A diamond powder certificate lists a dozen numbers: D3, D10, D50, D90, D97, TAPS, specific surface area. Buyers look only at D50 — "the average size." This is a serious mistake. Two powders with identical D50 of 5 μm can behave completely differently during polishing, because one has D97 = 8 μm and the other D97 = 15 μm. It is the distribution "tail," not the median, that determines whether the powder leaves scratches on an optical surface.
What D50, D10 and D97 Actually Mean
D-values are percentiles on the cumulative particle size curve. D50 means 50% of particles (by volume) are smaller than the stated size. It is the median of the distribution, not the arithmetic mean.
| Parameter | Physical meaning | Why it matters |
|---|---|---|
| D3 / D10 | 3–10% fine fraction | Too much "dust" → fast wear, high load |
| D50 | Median size | Sets material removal rate |
| D90 | 90% of particles smaller | Coarse upper bound estimate |
| D97 | 97% of particles smaller | Critical for optics: rare large grains = scratches |
Why D97 Matters More Than D50 for Precision Polishing
When polishing optical glass or sapphire, it is the rare large particles (3 out of 100) that create scratches. These scratches are visible under reflection and make the part scrap. That is why optics grades specify not D50 but D97 — the upper bound on coarse inclusions.
TAPS: The Packing Density Everyone Forgets
TAPS (Tap Density) is the density of powder after standard tapping of a graduated cylinder, in g/cm³. It correlates directly with particle shape and size distribution: the rounder the shape and the broader the distribution, the higher the TAPS.
| Powder type | Typical TAPS | Application |
|---|---|---|
| Sharp diamond W3.5–W5 | 1.6–1.9 г/см³ | Rough grinding of carbides |
| Semi-rounded M40/M42 | 1.9–2.2 г/см³ | Intermediate lapping |
| Spherical ACM | 2.2–2.6 г/см³ | Final optical polishing |
How to Compare Two Certificates
Do not compare D50 alone. Full comparison: D97 (scratch risk), D10 (fines), TAPS (packing), and specific surface area (m²/g). A 10% difference in D97 between "equivalent" powders from different suppliers is the difference between a scratching and a non-scratching polish.
DIAMTECH provides a full certificate for every batch listing D3/D10/D50/D90/D97/TAPS and specific surface area — request a sample and verify yourself.