Below one micron, diamond powder stops behaving like a powder and starts behaving like a colloid. The particles agglomerate, the laser reads the clumps instead of the crystals, and a grade that looks correct on paper scratches the surface it was bought to polish. The fix is mostly in how the powder is dispersed and measured, not in the grade itself.
Where The Trouble Starts
As particle size falls, surface area per gram rises sharply. Van der Waals forces scale with that surface, and below roughly one micron they are strong enough to hold particles together in clusters that behave like much larger grains.
The cluster is not a defect in the diamond. It is a physical state. But a polishing process cannot tell the difference: a 5 micron agglomerate of 0.5 micron crystals cuts like a 5 micron particle, and leaves a scratch to match.
Why The Measurement Lies
Laser diffraction measures what is suspended in the instrument, not what is in the drum. If the sample was dispersed gently, the instrument reports the agglomerate size. If it was ultrasonicated properly, it reports closer to the primary particle size. The same powder can produce two very different distributions depending on the prep.
This is the single most common source of disagreement between a supplier’s COA and a customer’s incoming test. Neither party is necessarily wrong; they measured different things.
Dry Powder Versus Slurry
Dry sub-micron powder is the harder product to use, because the customer has to disperse it. It stores well and ships cheaply, and it gives the formulator full control over the carrier.
A pre-dispersed slurry costs more to ship and has a shelf life, but it removes the dispersion step from the customer’s process and makes the particle size much more predictable in use. For a line that is already struggling with scratches, switching from dry powder to a matched slurry is often faster than reformulating the dispersion in-house.
What Actually Disperses It
Ultrasonication is the workhorse. Power, duration and bath temperature all matter, and over-treatment can fracture the crystals as well as separate them. Mechanical milling works but changes the particle shape, which changes the cut.
Surface treatment is the other lever. A light chemical or polymer treatment changes the surface charge so the particles repel instead of attracting. It is invisible in a size report but obvious in a settling test.
Dispersant choice is system-specific: what works in a water-based slurry can flocculate in a glycol carrier. There is no universal answer, which is why the carrier should be specified before the powder is ordered, not after.
What To Ask Before Ordering
Is the quoted particle size measured after ultrasonication, and at what power and duration? Is the product supplied dry or as a slurry? Has the surface been treated, and with what chemistry? What carrier will it be used in? Can you supply a settling or dispersion test alongside the size report?
Those five answers tell you more about how the powder will behave than the grade name does. A supplier who can answer all five has measured the product in the state it will be used.
Next Step
Nano and sub-micron grades: HDYM nano diamond micron powder, HDN nano monocrystalline, HDPN nano detonation polycrystalline.
Where dispersion decides the result: optical glass polishing, SiC and sapphire wafer polishing, and slurry selection in diamond slurry and paste.