Alumina ceramic
Grade-specific alumina properties, processing and joining constraints, and mechanical, thermal and electrical service limits.
Tags: materials, ceramics, alumina
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CoorsTek reports typical four-point flexural strength and thermal conductivity at 20°C of 343 MPa and 24.7 W/(m·K) for AD-96 (96% alumina) and 300 MPa and 30.0 W/(m·K) for AD-995 (99.5% alumina). The higher-purity grade conducts more heat but has lower measured flexural strength in this comparison. Use finished-part evidence for design allowables. CoorsTek Advanced Alumina chart
Identify the ceramic grade
Alumina is aluminum oxide, Al₂O₃, used in technical ceramics for electrical insulation, wear resistance and mechanical components. “Alumina ceramic” describes a material family rather than a complete engineering specification. Kyocera, alumina material overview
Specify the manufacturer, grade, nominal/minimum purity, supplied form, density/porosity requirements, dimensions, finish and acceptance tests. Purity alone does not establish strength, thermal conductivity or application suitability. A bulk wear component and an electronic substrate can require different material/process controls even when their nominal alumina percentages match.
Keep polycrystalline alumina separate from single-crystal sapphire and zirconia-toughened alumina. Kyocera lists sapphire separately and identifies ZTA as an alumina/zirconia composite. Do not transfer a composite's toughness or a single crystal's optical behavior to ordinary sintered alumina. Kyocera ceramic material families; Kyocera ZTA
Named grades: compare properties with their conditions
CoorsTek's Advanced Alumina chart, ©2023, document 01023L, gives these typical values:
| Named grade | Alumina content | Density | Four-point flexural strength at 20°C | Thermal conductivity at 20°C |
|---|---|---|---|---|
| AD-96 | Nominal 96% | 3.72 g/cm³ | 343 MPa | 24.7 W/(m·K) |
| AD-995 | Nominal 99.5% | 3.90 g/cm³ | 300 MPa | 30.0 W/(m·K) |
The manufacturer warns that manufacturing method, part size and shape affect values. These are not guaranteed design minima. CoorsTek Advanced Alumina chart
Record the method, specimen dimensions, surface preparation, temperature and statistical basis for mechanical results. Flexural strength, compressive strength, hardness and fracture toughness describe different responses. Use fracture evidence for brittle parts, where a metal-style yielding margin cannot be assumed.
Design for brittle behavior and thermal gradients
Kyocera recommends avoiding impact, tensile loading and stress concentrations, using rounded/chamfered edges where appropriate, and considering thermal-expansion mismatch with other materials. It also warns about rapid temperature changes and recommends attention to section uniformity. These are design principles, not load ratings for an arbitrary ceramic shape. Kyocera design guidance
Define contact faces, edge condition, supports and assembly load paths. A nominally compressive mounting can still create local tensile stress through uneven seating, point contact, bending or restraint. Qualify the actual fixture and clamping method rather than assigning a universal tightening torque to an alumina insert.
Thermal resistance and thermal-shock resistance are separate. Kyocera explains that rapid cooling produces surface/interior temperature differences and resulting stresses; heat transfer conditions and material properties affect the outcome. A quoted thermal-shock test cannot be converted into one permitted temperature ramp for every component. Kyocera thermal-shock explanation
Morgan's AL 995 sheet, ©2013, lists a 1725°C maximum no-load temperature. The no-load qualification is essential: this is not a loaded-component rating, a cycling-life guarantee or a safe brazed-assembly temperature. Morgan AL 995 datasheet
Assess atmosphere, duration, temperature gradients, mechanical load and attached materials together. Obtain grade- and application-specific limits for creep, corrosion and cyclic operation. A melting point or furnace-use description does not establish the allowable service temperature of a precision assembly.
Forming, machining and dimensional acceptance
Morgan describes AL 995 production by isostatic/dry pressing and green machining, with CNC grinding/lapping and metallizing capabilities. These are supplier processes for that grade, not evidence that a fired blank can be machined like aluminum. Morgan AL 995 production capabilities
Agree the manufacturing route with the ceramic supplier early. Identify which features are formed before firing and which require final machining. Put functional tolerances, datums, edge breaks and surface finish on the drawing; do not require precision everywhere without identifying its purpose.
CoorsTek's Thick-Film Ceramic Substrates Design Guide, ©2021, document 01537I, separates as-fired dimensions, laser-machined features and scribed/broken edges. Its published tolerances apply to that substrate offering and process. They are not universal tolerances for bulk alumina parts. CoorsTek substrate design guide
Specify allowable chips, cracks, porosity, camber and critical edge condition according to function and supplier capability. Qualification should represent final geometry and surface condition, including holes, slots and interfaces that concentrate stress. A polished test bar does not automatically represent an as-fired production part.
Electrical and electronic applications
The CoorsTek bulk-grade chart specifies dielectric strength for a 6.35 mm specimen, dielectric constant/loss at 1 MHz and 25°C, and volume resistivity at several temperatures. Its AD-995 resistivity is reported above 10¹⁴ Ω·cm at 25°C but 2 × 10¹⁰ Ω·cm at 500°C. Preserve the temperature and electrical test conditions with the value. CoorsTek Advanced Alumina electrical table
Do not multiply a catalog dielectric-strength value by an arbitrary wall thickness to assign a safe working voltage. Electrode geometry, surface contamination, creepage, clearance, atmosphere and thermal conditions belong in the actual insulation design and test plan. AC and DC test values from different suppliers are not directly interchangeable ratings.
For electronic substrates, CoorsTek identifies separate material/process combinations for thick-film circuitry and warns that property values depend on manufacturing and form factor. Evaluate substrate surface, thickness, metallization/paste compatibility and firing route together. A general-purpose alumina wear grade is not automatically a qualified circuit substrate. CoorsTek thick-film substrate guide
Joining, chemical exposure and release
Morgan states that AL 995 accepts molybdenum–manganese metallizing for high-temperature brazing of vacuum-tight assemblies. This is a named material/process capability, not a guarantee that any alumina/metal combination will be leak-tight. Morgan AL 995 joining description
Specify joint materials, expansion compatibility, geometry, thermal cycle and acceptance criteria with the joining supplier. Define leak testing, thermal cycling or electrical testing where required. A sound ceramic component does not establish the reliability of its braze or adhesive interface.
Chemical-resistance and low-contamination claims also need the actual medium, concentration, temperature, pressure and cleaning process. Semiconductor, food-contact, medical and vacuum application descriptions do not confer universal approval or cleanliness. Identify the grade and finished-assembly requirements directly.
Retain lot identity, source revision, manufacturing route, inspection records and final surface condition. Keep typical properties distinguishable from specification limits, batch results and demonstrated assembly performance. Qualify substitutions by their complete grade/process/service requirements rather than matching alumina percentage alone.