Lekhraj Verma Google 'comminution limit' and take a look at these webinars (free registration required):

Controlling powder bulk density by optimizing particle size and shape distribution

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W161006PowderParticleSizeShape

Adhesion and cohesion

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W180725AdhesionCohesion

Comminution - history and laws

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W100802MaterialComminution.html

Good luck with your research.

Alan Rawle provided good background - would like to add that if you can cryo-mill at -78 C (dry ice) or 77 K (liquid nitrogen) it can alter some behaviors of the material being milled.

No, ball milling is generally not very effective for reducing the particle size of ductile materials. Due to their plastic deformation behavior, ductile materials tend to flatten and cold-weld rather than fracture, which leads to particle agglomeration instead of size reduction

Try cryogenic ball milling (milling at very low temperatures using nitrogen) it can reduce ductility temporarily, or add a process control agent

Lekhraj Verma A slide from the last webinar I mentioned above. Another comment is that milling Zn metal in air will produce ZnO from heating and oxidation plus the metal will plastically deform and reform as the size is reduced. A limit is reached...