Cyclic nanoindentation shakedown of muscovite and its elastic modulus measurement

Abstract

A series of cyclic loading nanoindentation experiments with varied maximum loads (Fmax) of 0.05 to 2.0 mN were performed on a nanostructured, layered muscovite with loading direction normal to its basal plane. A critical load (e.g., 0.5 mN) exists that leads to distinct load-displacement curves: when Fmax is greater than this load, the loading/unloading curves, after a few initial cycles, become characteristic closed hysteresis loops, suggesting that shakedown process occur quickly; otherwise, only nonlinear elastic, completely overlapped hysteresis loops were observed. These phenomena result in two representative elastic moduli that depend on indentation depth. For F max (e.g., 0.05 and 0.1 mN) smaller than the critical load, the obtained elastic modulus at the nonlinear elastic state is nearly 88 GPa, which agrees with the reported Young's modulus of the material; However, when F max exceeds the critical value, the measured modulus decreased to a lower constant value around 55 GPa, close to the bulk modulus of muscovite. The transition from a higher, true Young's modulus to a lower bulk modulus can be attributed to the three dimensional confinement around the indenter tip after plastic shakedown at relatively larger depth, where significant alteration to the originally layered structure of muscovite has taken place.