以分子動力模擬探討奈米顆粒的滾動阻力機制與理論

Abstract

近來無機類富勒烯奈米顆粒的極低摩擦係數受到廣泛的關注。由於其極低的摩擦係數，無機類富勒烯奈米顆粒具有優良的磨潤性質，而被作為固態潤滑劑應用。從原子力顯微鏡的實驗觀察指出此類奈米顆粒具有極低摩擦係數的主要原因為其滾動行為。為了進一步了解此議題，本研究將聚焦於奈米顆粒與基板之黏著性接觸條件下，滾動阻力的發展與機制，並探討理論與模擬結果中滾動阻力與壓力分佈的關係。對於球體與彈性半平面之接觸問題，排斥力的影響在巨觀尺度下會較為顯著，因此可以用非黏著Hertz模型描述；然而，當大小向微觀尺度趨近時，黏著力的影響會逐漸顯露，在這種情況下，黏著接觸理論會比非黏著接觸理論更為適用。對於黏著接觸問題來說，Johnson-Kendall-Roberts模型、Derjaguin-Muller-Toporov模型與Maugis-Dugdale模型是目前最有影響力的連體力學理論。由前述力學理論所推得的壓力分佈將與分子動力學模擬所得的資料相互比較。另外，本研究以分子動力學模擬作為工具，來詳細探討當奈米顆粒處於預滾動階段時接觸面上的壓力分佈，並且根據模擬結果，為奈米顆粒預滾動問題提出新的連體理論模型。

Ultra-low friction coefficients of inorganic fullerene-like nanoparticles have received widely attention recently. Due to their ultra-low friction coefficients, the inorganic fullerene-like nanoparticles have been used as solid lubricants with excellent tribological performance. Recent experimental observations from atomic force microscope indicated that the ultra-low friction coefficients are mainly due to rolling behavior of nanoparticles. In this research, we focus on investigating the development and mechanism of rolling resistance in an adhesive contact of a nanosphere on an atomically flat surface. The pressure distribution at contact and its contribution to rolling resistance in both atomistic simulations and continuum models are discussed. For the contact problem between a sphere and an elastic half-space, repulsive force dominates in the macroscopic scale as in the non-adhesive Hertz model. When the scale goes down to the microscopic level, adhesive force manifests itself and thus adhesive contact models are more applicable. Johnson-Kendall-Roberts model, Derjaguin-Muller-Toporov model and Maugis-Dugdale model have been the most influential continuum approximations for such problems. The pressure distributions from these models are compared with data derived from molecular dynamics simulations. Furthermore, pressure distributions from molecular dynamics simulations of the pre-rolling stage are discussed. Their possible implications to a novel continuum rolling model for nanoparticles are also addressed.