Collaborative Research: Converging on a Physical Basis for Rate and State Friction through Nano-to-Macro-Scale Friction and Adhesion Experiments on Geological Materials

该项目的意义和重要性。地震在地球构造尺度断裂上的成核作用？S地壳明显地受控于断裂面之间微纳尺度接触的摩擦过程。地震周期通常是通过计算机模型来研究的，这些模型包含了所有的经验摩擦定律。这些模型再现了大量观测到的地震现象，尽管它们所依据的摩擦定律缺乏物理基础。简单地说，在断层材料之间的纳米级接触中发生的物理机制的身份是未知的。没有可靠的物理基础，研究人员从实验室测量到自然系统可靠地推断现有摩擦定律，并最终可靠地预测即将到来的地震的能力受到严重限制。摩擦定律缺乏物理基础，这在很大程度上反映了分离和研究纳米级断层接触过程的困难。在这项革命性的研究中，研究人员将使用材料科学的尖端方法，主要是原子力显微镜、纳米压痕和微压痕，以分离在岩石和自然界中的断层上实验中发生的摩擦机制。使用这些方法，研究人员将隔离断层表面上单一接触处发生的摩擦机制，而不是同时测量多个接触的综合行为(如在岩石上的实验室实验)。研究人员的目标是使用这种？自下而上？为现有摩擦定律建立一个强有力的、以物理为基础的基础，并禁止其适用范围。这项研究最终可能会让他们确定，他们是否能够在地震发生前几天到几个小时检测到断层上加速的蠕变，这将拯救许多人的生命，并减轻对人类基础设施的破坏。从固体力学和材料科学的科学学科的角度来看，通过识别和连接许多长度尺度上的摩擦行为所获得的见解具有远远超出地球物理的潜在应用，例如，在许多工程系统中，包括硅基微机械设备。技术说明。这项研究的主要目标是隔离和识别纳米级粗糙接触的物理机制，这些接触包括宏观摩擦界面。更具体地说，研究人员试图回答关于现有的与地震周期有关的速率和状态可变摩擦定律的最基本的问题？产生摩擦随时间变化的物理机制是什么(S)？界面的摩擦稳定性？即摩擦力是随着滑移率的增加而减小还是增大，从而地震能否孕育成核？这在很大程度上取决于摩擦的时间依赖性，也就是所谓的摩擦老化。在我们之前的工作中，他们建立了从岩石和其他工程材料的摩擦实验中观察到的典型现象？摩擦力随着固定接触时间的对数线性增加？可以用1)在足够高的接触应力下接触的蠕变来充分地定量解释(Goldsby等人，J.Mater)。研究结果，2004)或2)在没有接触蠕变的情况下，增加了接触的粘接强度(更强的化学结合)(Li等人，自然，2012)。解释2是基于我们对单纳米二氧化硅-二氧化硅接触的原子力显微镜(AFM)摩擦测试(Li等人，《自然》，2012年)。有趣的是，原子力显微镜测试中的老化程度远远大于实验室岩石摩擦实验中的老化程度，最高可达100倍。这种差异很容易用接触力学模型解释，该模型允许多个粗糙界面上的不均匀滑移(Li等人，自然，2012年)。此外，在低(2.2)pH和中性(7)pH下对石英进行的微压痕实验和互补摩擦实验表明，在这两种pH下的压痕尺寸没有差别，在pH 2.2的岩石摩擦试验中没有老化，但在pH 7的强烈老化。这些观察结果强烈地表明，老化是由于依赖时间的粘附力而不是接触蠕变，这一结论与普遍的看法相反。然而，需要进一步的工作来确定是否存在这两种机制都可能发生的条件。在这项新工作中，更复杂的实验将使我们能够区分塑性变形和粘着对摩擦老化的影响。研究人员将利用原子力显微镜、界面力显微镜、纳米压痕、微压痕和岩石摩擦实验来研究水、温度和化学环境(即pH)对粗糙度蠕变和粘附性的影响。研究人员还将在透射电子显微镜中使用复杂的原位纳米压痕，利用高分辨率成像、电子衍射、电子能量损失光谱和能量色散光谱实时研究塑性变形和化学键的变化。

Significance and importance of the project. Nucleation of earthquakes on tectonic-scale faults in the Earth?s crust is controlled, remarkably, by frictional processes that originate at micro- and nano-scale contacts between fault surfaces. The earthquake cycle is typically studied via computer models incorporating any of everal empirical friction laws. Such models reproduce a rich variety of observed earthquake phenomena, despite the fact that the friction laws upon which they are founded lack a physical basis. Stated simply, the identities of the physical mechanisms that occur at nanoscale contacts between the fault materials are unknown. Without a sound physical basis, the researchers are severely limited in our abilities to reliably extrapolate existing friction laws from laboratory measurements to natural systems, and ultimately to reliably predict approaching earthquakes. That the friction laws lack a physical basis largely reflects the difficulty of isolating and studying processes that occur at nanoscale fault contacts. In this transformative study, the researchers will employ cutting-edge methods of materials science, principally atomic force microscopy, nanoindentation, and microindentation, to isolate the frictional mechanisms that occur in experiments on rocks and on faults in nature. Using these methods, the researchers will isolate the frictional mechanisms occurring at a single contact on a fault surface, rather than measure the integrated behaviors of many contacts at once (as in laboratory experiments on rocks). The researchers aim to use this ?bottom-up? approach to establish a robust, physics-based foundation for existing friction laws and to proscribe their limits of applicability. The research may ultimately allow them to determine whether they are able to detect accelerating