Alumina Ceramic Properties

Engineering variability

• Strength follows flaw statistics and depends on specimen size, surface finish and machining damage.
• Porosity and purity strongly affect thermal conductivity and dielectric properties.
• CTE and thermal conductivity vary with temperature.
• Design of brittle ceramic components should use Weibull/statistical strength methods when failure probability matters.

Selection note

Common for wear parts, electrical insulators, substrates, valve components and precision mechanical parts needing hardness and chemical stability.

Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.

Where it is specified

Electrical insulators and substrates (spark plugs, feedthroughs, thick-film circuit substrates), wear parts such as pump seals, valve seats and cyclone liners, grinding media, furnace tubes and thermocouple sheaths, ballistic armour plate, and biomedical implant bearings.

Purity is the specification variable and it changes the material markedly. 96% alumina contains a glassy grain-boundary phase that lowers cost, sinters easily and takes metallisation for brazing; 99.5–99.8% has better strength, wear and corrosion resistance and higher maximum temperature; 99.9% is used for optical and semiconductor duty.

Chosen over what

Against silicon carbide: alumina is far cheaper and easier to make in complex shapes, but SiC has better thermal conductivity, thermal shock resistance and hardness. Alumina wins on cost; SiC on performance.

Against zirconia: alumina is stiffer, harder and much cheaper but far less tough. Zirconia is specified when the part must survive impact or tensile stress.

Against hardened steel for wear: alumina lasts far longer against abrasive slurry but cannot take impact or bending, so it is used as a liner supported by a steel backing rather than as a structural part.

How it fails

Brittle fracture from pre-existing flaws. Alumina has no plasticity to blunt a crack, so strength is governed by the largest flaw in the stressed volume, not by a material constant. This is why ceramic strength is statistical and quoted as a Weibull modulus (typically 10–15 for alumina) alongside a characteristic strength, and why larger parts are weaker than small ones tested in the laboratory.

Thermal shock. Low thermal conductivity and moderate expansion mean a rapid temperature change generates surface tension that cracks the part. Critical quench ΔT is around 200 K — far below silicon carbide or fused silica.

Tensile and bending loads. Compressive strength is perhaps 15–20 times tensile strength, so designs must load ceramic in compression wherever possible.

Slow crack growth under sustained stress in humid environments, which makes static fatigue a real consideration for long-life parts.

Fabrication notes

Shape is set before firing. Parts are formed green by dry pressing, isostatic pressing, injection moulding or slip casting, machined in the green or bisque state where economical, then sintered at 1500–1700 °C with roughly 15–20% linear shrinkage. Predicting that shrinkage is the core difficulty in ceramic tooling.

After firing, only diamond grinding works, and it is slow and expensive. The design rule is to accept as-fired tolerances everywhere except the few surfaces that genuinely need precision.

Joining to metal is by moly-manganese metallisation followed by nickel plating and brazing, or by active-metal brazing with titanium-bearing filler. The joint must accommodate the thermal expansion mismatch, which is the usual cause of failure in ceramic-to-metal seals.

Design for ceramics means generous radii, no sharp internal corners, uniform section thickness, and compressive preload — the same discipline as designing in glass.