Material
Alumina Al2O3
The volume workhorse of technical ceramics: cheap, well understood, electrically insulating and available from many sources. Heavier and less hard than the carbides, which is why it loses armor work to them and keeps almost everything else.
Overview
Alumina is the default technical ceramic and by far the highest volume one. It is a fraction of the cost of silicon or boron carbide, is produced by many suppliers rather than a handful, and does the enormous range of jobs that need hardness, electrical isolation and chemical inertness without extreme requirements. In armor it is the budget option, used where weight is less constrained than cost. In electronics it is the standard substrate material, carrying metallised conductor patterns in hermetic packages. Its ceiling is that it is denser than the carbides and less hard, so weight-driven applications move away from it.
Supply chain
What feeds what. Click any box with suppliers on file.
Buy a product
Forms you purchase by the pound or the piece. Each page separates mills from distributors.
Buy a service
Processes performed on material you already own.
Defense applications
Cost-driven armor
The economical strike face where areal density is less constrained than budget, typically in vehicle and fixed installation protection rather than on a soldier.
Vehicle appliqué, Fixed installation protection, Blast panels, Training targets
Electronic substrates and packaging
The standard carrier for metallised conductor patterns in hermetic microwave and power modules, chosen for electrical isolation, thermal conductivity and expansion match.
Radar module substrates, Hermetic feedthroughs, Power semiconductor carriers, RF packages
Wear, seal and insulator hardware
Pump seals, valve seats, thread guides and high voltage insulators across essentially every platform.
Seal faces, Valve seats, High voltage insulators, Wear liners
Radomes and windows
Lower cost radome and window material where the electrical performance requirement is moderate.
Ground radar radomes, Antenna windows, Sensor housings
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| 94% Alumina | Alumina, standard purity | The economical substrate and insulator grade, with a glassy grain boundary phase that aids sintering and metallisation. | Ceramic Substrate, Ceramic Component | |
| 99.6% Alumina | Alumina, high purity | Higher strength, hardness and dielectric performance than 94 percent, at higher cost and more difficult processing. | Ceramic Substrate, Ceramic Component, Armor Tile | |
| 99.8% Alumina | Alumina, very high purity | Near-theoretical purity for the most demanding electrical and vacuum applications. | Ceramic Component, Optical Window and Dome |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 3.7 to 3.95 | g/cm3 | Heavier than silicon or boron carbide |
| Vickers hardness | 15 to 20 | GPa | Substantially softer than the carbides |
| Elastic modulus | 300 to 380 | GPa | Varies with purity |
| Flexural strength | 300 to 400 | MPa | |
| Fracture toughness | 3 to 4.5 | MPa m^0.5 | Comparable to silicon carbide |
| Dielectric strength | 10 to 17 | kV/mm | The property behind its electronics use |
| Maximum service temperature | approximately 1600 | degrees C | In air, unlike the carbides |
| Relative cost | The baseline | Silicon carbide runs 2 to 3 times this; boron carbide far more |
Advantages
- Lowest cost technical ceramic by a wide margin
- Broad supplier base rather than a concentrated one
- Excellent electrical insulation and dielectric strength
- Stable in air to very high temperature, unlike the carbides
- Well characterised with decades of design data
- Available in purities from 94 to over 99.8 percent, trading cost against performance
Limitations
- Denser and softer than silicon or boron carbide, so it loses weight-driven armor work
- Lower thermal conductivity than SiC, giving worse thermal shock resistance
- Brittle, with all the backer and handling implications that carries
- Requires diamond abrasive for any post-fired machining
- Properties vary substantially with purity grade, so specifying alumina alone is insufficient
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM D2442 | ASTM International | Alumina ceramics for electrical and electronic applications |
| MIL-I-10 | U.S. Department of Defense | Insulators, ceramic, electrical and electronic |
| ASTM C1161 | ASTM International | Flexural strength of advanced ceramics |
| NIJ Standard 0101.06 | National Institute of Justice | Ballistic resistance of body armor, where alumina is used as strike face |