Effects of liquid phases on interfacial sliding in alkali halide crystals

The effects of liquids on intrinsic interfacial sliding have been studied in NaCl crystals. The primary liquids in the study were water, methanol, and mixtures thereof. Sliding experiments were performed using a simple geometry in which a shear and normal compressive component of force were exerted on the interface. The geometry consisted of two single crystals joined at a boundary whose normal was inclined at an angle, $\theta$, to an axis along which a compressive load, P, was applied. The specimens were found to deform in two distinct ways: (1) by sliding along the interface, and (2) by indenting into one another in a direction normal to the interface. The introduction of liquids into the interface through channel-like defects was found to increase both the rate of sliding and indentation, with the increases being much greater for liquids with high water contents. It was found that the overall rate of displacement along the axis of the specimen was effectively independent of P but increased in roughly a linear fashion with $\theta$. A model for the process is developed in which displacement is produced primarily by interfacial sliding, with the liquid acting to promote the rate by undercutting the boundary and reducing the effective area of contact. The area of contact is determined by adaptations of friction theory, which lead to the observed P and $\theta$ dependence of the displacement rate. In addition, results of other experiments are presented which describe how grain boundaries in NaCl and KCl bicrystals are penetrated by water and methanol. Water is found to penetrate at much greater rates. This is discussed in terms of the differences in wetting and solubility exhibited by the two liquids. Both the sliding and penetration experiments are important in the understanding of liquid enhanced creep