Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University , Chengdu 610031, China.
Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University , University Park, Pennsylvania 16802, United States.
Langmuir. 2017 Sep 26;33(38):9673-9679. doi: 10.1021/acs.langmuir.7b02491. Epub 2017 Sep 7.
In order to understand the interfacial parameters governing the friction force (F) between silicon oxide surfaces in humid environment, the sliding speed (v) and relative humidity (RH) dependences of F were measured for a silica sphere (1 μm radius) sliding on a silicon oxide (SiO) surface, using atomic force microscopy (AFM), and analyzed with a mathematical model describing interfacial contacts under a dynamic condition. Generally, F decreases logarithmically with increasing v to a cutoff value below which its dependence on interfacial chemistry and sliding condition is relatively weak. Above the cutoff value, the logarithmic v dependence could be divided into two regimes: (i) when RH is lower than 50%, F is a function of both v and RH; (ii) in contrast, at RH ≥ 50%, F is a function of v only, but not RH. These complicated v and RH dependences were hypothesized to originate from the structure of the water layer adsorbed on the surface and the water meniscus around the annulus of the contact area. This hypothesis was tested by analyzing F as a function of the water meniscus area (A) and volume (V) estimated from a thermally activated water-bridge formation model. Surprisingly, it was found that F varies linearly with V and correlates poorly with A at RH < 50%; and then its V dependence becomes weaker as RH increases above 50%. Comparing the friction data with the attenuated total reflection infrared (ATR-IR) spectroscopy analysis result of the adsorbed water layer, it appeared that the solidlike water layer structure formed on the silica surface plays a critical role in friction at RH < 50% and its contribution diminishes at RH ≥ 50%. These findings give a deeper insight into the role of water condensation in friction of the silicon oxide single asperity contact under ambient conditions.
为了理解在潮湿环境下控制二氧化硅表面摩擦力（F）的界面参数，使用原子力显微镜（AFM）测量了在二氧化硅（SiO）表面上滑动的二氧化硅球（半径为 1μm）的滑动速度（v）和相对湿度（RH）对 F 的依赖性，并使用描述动态条件下界面接触的数学模型进行了分析。通常，F 随 v 的对数增加而减小，直到低于界面化学和滑动条件相对较弱的截止值。在截止值以上，对数 v 依赖性可分为两个区域：（i）当 RH 低于 50%时，F 是 v 和 RH 的函数；（ii）相反，在 RH≥50%时，F 仅取决于 v，而不取决于 RH。这些复杂的 v 和 RH 依赖性被假设源于吸附在表面上的水层和接触面积环周围的水弯月面的结构。通过分析从热激活水桥形成模型估计的水弯月面面积（A）和体积（V）与 F 的关系来检验该假设。令人惊讶的是，发现当 RH<50%时，F 与 V 呈线性变化，与 A 的相关性较差；然后，当 RH 高于 50%时，其 V 依赖性变得较弱。将摩擦数据与吸附水层的衰减全反射红外（ATR-IR）光谱分析结果进行比较，似乎在 RH<50%时，在二氧化硅表面上形成的固态水层结构在摩擦中起着关键作用，而其贡献在 RH≥50%时会减小。这些发现更深入地了解了在环境条件下硅氧化物单峰接触中冷凝水在摩擦中的作用。
