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: 1141142
• 负责人: Robert Carpick
• 金额: $ 28万
• 依托单位: University of Pennsylvania
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-09-01 至 2015-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1141142&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 several 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。 地壳构造尺度断层上的地震成核明显受到源自断层表面之间微米级和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。这些模型再现了观测到的各种地震现象，尽管它们所依据的摩擦定律缺乏物理基础。简而言之，故障材料之间纳米级接触处发生的物理机制的身份尚不清楚。 如果没有健全的物理基础，研究人员就无法可靠地从实验室测量结果推断现有的摩擦定律到自然系统，并最终可靠地预测即将发生的地震。摩擦定律缺乏物理基础很大程度上反映了分离和研究纳米级断层接触处发生的过程的困难。 在这项变革性研究中，研究人员将采用材料科学的尖端方法，主要是原子力显微镜、纳米压痕和微米压痕，来分离岩石和自然界断层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个接触处发生的摩擦机制，而不是同时测量许多接触处的综合行为（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”的方法。为现有摩擦定律建立坚实的、基于物理的基础并限制其适用范围的方法。这项研究最终可能使他们能够确定是否能够在地震前几天到几小时检测到断层上的加速蠕变，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，通过识别和连接多个长度尺度的摩擦行为获得的见解具有远远超出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。技术说明。拟议研究的总体目标是分离和识别在包含宏观摩擦界面的纳米级粗糙接触处发生的物理机制。 更具体地说，研究人员试图回答有关现有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，摩擦力是否随着滑动率的增加而减少或增加，从而分别判断地震是否会成核？关键取决于摩擦力时间依赖性的大小，也称为摩擦老化？ 在我们之前的工作中，他们通过岩石和其他工程材料的摩擦实验建立了规范的观察结果？摩擦力随着静止接触时间的对数线性增加？可以通过 1) 在足够高的接触应力下发生触点蠕变（Goldsby 等人，J. Mater. Res., 2004）或 2）在没有接触的情况下增加触点的粘合强度（更强的化学键合）来充分地定量解释蠕变（Li et al., Nature, 2012）。解释 2 基于我们对单个纳米级二氧化硅-二氧化硅接触的原子力显微镜 (AFM) 摩擦测试（Li 等人，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中的老化程度，高达 100 倍。这种差异很容易通过接触力学模型来解释，该模型考虑了多粗糙体界面上的不均匀滑移（ Li 等人，《自然》，2012）。此外，在低 (2.2) pH 和中性 (7) pH 下对石英进行的显微压痕实验和互补摩擦实验表明，在任一 pH 下的测试之间压痕尺寸没有差异，在 pH 2.2 下的岩石摩擦测试中没有老化，但在 pH 下有强烈老化7. 这些观察结果强烈表明，老化是由于时间依赖性粘附而不是接触蠕变造成的，这一结论与普遍观点背道而驰。然而，还需要进一步的工作来确定是否存在两种机制都可以发生的条件。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摩擦老化的粘附效应。研究人员将采用AFM、界面力显微镜、纳米压痕、微米压痕和岩石摩擦实验来研究水、温度和化学环境（即pH）对粗糙体蠕变和粘附的影响。研究人员还将在透射电子显微镜中采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来实时研究塑性变形和化学键的变化。