Two powders can share the same D50 and behave completely differently. D10 and D90 describe the width of the distribution around that middle value, and the width is usually what decides your surface finish, your scratch rate and your wheel life. Reading only D50 is the most common mistake in diamond powder purchasing.
What The Three Numbers Measure
D10 is the particle size below which ten percent of the volume sits. D50 is the median: half below, half above. D90 is the size below which ninety percent of the volume sits.
Note the word volume. These are volume-based figures, normally from laser diffraction, not counts of particles. That matters: one large particle carries far more volume than many small ones, so a small tail of oversize grains moves D90 much more than you would expect from looking at a microscope image.
D50 Is Not The Grade
D50 is the number buyers compare first, and it is the least informative on its own. A powder with D50 of 3 microns and a narrow distribution and a powder with D50 of 3 microns and a long coarse tail are the same number by that measure. They are not the same product.
The second one will scratch. The coarse tail contains particles several times the median diameter, and each of those cuts a groove that no amount of fine powder will polish out afterwards.
What D10 Controls
D10 controls the fine end, and the fine end controls how well the powder refines the surface. A low D10 means more very fine particles doing the final smoothing. That is helpful in polishing, and it can be a problem in grinding, where the fine fraction loads the bond and reduces cutting efficiency.
On a polishing slurry, D10 is the number that quietly sets your achievable Ra. On a bonded wheel, it is closer to a nuisance than a benefit.
What D90 Controls
D90 is the scratch number. It is the size of the largest particles you are sending into the contact zone, and it is directly tied to the deep scratches you cannot remove with the next finer step.
If you are seeing scratches that survive the polish, check D90 before you change anything else. A tighter D90, at the same D50, is often the whole fix.
Distribution Width
The width is usually expressed as a span: D90 minus D10, divided by D50. A narrower span means a more uniform powder. A wider span means a mixture that covers a range of sizes.
Narrow is not automatically better. A deliberately wide distribution can cut faster in a roughing step, because it engages several mechanisms at once. Narrow is what you want when the process is finishing, when the surface is the specification, or when the next step is expensive.
A distribution with two distinct peaks deserves a question. It can be a legitimate blend made on purpose, or it can be two batches that were combined. The COA will not always tell you which, so ask.
How To Read The COA
A particle size report without the measurement method is not comparable to anything. Laser diffraction results depend on the instrument, the dispersion method and the optical model. Sieve results depend on the sieve stack. Put two COAs side by side and the method line is the first thing to check.
Our own practice is to name the method next to each figure and to report the full distribution curve, not just three points. Three points can be reproduced from a curve; a curve cannot be reconstructed from three points.
What To Ask Before You Order
Ask for D10, D50 and D90 with the method stated. Ask for the span, or compute it yourself. Ask whether the figures come from a reference sample or from the batch you will receive. And if the surface is the specification, ask for a morphology image of the actual batch.
Those four questions separate a specification from a marketing sheet.
Next Step
Fine and narrow for finishing: HDY W50 to W1. Standard micron grades with full distribution reporting: HDG. Premium tier where the surface is critical: HDH.
Process context: optical glass grinding and polishing, SiC and sapphire wafer polishing, and the measurement side in quality control and PSD.