The mechanism of size reduction may different with the nature of material. There each drug may required a separated or different treatment.
General mechanism described in this figure.
The theoretical strength of a crystalline material can be calculated from interatomic attractive and repulsive forces. In theoretical value strength of material is very high, generally the strength of material in reality is found to be many times lesser than the theoretical value.
Particle, may be it amorphous or crystalline, will have flaws to a definite degree. These constitute many weak parts in the particle. When sufficient ammount of stress might be impact, shear and compression are applied, the weak parts or weak flaws develop into cracks, further impact or stress the particle cleavage to smaller size and obtain smaller particle with smaller surface area.
when the stress in the form of attrition is applied, the particle surfaces chip and produce small particle.