Role of oxygen functional groups in the friction of water-lubricated low-index diamond surfaces
Role of oxygen functional groups in the friction of water-lubricated low-index diamond surfaces
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DOI：
10.1103/physrevmaterials.2.073606
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发表时间：
2018-07-18
影响因子：
3.4
通讯作者：
Moseler, Michael
中科院分区：
文献类型：
--
作者：
Kuwahara, Takuya;Moras, Gianpietro;Moseler, Michael
大规模的量子分子动力学模拟揭示了水润滑低指数金刚石表面的八种摩擦机制。其中有四种摩擦机制是普遍存在的，即它们发生在钻石(111)、(001)和(110)上。干滑动导致立即冷焊并伴有非晶化(区I)。少量的水(低于8H2O/nm(2))可以通过局部剪切界面乙醚基团来保持结晶度和降低摩擦(区域II)。水表面密度的进一步增加导致氢/羟基层钝化(制度IV)，最后(超过20H2O/nm(2))在氢/羟基钝化钻石表面之间的自由水层(制度V)。其他四种摩擦机制是特殊的，即它们只发生在某些表面上。通过在金刚石(111)表面进行芳香族Pandey表面钝化，建立了一个超低摩擦区域(区域III)。在金刚石(110)表面，制度II与其他三种制度共存：通过C-C键(制度VI)或C-O-C键(制度VII)进行部分冷焊会产生介于冷焊制度(I和II)和非冷焊制度(IV和V)之间的摩擦剪应力，而由酮基和乙醚基团组成的氧化碳单分子层的形成则导致超低摩擦(制度VIII)。对于金刚石(001)表面，也观察到了第八区。这些发现被不同低指数表面的结构和能量特性所合理。我们的研究为水或其他含氧润滑剂的边界润滑中碳表面含氧官能团的纳米级控制和操纵提供了指导。
Large-scale quantum molecular dynamics simulations unveil eight friction regimes of water-lubricated low-index diamond surfaces. Four of these friction regimes are universal, i.e., they occur on diamond (111), (001), as well as (110). Dry sliding leads to immediate cold welding accompanied by amorphization (regime I). Small amounts of water (less than 8 H2O per nm(2)) can preserve crystallinity and lower friction by localizing shear to interfacial ether groups (regime II). A further increase in water surface density results in passivating hydrogen/hydroxyl layers (regime IV) and finally (for more than 20 H2O per nm(2)) in free water layers between hydrogen/hydroxyl passivated diamond surfaces (regime V). The other four friction regimes are special, i.e., they occur only on certain surfaces. An ultralow friction regime is established by aromatic Pandey surface passivation on diamond (111) surfaces (regime III). On diamond (110) surfaces, regime II coexists with three other regimes: while partial cold welding via C-C bonds (regime VI) or C-O-C bonds (regime VII) leads to frictional shear stresses that are in-between the cold-welding regimes (I and II) and the non-cold-welding regimes (IV and V), the formation of an oxidized carbon monolayer consisting of keto and ether groups results in ultralow friction (regime VIII). Regime VIII is also observed for diamond (001) surfaces. These findings are rationalized by the structural and energetic peculiarities of the different low-index surfaces. Our study provides guidelines for nanoscale control and manipulation of oxygen functional groups on carbon surfaces in boundary lubrication with water or other oxygen-containing lubricants.