Deformation of NaCl particle in contact at the nano-scale

Large sodium chloride particles are known to be brittle when contacted at the micron scale, depending on the presence of crack initiating defects, whereas sub-micron particles behave plastically because there is insufficient elastic energy to drive a crack through a small particle. This paper shows that, as the particles are reduced to the nanometre scale, they again become brittle because there are then insufficient atoms to propagate plastic deformations through nanoparticles. A molecular dynamics model was established to simulate the individual ionic motions in a sodium chloride particle making and breaking contact with a clean NaCl flat surface. The geometry was arranged to model an atomic force microscopy (AFM) probe tip brought into contact with a flat surface at 300 K. Both cubic and pyramidal tips were investigated. It was demonstrated that the pyramid steps tended to cause plastic deformations, but below 0.5 nm, reversible brittle fracture occurred.