04Precision ceramics
Alumina plates, zirconia rings, SiC nozzles and machinable glass-ceramic fixtures. The pressing is the cheap half — the diamond grinding is where the tolerance, the price and the yield are actually decided.
01Materials
Ceramics are not interchangeable, and the wrong choice cannot be machined back. Alumina for cost and insulation, zirconia for toughness, SiC for wear and thermal shock, AlN when heat has to leave — and machinable glass-ceramic when you need three prototypes this week.
Table 1 · Technical ceramic properties, typical values
| Material | Density g/cm³ | Hardness HV | Flexural MPa | KIC MPa·m½ | CTE 10⁻⁶/K | k W/m·K | Max use °C, air |
|---|---|---|---|---|---|---|---|
| Alumina 99.5% Al₂O₃ | 3.90 | 1,600 | 360 | 4.0 | 7.2 | 30 | 1,650 |
| Alumina 96% cost grade | 3.70 | 1,600 | 300 | 3.5 | 6.5 | 25 | 1,600 |
| Zirconia 3Y-TZP ZrO₂ | 6.05 | 1,250 | 1,000 | 8.0 | 10.0 | 2.5 | 1,000 |
| ZTA alumina-toughened | 4.20 | 1,450 | 600 | 6.0 | 8.0 | 22 | 1,500 |
| Silicon nitride Si₃N₄ | 3.20 | 1,500 | 800 | 6.5 | 3.1 | 25 | 1,200 |
| Silicon carbide SSiC | 3.10 | 2,400 | 450 | 4.0 | 4.0 | 120 | 1,600 |
| Aluminium nitride AlN | 3.30 | 1,150 | 350 | 3.0 | 4.6 | 180 | 1,000 |
| Machinable glass-ceramic Macor-type | 2.52 | 250 | 100 | 1.5 | 9.3 | 1.6 | 800 |
Scroll the table sideways for all columns →
Why AlN is the exception. Its thermal conductivity is roughly six times alumina's — the reason it appears in LED bases, RF packages and heat spreaders. It is also brittle (KIC ~3) and expensive to grind, so it is specified only when the heat path is the problem.
02Tolerances
A ceramic part leaves the kiln 15–20% smaller than the mould and nowhere near final tolerance. Everything after that is diamond. The three columns below are three different processes, not three quality levels.
Table 2 · Achievable tolerance by process stage
| Attribute | As sintered net shape | Diamond ground standard | Lapped / polished precision |
|---|---|---|---|
| Dimensional tolerance | ±0.5% or ±0.2 mm | ±0.010 mm | ±0.005 mm |
| Flatness | 0.05 mm | 0.005 mm | 0.001 mm |
| Parallelism | 0.10 mm | 0.010 mm | 0.003 mm |
| Surface roughness Ra | 1.5–3 µm | 0.4 µm | 0.1 → 0.02 µm |
| Hole tolerance | ±0.2 mm | ±0.020 mm | ±0.010 mm |
| Minimum wall | 1.5 mm | 0.5 mm | 0.3 mm |
| Edge chipping | ≤0.5 mm | ≤0.2 mm | ≤0.05 mm |
| Typical use | Furnace furniture, spacers | Fixtures, seals, nozzles | Wafer contact, metrology, sealing faces |
Scroll the table sideways for all columns →
Grinding cost is not linear in tolerance. Going from ±0.05 mm to ±0.010 mm is a modest step; going from ±0.010 to ±0.005 roughly doubles the time again, because it usually means a second fixturing and a lapping operation. That is worth knowing before the drawing is frozen.
03Written into the order
Because the material is cheap and the machining is not, most of the risk sits in three places that a one-line price never mentions.
“99% alumina” is a purity claim, not a property. Density and flexural strength are what you actually design against — and they vary between one powder supplier and the next.
A flatness callout without a datum is unmeasurable on a flexible thin plate. On ceramics thinner than 2 mm, how the part is supported during measurement changes the reading.
Ceramic fails at edges. A part that measures perfectly and chips at the seal face leaks — so the acceptance number for edge damage belongs in the PO.
04Qualification
A ceramic workshop can be excellent at pressing and mediocre at grinding, or the reverse. The checks below separate the two, because the price you are quoted is almost always driven by the second.
Pressing, sintering and grinding in one plant — or pressing here and grinding subcontracted. Both models work; the second needs a named grinding shop and a tolerance conversation before quoting.
Archimedes density per batch, not a datasheet figure. Under-sintered bodies machine beautifully and fail in service, and the only early signal is density a few percent low.
On a surface plate, on three points, or on a CMM with a defined support. For thin plates the three answers differ — so the method must be written down, not assumed.
Chipping is judged at 10–20×, photographed, and counted against the PO number. “Small chips acceptable” is how a seal face becomes a leak two months later.
A worn wheel burns the surface and pulls grains; the part measures right and the Ra is wrong. We look at wheel logs on the machines running our parts.
Three parts, full CMM report, sectioned if the geometry is internal. Cheaper than discovering a shrinkage error after a thousand pieces are pressed.
05Where it goes wrong
“99.7% alumina” sounds precise and tells you almost nothing about strength. Two bodies of the same purity can differ by 30% in flexural strength depending on grain size and sintering aids.
A 0.5 mm alumina plate measured flat on a granite block and measured flat in free air are different parts. If the drawing does not say how, the supplier will choose the method that passes.
Glass-ceramic is superb for prototypes and vacuum fixtures, and wrong for wear parts: hardness is a fraction of alumina's and it generates dust. Cheap in ones, expensive in thousands.
Other categories
The parts these fixtures and spacers are built to hold.
View category 03 · WafersSilicon, SiC and sapphire with flatness and particle data.
View category All fivePiezoelectric and scintillation crystals, plus PFA carriers.
Back to overviewWear, heat, HF, 1,600 °C, or a seal that must not leak — the duty decides the body, and the body decides the price. We will come back with both, and with the grinding route written down.