creep on faults days to hours prior to an earthquake, which would save many lives and mitigate damages to human infrastructures. From the perspective of the scientific disciplines of solid mechanics and materials science, insights gained by identifying and connecting frictional behavior across many length scales have potential application well beyond geophysics, for example, in many engineered systems, including silicon-based micromechanical devices. Technical description. The overarching goals of the proposed research are to isolate and identify the physical mechanisms that occur at the nanoscale asperity contacts which comprise macroscopic frictional interfaces. More specifically, the researchers seek to answer arguably the most fundamental question regarding existing rate- and state-variable friction laws as they pertain to the earthquake cycle ? What is the physical mechanism(s) that gives rise to the observed time dependence of friction? The frictional stability of an interface ? i.e., whether friction decreases or increases with increasing slip rate, and therefore whether an earthquake can nucleate or not, respectively ? depends critically on the magnitude of the time dependence of friction, otherwise known as frictional ?ageing?. In our previous work, they established that a canonical observation from friction experiments on rocks and other engineering materials ? that friction increases linearly with the log of the time of stationary contact ? can be amply explained quantitatively by either 1) creep of contacts at sufficiently high contact stresses (Goldsby et al., J. Mater. Res., 2004) or 2) increased adhesive strength of contacts (stronger chemical bonding) in the absence of contact creep (Li et al., Nature, 2012). Explanation 2 is based on our atomic force microscopy (AFM) friction tests on single nanoscale silica-silica contacts (Li et al., Nature, 2012). Intriguingly, the magnitude of ageing in the AFM tests is far larger than in laboratory friction experiments on rocks, by up to a factor of 100. This discrepancy is readily explained by a contact mechanics model allowing for inhomogeneous slip on a multi-asperity interface (Li et al., Nature, 2012). In addition, microindentation experiments and complementary friction experiments on quartz at low (2.2) pH and neutral (7) pH reveal no difference in indentation size between tests at either pH, no ageing in rock friction tests at pH 2.2, but strong ageing at pH 7. These observations strongly suggest that ageing is due to time-dependent adhesion rather than contact creep, a conclusion that runs counter to the prevailing wisdom. However, further work is required to determine if there are conditions where both mechanisms can occur. In this new work, more sophisticated experiments will allow us to discriminate between plastic deformation and adhesion effects on frictional ageing. The researchers will employ AFM, interfacial force microscopy, nanoindentation, microindentation, and rock friction experiments to investigate the influences of water, temperature, and chemical environment (namely, pH) on asperity creep and adhesion. The researchers will also employ sophisticated in situ nanoindentation in the transmission electron microscope to study, in real time, plastic deformation and changes in chemical bonding using high resolution imaging, electron diffraction, electron energy loss spectroscopy, and energy dispersive spectroscopy.