Nanoindentation Studies of Plasticity and Dislocation Creep of Halite

Deformation experiments on halite have collectively explored different creep mechanisms, including dislocation creep and pressure solution. Here we use an alternative to conventional uniaxial or triaxial tests, nanoindentation, to measure the hardness and creep behavior of single crystals of halite at room temperature. The hardness tests, in which strain rates were varied by over two orders of magnitude, from 2 x 10^-3 to 4 x 10^-1 s^-1 , reveal two key phenomena: 1) an indentation size effect, whereby hardness increases with decreasing size of the indents, and 2) strain rate-dependent hardness characterized by a value of the stress exponent n of 25, indicating that plasticity is the dominant deformation mechanism. Indentation creep tests, in which strain rates varied by over 7 orders of magnitude in a single experiment, from 5 x 10^-7 to 10¹ s^-1 , were also conducted. In these tests, samples were loaded at rates of 10 to 1000 mN/s to a peak load in the range 60 to 120 mN, which was then held constant for a prescribed length of time. The use of the continuous stiffness method to measure the contact stiffness allowed the stress and strain rate throughout the creep tests to be determined, for hold times ranging from 3600 to 10⁶ s. For hold times < 10⁴ s, values of the strain rate, stress, and stress exponent n show no sensitivity to loading rate or peak load, and agree quantitatively with data from the hardness tests. For holds times longer than 10⁴ s, a transition from plasticity to power law creep is observed as the stress decreases during the hold, with the latter characterized by a value of n=4.87 +/- 0.91, in excellent agreement with published values for polycrystalline halite, n = 5. We employ an existing theoretical analysis to directly compare our indentation creep data with halite flow laws derived from uniaxial experiments on polycrystalline halite, and show excellent agreement between our data and the flow laws, with the strain rate at a given stress differing by less than a factor of 1.05. Our results underscore the utility of using nanoindentation to measure creep behavior of geological materials that is directly comparable to, and in excellent agreement with, that obtained using more conventional methods.