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

Abstract

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.

Thickness and Structure of Adsorbed Water Layer and Effects on Adhesion and Friction at Nanoasperity Contact

“…The variations in adhesion as a function of surface wettability obey the model in Equation 3. Many research groups have shown the possibility of controlling interfacial adhesion by adjusting the wettability of a surface [91][92][93]. As shown in Figure 12, upon either nor p-type doping with subsurface polyelectrolytes, graphene surfaces become more hydrophilic [59], water adsorption is enhanced, and F a (i.e., the pull-off force) increases significantly.…”

“…In this case, the sliding interface can be modeled by a single-asperity contact if the elastic deformation of the solid is larger than the topographic features of the initial surface [139,140]. As discussed in Sections 2 and 3, strongly H-bonded solid-like water can grow on hydrophilic surfaces, not only affecting F a [41] but also playing an important role in the resulting friction behavior [91].…”

“…Chen et al [91] recently investigated the friction behaviors between a silica microsphere and a Si substrate at single-asperity contact and found that F t strongly depends on the structure and amount of the adsorbed water layer. As shown in Figure 22a, F t generally decreases logarithmically with increasing sliding speed to a cutoff value.…”

“…Reprinted from Ref. [91], Copyright 2017, with permission from ACS publications. Reprinted from Ref.…”

“…Here, φ is the filling angle, R the sphere radius, h DMT the DMT indentation depth, h m the height of meniscus, r K the Kelvin radius (or meridional radius) of meniscus, r m the meniscus-only portion of the total radius from the contact center, and r DMT the DMT contact radius. Reprinted from Ref [91],. Copyright 2017, with permission from ACS publications.…”