| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 102 === Nanoparticle (NP) is a nano-scale material with diameters ranging from tens to hundreds of nanometer. Treating this nanoparticle as lubricant additives can further improve mechanical performance and durability. Hence the tribological properties of nanoparticle have gained much attention and the lubrication mechanisms of nanoparticle have been considered as an important issue. The single inorganic fullerene-like nanoparticle (IF-NP) would display rolling and sliding behavior when nanoparticle is subjected to lateral displacement under low contact stress. However, it has not been revealed that whether the rolling behavior of IF-NP is one of the significant lubrication mechanisms. Meanwhile, under contact stress, adhesion hysteresis play an important role in the rolling mechanism. However, it is not clear whether such effect influence NP rolling mechanism.
The objectives of this study are twofold, first to study the friction coefficients and tribological mechanisms of a single molybdenum disulfide (MoS2) IF-NP using molecular dynamics simulation. It has been interpreted in this work that the rolling behavior is indeed a remarkable lubrication mechanism. Through these results, we can provide a guideline which will enable the design of NP with high performance of lubrication. The second objective is to address the effect of adhesion hysteresis on the onset of the rolling of nanoparticles. The results from the atomistic studies are compared with analysis from the elasticity theory.
Until now, there are few appropriate molecular dynamics simulator which can describe the Mo-S covalent bond system. In the present study, a covalent bond potential describing the interaction between molybdenum and sulfur atoms has been implemented in LAMMPS. This implementation will benefit researchers to undertake the future studies in Mo-S system. It has been reported that rolling behavior could result in significant lubricant effects. From our results, the rolling behavior could reduce the friction for about 30%, which demonstrates that the rolling behavior is a significant lubricant mechanism for MoS2 nanoparticle. In addition, we observed the repeating behaviors when nanoparticle is rolling and categorized these behaviors into two rolling patterns.
The adhesion hysteresis plays an important role in the rolling mechanism. We therefore extended the continuum theory to atomic scale for interpreting the influence of adhesion hysteresis within rolling behavior. Present study indicates that the adhesion hysteresis represented by a difference of energy release rates, is apparent at the onset of rolling. The peak value of difference of energy release rates occurs when nanoparticle starts to roll. This result has agreed well with the rolling friction theory proposed by Krijt. Therefore, we conclude that the critical difference of energy release rates is a material property for the onset of rolling. This property can be used to determine whether a given nanoparticle is easy to roll.
From our study, the tribological mechanisms have been investigated using molecular dynamics simulation. A Mo-S system bond-order potential has been implemented in LAMMPS. We conclude that the rolling behavior is certainly an important lubricant mechanism, which has been demonstrated in present study. We also observe repeating behaviors when MoS2 nanoparticle is rolling. Moreover, how the adhesion hysteresis influences the onset of rolling of nanoparticle has been revealed as well.
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