Scanning force microscope observations of corrosive wear on single-crystalbrushite (CaHPO4 center dot 2H(2)O) in aqueous solution

We examine nanometer-scale corrosive wear on a model biomaterial, single-cr ystal brushite (CaHPO4. 2H(2)O) in aqueous solution. Wear was produced by u sing the tip of a scanning force microscope (SFM) to provide mechanical sti mulation. Monolayer deep, triangular etch pits form spontaneously on brushi te {010} surfaces in undersaturated aqueous solution, providing three cryst allographically distinct steps for study. Pronounced, localized wear is rea dily produced by drawing the SFM tip back and forth across the edges of the se pits. The wear rate depends strongly on the contact force and the crysta llographic orientation of the step. We analyze the length of the wear track in terms of the rate of mechanically enhanced double-kink nucleation (the removal of ions from initially complete or intact steps). The rate of subse quent material removal (after linear scanning) also depends strongly on the contact force due to mechanically enhanced rates of kink propagation. Mode ls of the bonding geometries for steps and various kinks explain several st rong asymmetries in step stability and vulnerability to dissolution and cor rosive wear.