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Tribological properties of nanodiamonds in aqueous suspensions: effect of the surface charge

机译：水性悬浮液中纳米金刚石的摩擦学特性：表面电荷的影响

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摘要

Surface uptake and nanotribological properties of positively and negatively charged 5-15 nm diameter nanodiamonds dispersed in water have been studied in real time by means of an in situ Quartz Crystal Microbalance (QCM) technique. The frequency and dissipative properties (mechanical resistance) of a QCM with gold surface electrodes immersed in water were monitored upon addition of the nanodiamonds. Measurements were also performed with the QCM electrode in macroscopic contact with stainless steel ball bearings in the presence or absence of the positively or negatively charged nanodiamonds dispersed in water surrounding the contact. The nanodiamonds were found to have a profound effect on the tribological performance at both nanometer and macroscopic scales. The tribological effects were highly sensitive to the sign of the nanodiamond electrical charge: negatively (positively) charged particles exhibited weaker (stronger) adhesion. Positively charged particles consistently increased friction at the solid-liquid interface, while negatively charged particles of comparable size were observed to decrease interfacial friction. For the macroscopic contacts with the gold electrodes, negatively charged nanodiamonds appeared to be displaced from the contact, while the positively charged ones were not. Overall, the negatively charged nanodiamonds were more stable in an aqueous dispersion over extended time periods while the positively charged nanodiamonds coagulated into larger particles and formed precipitates more quickly.
机译：通过原位石英晶体微天平（QCM）技术实时研究了分散在水中的带正电和带负电的5-15 nm直径的纳米金刚石的表面吸收和纳米摩擦学特性。添加纳米金刚石后，将金表面电极浸入水中的QCM的频率和耗散特性（机械电阻）进行监测。在存在或不存在分散在接触周围水中的带正电荷或带负电荷的纳米金刚石的情况下，还使用QCM电极与不锈钢滚珠轴承宏观接触进行测量。发现纳米金刚石对纳米和宏观尺度的摩擦学性能都有深远的影响。摩擦学效应对纳米金刚石电荷的信号高度敏感：带负电（正电）的颗粒表现出较弱（强）的附着力。带正电的颗粒在固液界面处持续增加摩擦，而观察到具有可比尺寸的带负电的颗粒则减少了界面摩擦。对于与金电极的宏观接触，带负电的纳米金刚石似乎从接触中移出，而带正电的纳米金刚石却没有。总体而言，带负电的纳米金刚石在延长的时间段内在水分散体中更稳定，而带正电的纳米金刚石则凝结成较大的颗粒，并更快地形成沉淀。

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