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.

该项目的意义和重要性。 地震中地震上地震的成核是通过摩擦过程来控制的，这些过程源于断层表面之间的微观和纳米级接触。通常通过结合几种经验摩擦的任何计算机模型来研究地震周期。尽管事实上建立的摩擦定律缺乏物理基础，但这种模型还是重现了许多观察到的地震现象。简而言之，断层材料之间纳米级接触的物理机制的身份尚不清楚。 没有合理的身体基础，研究人员在我们的能力上受到了严格的限制，从实验室测量到天然系统，最终可靠地预测地震。摩擦法律缺乏物理基础，在很大程度上反映了隔离和研究纳米级断层接触时发生的过程的困难。 在这项变革性的研究中，研究人员将采用材料科学，主要是原子力显微镜，纳米识别和微观识别的最先进方法，以隔离在自然界岩石和断层的实验中发生的摩擦机制。使用这些方法，研究人员将分离出在断层表面的单个接触处发生的摩擦机制，而不是一次测量许多接触的综合行为（如岩石上的实验室实验中）。 研究人员的目标是使用它？自下而上？为现有摩擦法建立强大的，基于物理的基础的方法，并禁止其适用性限制。这项研究最终可能使他们能够确定地震前几天到几个小时的断层上是否能够发现加速蠕变，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，通过识别和连接许多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，包括基于硅的微机械设备。技术描述。拟议的研究的总体目标是隔离和确定纳米级垂体接触的物理机制，包括宏观摩擦接口。 更具体地说，研究人员试图回答与地震周期有关的现有速率和州可变性法律的最基本问题？产生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，摩擦是随着滑移速率的增加而降低还是增加，因此地震能否分别分别核定？批判性地取决于摩擦的时间依赖性的大小，否则称为摩擦？ 在我们以前的工作中，他们确定了对岩石和其他工程材料的摩擦实验的规范观察？这种摩擦随固定接触时间的日志线性增加吗？可以通过1）在足够高的接触应力下进行定量解释（Goldsby等人，J。Mater。Res。，2004）或2）在没有接触蠕变的情况下，接触的粘附力强度提高了（Li等人，自然，2012年）。解释2基于我们的原子力显微镜（AFM）摩擦测试，对单纳米二氧化硅触点（Li等，Nature，2012）。有趣的是，AFM测试中老化的幅度远大于在岩石上的实验室摩擦实验中，高达100倍。这种差异很容易通过接触力学模型来解释，允许在多种耐受性界面上进行不均匀的滑移（Li等人，自然，2012年）。此外，在低（2.2）pH（2.2）和中性（7）pH值下对石英的微观调节实验和互补摩擦实验表明，这两种pH值的测试之间的凹痕尺寸均无差异，在pH 2.2处的岩体摩擦测试中没有老化的衰老，但在pH 7处的强劲衰老。这些观察力强烈表明，这些观察力强烈地表明，衰老是由于时间依赖性而不是依赖于时间依赖的粘附，这是一个criperive的次数。但是，需要进一步的工作来确定是否有两种机制发生的条件。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对摩擦衰老的粘附效应。研究人员将采用AFM，界面力显微镜，纳米识别，显微识别和岩石摩擦实验来研究水，温度和化学环境（即pH）对浅层蠕变和粘附的影响。研究人员还将使用高分辨率成像，电子衍射，电子能量损失光谱和能量分散光谱法实时研究透射电子显微镜中的精致原位纳米引导。