Milling Mentor: Size reduction -- It's just not that simple
Grinding large particles to small ones can be a complicated process. It's not as simple as just dumping some powder through a grinding mill. Multiple variables in both the equipment and the particle characteristics can cause some surprising results.
The principle of particle size reduction follows crack propagation theory. During World War I, engineer A.A. Griffith developed the ideas of fracture mechanics of brittle materials. His theory was that minor flaws or cracks occur on a surface; when stress is applied to the surface, the stress concentrates at the flaws and extends them, causing failure (breakage). An example can be seen in automobile windshields: If you've ever had a tiny chip in your windshield, you might have had the misfortune of experiencing crack propagation. That tiny chip is a flaw. Any stress applied to the windshield, such as a high wind load or thermal expansion in winter, will concentrate on the flaw. The windshield will crack, sometimes into an elaborate spiderweb, and the starting point will have been the tiny chip.