, all at once.
Missile seeker domes, Targeting pod windows, IRST sensor windows, Countermeasure apertures
Night vision and electro-optics
Protective windows and lens elements in night vision and thermal sighting systems.
Night vision objective windows, Thermal sight windows, Periscope optics
Laser systems
Windows, mirror substrates and laser host material where thermal conductivity and optical damage threshold govern.
Laser windows, Mirror substrates, High energy laser optics
Armored and abrasion-resistant transparency
Transparent armor layers and scratch-resistant display covers where hardness matters more than cost.
Transparent armor face plies, Instrument covers, Ruggedised displays
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| Czochralski Sapphire | Single crystal alumina, Czochralski grown | Pulled from a melt on a seed crystal. High optical quality, with size limited by the puller. | Optical Window and Dome | |
| EFG Sapphire | Single crystal alumina, edge-defined film-fed growth | Grown to near-net shape including tubes and sheets, reducing the very expensive machining step at some cost in optical quality. | Optical Window and Dome, Ceramic Component | |
| HEM Sapphire | Single crystal alumina, heat exchanger method | Grown as large boules in a crucible, favoured for large aperture windows. | Optical Window and Dome |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 3.98 | g/cm3 | Slightly denser than sintered alumina because it is fully dense |
| Transmission range | 0.15 to 5.5 | micrometre | Ultraviolet through mid infrared |
| Vickers hardness | 18 to 22 | GPa | Second only to a few carbides and nitrides; the basis of its erosion resistance |
| Elastic modulus | 345 to 435 | GPa | Varies with crystal orientation |
| Maximum service temperature | approximately 1800 | degrees C | Melting point around 2050 degrees C |
| Optical anisotropy | Birefringent | Properties vary with crystal axis; parts must be cut on the correct orientation | |
| Thermal conductivity | 25 to 40 | W/m-K | High for an optical material, aiding thermal shock resistance |
| Relative cost | Very high | Driven by boule growth time and low machining yield |
Advantages
- Transparent from ultraviolet through mid infrared with no grain boundary scattering
- Hard enough to survive rain and sand erosion at flight speed
- Chemically inert and stable to very high temperature
- High thermal conductivity for an optical material
- Mechanically strong enough to serve as structure as well as window
Limitations
- Very expensive, driven by slow crystal growth and low yield in finishing
- Size limited by achievable boule diameter
- Birefringent, so crystal orientation must be specified and controlled
- Transmission falls off above roughly 5 micrometre, ruling out long wave infrared
- Extremely difficult to machine and polish; lapping to optical figure dominates cost
- Boule growth capacity is concentrated in few suppliers
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| MIL-PRF-13830 | U.S. Department of Defense | Optical components: surface quality, scratch and dig |
| ASTM F2094 | ASTM International | Related sapphire and ceramic ball specifications |
| MIL-STD-810 | U.S. Department of Defense | Environmental test methods including rain and sand erosion |