Alumina is the default technical ceramic for substrates, insulators and wear parts because it is hard, chemically stable, electrically insulating and available at scale. The purchasing decision usually comes down to one question: 96 percent or 99.6 percent. The answer is not that the higher purity is better, because the two grades solve different problems and the more expensive one can be the wrong choice. This guide sets out what actually changes between grades and how to verify the claim.
Why the grade number is not the whole story
The percentage describes the alumina content of the fired body. The remainder is a controlled mix of sintering aids, typically silica, magnesia and calcia, which form a glass phase at the grain boundaries. That glass phase is not a defect: it lowers the sintering temperature, promotes densification and raises the strength and toughness of the body relative to a very high purity material of the same grain size. What it costs is dielectric and thermal performance, because the glass phase has higher dielectric loss and lower thermal conductivity than pure alumina.
Where 96 percent alumina wins
- Cost per part at any given geometry, driven by lower sintering temperature and easier densification.
- Strength and toughness for the same grain size, because the glass phase at the boundaries relieves stress and blunts crack propagation.
- Thick film compatibility for screen printed resistors and conductors, where firing temperatures and paste systems are normally qualified against 96 percent substrates.
- Wear and structural parts such as liners, seal rings, nozzles and insulators, where dielectric loss is irrelevant.
Where 99.6 percent and higher alumina wins
- Thermal conductivity, which is markedly higher without a glass phase, and which matters for power dissipating substrates and for laser diodes.
- Dielectric loss and stability at high frequency, which is why microwave and RF substrates, filters and thin film circuits are built on high purity alumina.
- Thin film compatibility, where sputtered metallisation and photolithography need a very smooth, chemically uniform surface and a substrate that can tolerate high temperature vacuum processing.
- Contamination sensitive processes, including semiconductor equipment parts and analytical components, where leachable alkalis and silica are unacceptable.
Grades of 99.5, 99.6, 99.7 and 99.9 percent are all sold as high purity. They are not identical, and the figures that matter are the measured content, the density as a percentage of theoretical, and the size and level of porosity.
The parameters to write into the specification
1. Alumina content and impurity profile
Specify the minimum alumina content and require the supplier to report the measured value by X-ray fluorescence for each lot, together with the major impurities: silicon, magnesium, calcium, iron, sodium and potassium. A supplier who quotes a grade name without a per lot measured content is asking you to take the purity on trust.
2. Density and porosity
Density as a percentage of theoretical is the single best indicator of overall quality, because it captures sintering quality and it correlates with strength, thermal conductivity and dielectric loss. It is measured by the Archimedes method in water. Also specify open porosity, or the water absorption figure, and require it to be effectively zero for a dense substrate.
3. Grain size
Grain size controls strength through the Hall Petch relationship and it also sets the achievable surface roughness after polishing. Specify a grain size range and the measurement method, since a fine grained body and a coarse grained body of identical purity behave very differently under load and under polishing.
4. Thermal and dielectric properties
Specify thermal conductivity with the measurement method and the temperature, since values are quoted at different temperatures. For RF and microwave parts, specify relative permittivity and loss tangent at the operating frequency, together with the measurement method, because loss tangent is frequency dependent and a value quoted at one megahertz does not predict behaviour at ten gigahertz.
5. Mechanical properties
Specify flexural strength with the test method, since three point and four point bending, and different specimen sizes, give different numbers. Add hardness and elastic modulus where the part is used as a wear surface or as a structural member.
6. Geometry and surface finish
Thickness and its tolerance, flatness or camber, edge condition and surface roughness are all specifications. Surface roughness must be specified in the appropriate band with the measurement method and the evaluation length, since a root mean square value and a peak to valley value describe different risks. For thin film use, specify the polishing route and the surface defect requirement, not just the roughness number.
7. Metallisation compatibility
State the metallisation route the substrate must accept: thick film screen printing, thin film sputtering, direct bonded copper or active metal brazing. Surface finish, camber and the level of surface defects all interact with the bonding process, and a substrate that is acceptable for screen printing may be unsuitable for thin film.
Incoming inspection
- Verify alumina content on a sample per lot by X-ray fluorescence, and compare against the certificate rather than trusting the grade label.
- Check density by the Archimedes method, and check water absorption, since these catch under sintered material that passes a visual inspection.
- Measure thickness and camber across the lot, because camber is the parameter most likely to cause downstream yield loss in large substrates.
- Measure surface roughness by the agreed method on the agreed evaluation length, and inspect under illumination for pits, scratches and embedded particles.
- Check the edge for chipping and micro cracks, since edge damage propagates during thermal cycling and during assembly handling.
- Confirm grain size by a micrograph on a fracture surface or a polished and etched section, at least for the qualification lot and periodically thereafter.
Common sourcing mistakes
- Buying high purity for a thick film or wear application where 96 percent performs adequately at lower cost, and paying for a property the design does not use.
- Buying 96 percent for microwave, thin film or semiconductor equipment use, where loss, thermal conductivity and leachables matter.
- Treating 99.5, 99.6 and 99.9 as interchangeable, when they differ in content, in sintering behaviour and in cost.
- Specifying a strength figure without the test method, then comparing two suppliers whose numbers were measured differently.
- Ignoring camber and edge quality, then losing yield at the metallisation and assembly step rather than at the ceramic supplier.
- Accepting a roughness value without the evaluation length, since the same surface can report very different numbers.
Sourcing alumina substrates from Hunan and China
China produces a large share of the world technical alumina, spanning tape cast substrates, dry pressed parts and precision machined components, with a deep domestic supply base for alumina powder and sintering. Hunan hosts advanced ceramics manufacturers serving electronics, power and industrial customers in both domestic and export markets, within the province new materials industrial base. LiiFoo maintains verified supplier profiles in the advanced ceramics cluster with documented grade capability and certification.
For grade matched samples with density and roughness data, a datasheet cross reference or a metallisation compatibility review, see the cluster above or contact our sourcing team.
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