Mechanical characterization of shale matrix minerals using phase-positioned nanoindentation and nano-dynamic mechanical analysis

The accurate determination of mechanical parameters (namely, Young's modulus and hardness) of shale-constitutive minerals is crucial for predicting the macroscale mechanical parameters of shale composites. This study employed an advanced nanoindentation apparatus equipped with a high-resolution microscope (4000 x) and a newly emerging nano-dynamic mechanical analysis (nano-DMA) module to conduct mineral-positioned indentation and investigate the depth profiles of mechanical parameters of shale matrix minerals, with the objective of obtaining their intrinsic mechanical values. To conduct mineral-positioned nanoindentation, various matrix minerals were pre-discerned under the optical microscope based on their particle shapes, surface features, and reflection colors. The mechanical response curves show that silicates (quartz and feldspar) exhibit significant elastic characteristics, whereas carbonates (dolomite and calcite) and clay minerals exhibit a combined deformation behavior of elasticity and plasticity based on the elastic recovery ratio and plastic work ratio. The mechanical depth profiles produced by nano-DMA show that the mechanical parameters of all aforementioned minerals decrease rapidly with respective to the increase in indentation depth, before reaching stable values. This variation pattern for the mechanical parameters is a result of the indentation size effect (ISE), which comes from the internal structure (or texture) adjustment of the indented material in small deformed volumes during indenter invasion. Whereas the platform section represents the indenter overcoming the ISE layer and actually probing the material as it is, the mechanical values measured at this stage can be recognized as the true values for the indented materials. In addition, the tested mineral grains would be easily affected by the substrate phases at a much larger indentation depth, and the mechanical parameters would correspondingly change in the mechanical depth profiles after the plateau stage. Overall, our findings identify reliable Young's moduli and hardnesses for different matrix minerals: 30 GPa and 1.5 GPa for clay minerals, 105-110 GPa and 14-16 GPa for quartz, 75-85 GPa and 9-10 GPa for feldspar, 70-75 GPa and 2-3 GPa for calcite, and 115-120 GPa and 7-8 GPa for dolomite.