What Governs Friction of Silicon Oxide in Humid Environment: Contact Area between Solids, Water Meniscus around the Contact, or Water Layer Structure?

In order to understand the interfacial parameters governing the friction force (F_t) between silicon oxide surfaces in humid environment, the sliding speed (v) and relative humidity (RH) dependences of F_t were measured for a silica sphere (1 μm radius) sliding on a silicon oxide (SiO_x) surface, using atomic force microscopy (AFM), and analyzed with a mathematical model describing interfacial contacts under a dynamic condition. Generally, F_t 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_t is a function of both v and RH; (ii) in contrast, at RH ≥ 50%, F_t 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_t as a function of the water meniscus area (A_m) and volume (V_m) estimated from a thermally activated water-bridge formation model. Surprisingly, it was found that F_t varies linearly with V_m and correlates poorly with A_m at RH < 50%; and then its V_m 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.