Scanning force microscope observations of corrosive wear on single-crystal brushite (CaHPO4•2H2O) in aqueous solution

We examine nanometer-scale corrosive wear on a model biomaterial, single-crystal brushite (CaHPO4. 2H(2)O) in aqueous solution. Wear was produced by using the tip of a scanning force microscope (SFM) to provide mechanical stimulation. Monolayer deep, triangular etch pits form spontaneously on brushite {010} surfaces in undersaturated aqueous solution, providing three crystallographically distinct steps for study. Pronounced, localized wear is readily produced by drawing the SFM tip back and forth across the edges of these pits. The wear rate depends strongly on the contact force and the crystallographic 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 subsequent material removal (after linear scanning) also depends strongly on the contact force due to mechanically enhanced rates of kink propagation. Models of the bonding geometries for steps and various kinks explain several strong asymmetries in step stability and vulnerability to dissolution and corrosive wear.