Guides / Materials
Alumina 96, 99.5 or 99.8: Paying for Purity You Do Not Use
April 6, 2026
Alumina is the default technical ceramic, and the number in front of it decides more than most drawings admit. A part specified as 99.8% when 96% would do is not a safer part. It is the same part with a longer lead time, a smaller choice of stock blanks and a higher bill.
Purity here means the aluminium oxide content by weight. The remainder is a glassy phase, usually silica with magnesia and calcia, that helps the powder sinter dense at a lower temperature. That glassy phase is the reason the grades behave differently, and it is not always the enemy.
What actually changes with purity
| Property | 96% alumina | 99.5% alumina | 99.8% and above |
|---|---|---|---|
| Glassy grain boundary phase | Around 4% | Under 0.5% | Trace |
| Plasma and halogen resistance | Modest | Good | Best of the aluminas |
| Dielectric strength | Good | Higher | Highest |
| Metallising and brazing | Straightforward | Needs a tuned process | Usually active metal brazing |
| Stock availability | Widest | Good | Limited, often made to order |
| Relative blank price | Baseline | Step up | Larger step up |
Hardness and stiffness move much less than buyers expect. If your part is a wear plate or a fixture, going from 96% to 99.5% buys you very little mechanical performance for the money.
Where high purity earns its price
Plasma exposure. In an etch or deposition chamber the glassy phase is attacked first. It erodes, releases particles and roughens the surface, which is exactly what a fab does not want above a wafer. This is why plasma facing parts are specified at 99.5% and above, and why some are coated with yttria on top of that.
Dielectric work at voltage. The grain boundary phase lowers breakdown strength and raises loss. For high voltage feedthroughs, RF windows and insulators that sit near a hot electrode, purity is a functional requirement.
Chemical service. Strong acids and alkalis attack the glassy phase before they touch the alumina grains. In a pump or valve running aggressive chemistry, the purer grade lasts longer even though both look identical on a hardness chart.
Contamination limits. Medical, analytical and semiconductor buyers often care less about the headline percentage than about what the remaining fraction is. Sodium, iron and calcium limits belong on the drawing if they matter.
Where 96% is the better engineering answer
Metallised and brazed assemblies. The classic moly-manganese metallising process relies on the glassy phase migrating into the metallised layer to form the bond. On a very pure alumina there is little glass to work with, so shops either add a glass frit or move to active metal brazing with a titanium bearing filler. Both are viable and both cost more. If your part is a hermetic feedthrough with no plasma or high voltage requirement, 96% is often the sensible specification.
Structural and wear parts. Fixtures, standoffs, guides, nozzles and wear plates rarely see the conditions that punish the glassy phase. Buyers who specify 99.5% out of habit here are paying for nothing.
Thermal shock. Neither grade is good at thermal shock, but the purer grades are marginally worse because of their larger grain size in some sintering routes. If the part cycles hard and fast, the real answer is usually silicon nitride rather than a purer alumina. See silicon carbide vs silicon nitride.
Density and grain size matter as much as purity
Two 99.5% aluminas from different suppliers can behave differently. What separates them is density, expressed as a percentage of theoretical, and average grain size.
- A denser part has fewer pores, which means fewer places for a crack to start and a better ground finish.
- Finer grain gives higher strength and a cleaner edge when ground, which matters on thin walls and sharp features.
- Coarse grain erodes faster under plasma and generates more particles.
If your application is sensitive, put density and grain size on the drawing as a minimum, not just the purity. A shop that machines ceramic every week will know which blank meets it.
How to specify it without overpaying
- Start from the failure mode. Is it erosion, breakdown voltage, chemical attack, wear or just structure?
- Specify purity only where that failure mode is driven by the glassy phase.
- Add density and grain size if the part is plasma facing, sealing or thin.
- Say if the part will be metallised or brazed later, because that can argue for a lower purity.
- Ask whether the grade is available as stock plate, rod or tube in a size near your part. Grinding 5 mm off a blank because nothing closer exists is a real cost. This is covered in what drives ceramic machining costs.
If you are not sure
Describe the environment instead of the grade. Temperature, chemistry, voltage, load, vacuum level and whether the part is joined to metal are enough for a shop to propose a grade and tell you what it will cost. Our buyer guide to machined ceramic parts covers the rest of the RFQ.
Purity is one decision among five families and several routes; see ceramic material selection for where it sits. A purity figure alone does not fix the body; see how to write a ceramic material specification line.
See also: the 99.5% alumina reference page.
Send your drawing and we match it with up to 3 shops that grind that grade as routine work. Reply within 24 business hours.
Frequently asked questions
Is 99.8% alumina always better than 96%?
No. The glassy phase in 96% alumina is what makes moly-manganese metallising straightforward, so a brazed feedthrough is often better in 96% than in a purer grade. Purity is only better where plasma, corrosion or dielectric strength is the limit.
How much more does high purity alumina cost?
Expect a step up in blank price moving from 96% to 99.5%, and a larger one again to 99.8% and above, partly because stock sizes are fewer so more material gets ground away.
Which alumina grade is standard for semiconductor parts?
99.5% and above is the usual starting point, with 99.6% to 99.8% common for plasma facing parts. Fabs specify purity, density, grain size and often metal contamination limits, not just the name alumina.
Need a quote for this part?
Send the drawing. We match you with up to 3 shops that grind this grade and reply within 24 business hours.
Get matched with a ceramic shop