Studies Of Surface Atomic Layer Fatigue In Muscovite Mica

A typical layered crystal, muscovite mica, has attracted more attention as a source to produce two-dimensional(2D) layered materials. It has been found that the ultra-thin muscovite mica sheet has some novel electronic properties, which can significantly promote the performance of organic field-effect transistors. However, the mechanical behavior of an ultra-thin muscovite mica sheet is still thought to follow continuum principles derived for macroscopic systems. Given the mechanisms of stress generation and propagation, it might be expected that 2D layered materials may also exhibit fundamentally different mechanical behaviors. Such novel mechanical properties hold the promise for new strategies to alleviate mechanical failures in micro- and nano-electro-mechanical systems. It has been known that ~90% mechanical failures stem from material’s fatigue. As a crucial factor that leads to the failure of devices, material’s fatigue is closely associated with the durability and reliability of the system. Thus, it has become necessary to study fatigue properties of 2D layered materials. Here, this dissertation has systematically studied the surface atomic layer fatigue within a single muscovite mica sheet, based on atomic force microscopy with angstrom-scale precisions.This research has applied a defined normal load to the mica surface by an AFM probe and performed a series of independent experiments, while continuously monitoring for surface changes. These AFM studies have revealed that surface atomic layer fatigue has some novel properties. First, different from the macro/microscopic fatigue which monotonically depends on the applied normal load, atomic layer fatigue discontinuously depends on the applied stress. These striking discontinuities demarcate three unique fatigue regimes: 2.3 ?, 4.4 ? and 10 ? atomic layer fatigue. Most notably, this research has found that the fatigue lifetime of muscovite mica tends to infinity at two specific stresses: 4.17 ± 0.01 GPa and 4.84 ± 0.01 GPa. Though above the yield stress, the surface appears essentially indestructible at these values. This novel phenomenon has never been described before, so it is a distinct feature of nanoscale fatigue. Meanwhile, this study has also found a nonlinear dependence of the fatigue lifetime on the tip scanning velocity. But the velocity didn’t change the discontinuities in the normal load dependence. We also note that the depth of the atomic pits, the yield stresses, the two specific stresses, and the corresponding Basquin’s exponents remain unchanged at different sliding velocities.To our knowledge, these unique atomic phenomena have no macroscopic equivalent and are unanticipated by available fatigue models. So, this dissertation has presented theoretical inquiries on the surface atomic layer fatigue based on these AFM results. This research has proven that frictional shear stress is the dominant stress during the fatigue process by considering the origin and distribution of all kinds of stresses. Moreover, this dissertation has also designed and performed an AFM experiment to verify this conclusion. Furthermore, this dissertation speculates that the atomic holes and asymmetric charge distribution within a single muscovite mica sheet are the main reasons for the novel fatigue properties. Therefore, this research has compared the fatigue behavior of two different kinds of mica: muscovite mica and phlogopite mica, and the experimental results have demonstrated that fatigue behavior is strongly dependent on the atomic structure. Eventually, this research indicates that only under the action of frictional shear stress and the atomic holes within a single mica sheet, will there be the unique behavior “The surface appears essentially indestructible above the yield stress”, and thus surface atomic layer fatigue will significantly depend on the applied normal stress and the tip velocity. Meanwhile, it also suggests that prevention of fatigue for some materials will require strengthening of subsurface interactions, besides strengthening of topmost surface interactions.Above all, this dissertation has demonstrated that 2D layered materials have novel fatigue properties with angstrom-scale precision. At present, 2D layered materials have very extensive application prospects in electronic fields due to their novel electronic and thermal properties. Similarly, these novel mechanical properties will be helpful to understanding the physical principles operating at these length-scales and to develop reliable nano-electro-mechanical devices. Thus, this study not only has great significance for science, but also has high practical values.The main innovation of this dissertation is:(1) directly observe surface fatigue of three different atomic layers with an angstrom-scale precision in real-time, and obtain AFM height images with atomic resolution;(2) detect the unique atomic fatigue behavior “No damage happens when the normal load is beyond the yield strength”; and(3) theoretically explore the origin of these novel atomic layer fatigue behaviors, and reveal the internal relation between surface atomic layer fatigue and the dominant stress as well as the atomic structure.