See title.
Depends how you define powder, I think.
What counts as a powder? Would you count the solid component of a colloidal solution of solid in a gas as a powder? If so, the answer’s probably monatomic Lithium.
Cocaine? 
graphite is fine enough to use as a lubricant.
Graphite’s lubricating properties don’t really have anything to do with particle size - it’s because its structure is of linked carbon atoms in “sheets” one atom thick - i.e. the bond between atoms in a sheet is much stronger than the bond between sheets. So the sheets can slide over one another easily.
It seems from the British Pharmaceutical Codex that finer powders are derived at will.
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- Depends on what you want to make into a powder. Cigarette smoke is a pretty-fine particle size, and it’s easy to do. Many elemental substances can be boiled into a vapor in a vacuum, and the resulting particles released from the boil can be only a few dozen atoms large for some elements (-important safety note: I have read that many ordinary substances become both highly toxic to humans if powdered on the near-atomic level, and at the same time many ordinarily non-flammable substances become highly explosive).
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- Depends on what you want to make into a powder. Cigarette smoke is a pretty-fine particle size, and it’s easy to do. Many elemental substances can be boiled into a vapor in a vacuum, and the resulting particles released from the boil can be only a few dozen atoms large for some elements (-important safety note: I have read that many ordinary substances become both highly toxic to humans if powdered on the near-atomic level, and at the same time many ordinarily non-flammable substances become highly explosive).
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This device can measure particles to a tenth of a nanometer accuracy.
Solid helium
Sintering is a process whereby exceedingly fine powders are made. Basically, what happens is that you take a powder, stick it in a paint can with a titanium ball, load it into an industrial paint mixer, and let it shake the crap out of it. (I’m simplifying the process slightly.) At present, that’s the primary method for making very fine powders, however, Nanophase Technologies has supposedly invented a process using vacuum, lasers, and helium, that can create the finest powders possible (down to individual atoms), but it is only practical for a few materials at this time.
I think I read that as a tenth of a micron. A tenth of a nanometer is an angstrom which would be quite a feat; a carbon-carbon bond is 1.3 angstoms if I remember correctly
It’s just that damned good!! Well… no, not it’s not. :o
I read it as 10 nm. The spec sheet says it can determine the size of a 0.1 [symbol]m[/symbol]m sphere to 10% accuracy, meaning 10 nm accuracy.
We recently used 50 nm polystyrene spheres for a study. They’re delivered in a water suspension, but if you evaporate the water, you’re get a powder of 50 nm spheres. Somewhat smaller sizes were available in the market, but I don’t remember exactly how small they got. (I think one company offered 5 nm ones.)
Tuckerfan: I think you’re talking about a ball mill. Sintering may involve using a ball mill but heating a substance to just under its melting point is the essence of sintering. When crystals or finely powdered solids are heated, they partially liquefy and clump together (sinter) as they approach their melting point.
Ball mills are probably the most practical way to make a very fine powder, even from a hard substance. A mortar and pestle can make fine powders too, with enough work, and particularly if an agate mortar and pestle is used.