Origins of Folding Instabilities on Polycrystalline Metal Surfaces
Origins of Folding Instabilities on Polycrystalline Metal Surfaces
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DOI：
10.1103/physrevapplied.2.064004
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发表时间：
2014-12-04
影响因子：
4.6
通讯作者：
Gumbsch, P.
中科院分区：
文献类型：
--
作者：
Beckmann, N.;Romero, P. A.;Gumbsch, P.
多晶金属表面材料的磨损和去除本质上与塑性流动有关。通过大规模分子动力学模拟和原子力显微镜划痕实验，研究了在纳米晶铜表面耕作诱导的无约束表面塑性流动。与实验结果一致的是，在模型凸起处形成涡状褶皱，标志着层流的中断。我们认为错位介导的塑性流动在具有适当取向滑移系统的晶粒中是胀形和褶皱形成的基本机制。观察到的褶皱可以从根本上解释为多晶表面塑性的不均匀性，这种不均匀性有利于在具有适当取向滑移系统的晶粒上形成凸起。这一过程明显不同于流体中的开尔文-亥姆霍兹不稳定性，后者先前被认为类似于形成的表面褶皱模式。生成的船头生长成粗糙的切屑，有分层的薄片，被认为是磨损碎屑的前兆。我们的发现证明了表面纹理和晶粒结构工程对于实现金属的超低磨损的重要性。
Wear and removal of material from polycrystalline metal surfaces is inherently connected to plastic flow. Here, plowing-induced unconstrained surface plastic flow on a nanocrystalline copper surface has been studied by massive molecular dynamics simulations and atomic force microscopy scratch experiments. In agreement with experimental findings, bulges in front of a model asperity develop into vortexlike fold patterns that mark the disruption of laminar flow. We identify dislocation-mediated plastic flow in grains with suitably oriented slip systems as the basic mechanism of bulging and fold formation. The observed folding can be fundamentally explained by the inhomogeneity of plasticity on polycrystalline surfaces which favors bulge formation on grains with suitably oriented slip system. This process is clearly distinct from Kelvin-Helmholtz instabilities in fluids, which have been previously suggested to resemble the formed surface fold patterns. The generated prow grows into a rough chip with stratified lamellae that are identified as the precursors of wear debris. Our findings demonstrate the importance of surface texture and grain structure engineering to achieve ultralow wear in metals.