Alumina is specified for properties no metal offers — hardness, insulation, high-temperature stability — and each of those properties is also why it is unforgiving to machine.
Alumina (Al2O3) is a technical ceramic with very high hardness, excellent wear resistance, good high-temperature capability and strong electrical insulation. It appears in semiconductor process equipment, vacuum components, insulators, wear parts and medical or analytical hardware.
Alumina grades are classified by alumina content rather than by a single alloy name:
| Grade | Al2O3 content | Typical use |
|---|---|---|
| 95% alumina | ≥ 95% | General purpose, cost-driven parts |
| 96% alumina | ≥ 96% | Common industrial ceramic |
| 97% alumina | ≥ 97% | Better strength and insulation |
| 99% alumina | ≥ 99% | High-purity parts |
| 99.5% alumina | ≥ 99.5% | High purity, wear and insulation |
| 99.7% alumina | ≥ 99.7% | High-end applications |
| 99.9% alumina | ≥ 99.9% | Ultra-high purity |
Higher alumina content generally improves dielectric strength, wear resistance and thermal performance, at higher material cost and often greater machining difficulty.
Metals deform before they break, which gives a machinist warning. Alumina does not: it has almost no plastic deformation, so it fractures at the edge and chips rather than forming a chip. The practical consequences are that material is removed with diamond tooling at small depths, that vibration and point loads are dangerous, and that process time is measured in grinding passes rather than milling passes.
Diamond tooling with small depth of cut and controlled feed; fixturing that distributes force instead of concentrating it, so edges are supported; moderate, controlled cooling rather than thermal shock; and deliberate chamfers and radii so internal corners do not concentrate stress during and after machining.