Collaborative Research: A Multidiscilpinary Study to Determine the Fundamental Mechanisms of Rock Friction through Coordinated Experiments and Simulations

地震是最具破坏性的自然灾害之一，在某些情况下会造成极端的生命损失和对基础设施的难以估量的破坏。地震是沿着裂缝快速滑动的结果吗？所谓的断层--在地球内部？S内部。地震相当有规律的反复发生源于向锁定断层施加力的岩石之间的相互作用，以及断层岩石开始彼此滑动时的力学行为？即他们的摩擦行为。现有的用于地震计算机模型的岩石摩擦行为的数学描述都是基于实验的估计。如果这样的描述是基于对基本物理的基本理解，那就更好了，因为这将使它们在地震模型中的使用更加可靠，并可能导致预测地震时间的能力，而这在目前是不可能的。令人难以置信的是，地震的发生依赖于发生在断层上的非常小规模的物理和化学过程？在断层表面实际接触的微米到纳米大小的区域。这一极端范围的相关长度刻度？从千米级到纳米级--因此需要一种多学科的方法，包括对岩石摩擦行为进行实验和计算机模拟，精确到纳米级。在这个高度跨学科的项目中，摩擦学家、地球物理学家和材料科学家将融合他们的专业知识，在小尺度上研究岩石的摩擦行为，并将由此获得的摩擦机制知识与现有的岩石摩擦的数学描述相结合，以修订，如果不是取代的话，这些模型。拟议工作的最终目标是更好地了解地震过程，并最终通过应用于地震预测将这种理解转化为社会效益。该项目将促进三名博士生和几名本科生的教育，如果可能的话，他们是从代表性不足的群体中招募的。科学成果将被纳入课程作业，在材料科学、工程学和地球科学会议上报告，并在多个学科的高影响力期刊上发表。速率和状态可变的摩擦力？定律？形成了我们在实验室中对岩石摩擦行为和稳定性有限了解的基础，也是地震成核和复发模型的基础。然而，这些定律的物理基础仍然不完整到未知，这使得它们可以可靠地外推到地球上，超出了形成它们基础的实验中探索的有限条件范围，充满了不确定性。令人难以置信的是，断层规模滑动事件(地震)的成核依赖于发生在断层摩擦接触处的微观到纳米级的物理化学过程。因此，这种相关长度尺度的极端范围需要一种多学科的方法，包括对精确到纳米级的地球材料进行实验和原子模拟。该项目的目标是为岩石的摩擦行为开发基于物理的本构定律，可以放心地将其外推到地球上。为了实现这些目标，私人投资者将采用高度跨学科的方法，利用包括原子力显微镜(AFM)、纳米压痕和纳米光刻在内的广泛尖端实验方法，在广泛的长度范围内探索岩石摩擦的物理机制。至关重要的是，这些实验将与对地球材料摩擦行为起作用的关键物理过程的原子模拟相结合，并将提供信息和进行验证。具体地说，该团队将1)使用原子力显微镜、原位电子显微镜、纳米压痕和原子模拟来研究二氧化硅和石英的粘合和压痕蠕变，以确定界面化学键和塑性粗糙度蠕变在摩擦老化中所起的作用(演化效应)，2)确定直接影响的物理机制。通过将单接触AFM实验和模拟中揭示的机制外推到岩石的宏观摩擦行为，通过多接触模型，以及3)使用微观结构分析和创新的纳米光刻技术来确定加载岩石界面中弹性接触和塑性接触的相对比例，并提供单接触和多接触老化行为之间的定量联系。

Earthquakes are among the most devastating natural disasters, in some cases causing extreme loss of life and untold damage to infrastructure. Earthquakes are the result of rapid sliding along fractures ? called faults - within Earth?s interior. The fairly regular recurrence of earthquakes stems from the interaction of the rocks that apply forces to a locked fault, and the mechanical behavior of fault rocks as they begin to slide past each other ? i.e., their frictional behavior. Existing mathematical descriptions of the frictional behavior of rock, which are employed in computer models of earthquakes, are based on estimates from experiments. It would be far better if such descriptions were based on a fundamental understanding of the underlying physics, as this would make their use in earthquake models far more reliable and potentially could lead to the ability to predict the timing of earthquakes, which is impossible presently. Incredibly, the initiation of earthquakes depends on physical and chemical processes that occur at very small scales on faults ? at micrometer- to nanometer-sized regions where fault surfaces are actually touching. This extreme range of relevant length scales ? from the kilometer to the nanometer scale - thus necessitates a multi-disciplinary approach, including experiments and computer simulations of rock friction behavior down to the nanometer scale. In this highly interdisciplinary project, tribologists, geophysicists, and materials scientists will merge their expertise to study the frictional behavior of rocks at small scales, and will integrate the knowledge of friction mechanisms thus gained with existing mathematical descriptions of rock friction to revise, if not replace, those models. The ultimate goal of the proposed work is to better understand the earthquake process, and ultimately translate that understanding to societal benefit through applications to earthquake prediction. The project will further the education of three PhD students and several undergraduates, recruited from underrepresented groups when possible. Scientific results will be incorporated into coursework, reported at materials science, engineering, and earth science meetings, and published in high impact journals across multiple disciplines. Rate- and state-variable friction ?laws? form the basis for our limited understanding of the frictional behavior and stability of rocks in the laboratory, and are the foundation for models of earthquake nucleation and recurrence. The physical basis for these laws, however, remains incomplete to unknown, making a reliable extrapolation of them to the Earth, beyond the limited ranges of conditions explored in the experiments that form their basis, fraught with uncertainty. Incredibly, the nucleation of fault-scale slip events (earthquakes) depends on micro-to-nanoscopic physico-chemical processes that occur at frictional contacts on faults. This extreme range of relevant length scales thus necessitates a multi-disciplinary approach, including experiments and atomistic simulations of Earth materials down to the nanoscale. The goal of this project is to develop physically-based constitutive laws for the frictional behavior of rocks that can be extrapolated to the Earth with confidence. To achieve these goals, the PIs will apply a highly interdisciplinary approach that explores the physical mechanisms of rock friction over a wide range of length scales using a broad range of cutting-edge experimental methodologies, including atomic force microscopy (AFM), nanoindentation, and nanolithography. Critically, these experiments will be coupled with, and will inform and validate, atomistic simulations of the key physical processes that contribute to the frictional behavior of Earth materials. Specifically, the team will 1) Study adhesion and indentation creep of silica and quartz using AFM, in situ electron microscopy, nanoindentation, and atomistic simulations to determine the role that interfacial chemical bonding and plastic asperity creep play in frictional aging (the ?evolution effect?), 2) determine the physical mechanism underlying the ?direct effect? by extrapolating the mechanisms revealed in single-contact AFM experiments and simulations to macroscopic friction behavior of rocks via multi-contact models, and 3) use microstructural analyses and innovative nanolithographic techniques to determine the relative proportions of elastic and plastic contacts in loaded rock interfaces, and provide quantitative links between single-contact and multi-contact aging behavior.