Extracting hydromechanical properties of montmorillonite quasi-crystals via nanoindentation of oriented thin films

Abstract

This paper presents the first effort to extract the hydromechanical properties of montmorillonite quasi-crystals via nanoindentation testing of highly oriented montmorillonite thin films with varying degrees of water intercalation. By preparing highly preferentially oriented montmorillonite thin films, the evolution of basal spacing, isothermal adsorption of water, microstructure, and mechanical properties of montmorillonite quasi-crystals equilibrated with different relative humidity (RH) was examined by the grazing incidence X-ray diffraction (GIXRD), sorptometry, environmental scanning electron microscopy (ESEM), and nanoindentation, respectively. Results indicate that the montmorillonite thin films exhibit depth-dependent mechanical properties across different RH. At a 40 % RH, the Young's modulus and hardness of a montmorillonite quasi-crystal consisting of ∼60 platelets are 4.85 and 0.20 GPa, which remain relatively stable upon increasing the RH to 50 % but experienced a remarkable decrease to 3.31 and 0.12 GPa when the RH reaches 65 %. Simultaneous GIXRD and isothermal adsorption results reveal the interlayer swelling as the key factor contributing to the mechanical property variation of montmorillonite quasi-crystal, while the capillarity dominates the mechanical behavior at high RH. This study shed light on the hydromechanical behavior of montmorillonite upon hydration and swelling, and paves a solid foundation for the multiscale modeling of unsaturated compacted bentonites.