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

合作研究：通过地质材料的纳米到宏观摩擦和粘附实验，汇聚速率和状态摩擦的物理基础

• 批准号: 1464714
• 负责人: David Goldsby
• 金额: $ 29.34万
• 依托单位: University of Pennsylvania
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-07-01 至 2017-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1464714&HistoricalAwards=false
• 关键词: Collaborative; Research; Converging; Physical; Basis

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.

项目的意义和重要性。地球构造尺度断层上的地震成核？值得注意的是，美国地壳受到源自断层表面之间微纳米级接触的摩擦过程的控制。地震周期通常是通过计算机模型来研究的，该模型结合了几种经验摩擦定律中的任何一种。这些模型再现了各种观测到的地震现象，尽管它们赖以建立的摩擦定律缺乏物理基础。简单地说，故障材料之间纳米级接触的物理机制的身份是未知的。如果没有坚实的物理基础，研究人员在从实验室测量到自然系统可靠地推断现有摩擦规律，并最终可靠地预测即将来临的地震方面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隔离和研究纳米级故障接触发生的过程的困难。在这项变革性的研究中，研究人员将采用材料科学的前沿方法，主要是原子力显微镜、纳米压痕和微压痕，来分离岩石和自然界断层实验中出现的摩擦机制。利用这些方法，研究人员将分离出断层表面上单个接触点的摩擦机制，而不是一次测量多个接触点的综合行为（就像在岩石的实验室实验中那样）。研究人员的目标是使用这种自下而上的方法。为现有的摩擦定律建立一个坚实的、基于物理的基础，并禁止其适用范围的方法。这项研究可能最终使他们能够确定他们是否能够在地震发生前几天到几小时检测到断层的加速蠕动，这将挽救许多生命并减轻对人类基础设施的损害。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获得的见解远远超出了地球物理学的潜在应用，例如，在许多工程系统中，包括硅基微机械设备。技术描述。提出的研究的总体目标是分离和识别发生在纳米尺度的粗糙接触，包括宏观摩擦界面的物理机制。更具体地说，研究人员试图回答有关现有的速率和状态变量摩擦定律的最基本问题，因为它们与地震周期有关。引起观察到的摩擦的时间依赖性的物理机制是什么？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，从而地震是否能成核？关键取决于摩擦的时间依赖性的大小，也称为摩擦老化。在我们之前的工作中，他们从岩石和其他工程材料的摩擦实验中得出了一个典型的观察结果。摩擦力随静止接触时间的对数线性增加？可以充分地定量解释1)接触蠕变在足够高的接触应力(Goldsby等人，J. Mater；Res., 2004)或2)在没有接触蠕变的情况下增加接触的粘合强度（更强的化学键合）（Li et al., Nature, 2012）。解释2基于我们的原子力显微镜（AFM）对单个纳米级二氧化硅-二氧化硅接触的摩擦测试（Li et al., Nature, 2012）。有趣的是，AFM测试中的老化程度远远大于实验室岩石摩擦实验，高达100倍。这种差异很容易通过接触力学模型来解释，该模型允许在多粗糙界面上进行非均匀滑动（Li et al., Nature, 2012）。此外，石英在低pH（2.2）和中性pH(7)条件下的微压痕实验和互补摩擦实验表明，两种pH条件下的压痕尺寸没有差异，在pH 2.2条件下岩石摩擦试验没有老化，但在pH 7条件下有强烈的老化。这些观察结果有力地表明，老化是由于与时间相关的粘附而不是接触蠕变，这一结论与流行的智慧背道而驰。然而，需要进一步的工作来确定是否存在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摩擦老化中的塑性变形和粘附效应。研究人员将采用原子力显微镜、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值）对粗糙蠕变和粘附的影响。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。