Deformation in great detail: A nanoindentation workflow for investigating low-temperature plasticity in silicate minerals

Nanoindentation is a mechanical testing technique used to quantify material properties (e.g., hardness) and deformation behaviour (e.g., plasticity). By controlling the indenter tip with great precision in all dimensions, the range of available methods can be expanded to include rapid property mapping, constant-stiffness stress-strain curves, topography mapping, and scratch and frictional testing. We have set up a complete, affordable, and fast workflow centred around nanoindentation with a Bruker Hysitron TriboIndenter 990 and complemented by electron microscopy to tackle a variety of research questions concerning the behaviour of earth materials.

This contribution showcases this workflow as applied to several common rock-forming high-pressure metamorphic minerals. We begin with first-order sample characterisation of thin sections using optical microscopy and electron backscatter diffraction to quantify crystal orientations to determine which crystal axes are being indented. Transitioning to the mechanical testing phase, we use spherical tips to obtain stress-strain curves to analyse the transition from elastic deformation to low-temperature plasticity, and to quantify the yield hardness. Stress-strain curves can be calculated from regular constant loading rate indentation experiments, only valid within the elastic domain, or with constant stiffness measurements using tip oscillations to provide a full stress-strain curve including plastic behaviour. We image the residual indent sites with surface probe mapping, which measures surface topography with a vertical resolution down to 0.1 nm and thus produces 3D maps with which we can quantify the dimensions and geometries of indent pits.

The results of our case studies on glaucophane, omphacite, and garnet show that plastic yielding is controlled by the availability of nucleation points for dislocations, provided by pre-existing defects. The degree of this effect varies per mineral, and further depends on crystal orientation. Overall, we demonstrate an efficient workflow for mechanical and microstructural characterization of low-temperature plasticity with nanoindentation applicable to most silicates and other minerals. This workflow can also be adjusted to analyse and quantify many other aspects of the properties and behaviour of earth materials.