Alumina: structure, properties and engineering selection
A technical guide to alumina grades, mechanical and thermal behaviour, electrical insulation, processing and component qualification.
A public-source technical guide. Illustrative data are not guaranteed specifications for Robat products.
A family of materials, not one specification
Alumina is aluminium oxide, Al₂O₃. Dense engineering bodies commonly rely on α-alumina, or corundum. A sintered polycrystalline body includes grain boundaries, possible secondary phases and residual pores; single-crystal reference data are not interchangeable with commercial grade data.
Purity and relative density describe different things. Composition alone does not establish flaw population, grain structure or component reliability.
A grade-specific comparison
| Property | AD-96 | AD-995 |
|---|---|---|
| Nominal Al₂O₃ | 96% | 99.5% |
| Density, g/cm³ | 3.72 | 3.90 |
| Elastic modulus, 20°C, GPa | 303 | 370 |
| Four-point flexural strength, 20°C, MPa | 343 | 300 |
| Thermal conductivity, 20°C, W/(m·K) | 24.7 | 30.0 |
| Relative permittivity, 25°C, 1 MHz | 9.0 | 9.7 |
The density row has no separately stated temperature. Flexural results are specimen test data, not allowable component stresses.
Hardness, fracture and temperature
High hardness is not high fracture toughness. Surface, edge and internal flaws can control brittle failure; grinding conditions can introduce damage. The simplified relation KI = Yσ√(πa) explains why stress, crack geometry and flaw size must be considered together. It does not establish a safe load without valid inputs and an appropriate reliability assessment.
Room-temperature properties are not constants across the service temperature range. Thermal gradients, constrained expansion and contact stresses require component-level evaluation.
Processing and applications
Powder preparation, forming, organic removal where needed, sintering and finishing each influence the finished body. Alumina LPIM research shows that powder size, binder and solids fraction change feedstock flow. LPIM is a forming process; alumina is a material.
Wear components, insulators and electronic substrates are established application areas. This does not qualify every alumina grade for every environment. Electrical resistivity, permittivity, dielectric loss and breakdown strength are separate quantities; surface condition, thickness, temperature and frequency matter.
Specify and verify the component
Define the critical failure mode before choosing a grade. Specify composition, density/porosity acceptance, dimensions, surface finish, loads, temperature cycles, chemical exposure and test conditions. Evaluate strength distributions and inspect critical surfaces. A catalogue number does not replace qualification of the finished part.