Deformation by dissolution and plastic flow of a single crystal sodium chloride indenter. An experimental study under the confocal microscope

A halite cone-shaped indenter loaded against a flat silicate surface and immersed in saturated solution deforms in two distinct stages. The first stage is characterized by slow convergence rates and rapid contact area reduction, while the second stage is characterized by high convergence rates and fluctuations around a constant contact area. This sequence shows a progression of the system from a far from equilibrium state, through an overshoot point to a dynamic steady state. We propose that the evolution of the system is dictated by an interaction between two deformation mechanisms. The first is undercutting dissolution that reduces the area of the contact, and the second is probably plastic flow that increases it. Undercutting dissolution is driven by a horizontal thermal gradient and gradients in strain and surface energy. The feedback between these two mechanisms is responsible for the establishment of a steady state at the latter part of the second deformation stage, and is at the core of the free face pressure solution model. On the basis of this data we model the relation between undercutting rates at grain contacts and rates of granular compaction.