Due to the strong capillary condensation, the adhesion force between a Si3N4 atomic force microscope (AFM) tip and silicon oxide was observed to first increase and then decrease with an increase of humidity. In contrast, due to weak capillary condensation, the adhesion force between the AFM tip and the N-octadecyltrimethoxysilane (OTE, CH3(CH2)17Si(OCH2CH3)3) self-assembled monolayer (SAM) was found to be almost independent of humidity. It was found that the formulation commonly used for macroscopic objects fails to explain our data. Using an accurate formulation and an assumed tip shape, we can explain the observed decrease of adhesion for SiO2 at high humidity as being due to the decreased capillary pressure force when the dimension of the meniscus becomes comparable with the tip size. However, the observed late onset of adhesion of SiO2 cannot be understood within the framework of the classical continuum theory. We attribute this late onset to the properties of an ultrathin water film at molecular thickness. The new formulation predicts a vanishing water meniscus between the AFM tip and OTE over the entire humidity range and can also fully account for the humidity-independent adhesion results for OTE.

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The effects of counter-surface chemistry, relative humidity, and applied normal load on nanowear of single-crystalline silicon were studied with atomic force microscopy. In the absence of humidity, the silicon surface can resist mechanical wear as long as the contact pressure is lower than the hardness of silicon regardless of the counter-surface chemistry (diamond or SiO(2)) and ambient gas type (vacuum, N(2), O(2), air). In these conditions, the sliding contact region is protruded forming a hillock. However, when the relative humidity is higher than ~7%, the hillock formation is completely suppressed and, instead, tribochemical wear of the silicon surface takes place even at contact pressure much lower than the hardness. The tribochemical wear increases drastically in the relative humidity regime where the adsorbed water layer assumes the "solid-like" structure; further increase of wear is small in higher relative humidity regime where the "liquid-like" water layer is formed. It is also noted that the humidity-induced wear occurs only when the counter-surface is SiO(2); but not with the diamond counter-surface. This implies that the interfacial shear of the water-adsorbed SiO(2) surface with a chemically inert counter-surface is not sufficient to initiate the tribochemical wear; both substrate and counter-surface must be chemically reactive. A phenomenological model is proposed to explain the experimental observations.

The isotherm thickness and hydrogen-bonding interactions of water layers adsorbed on hydrophilic and hydrophobic surfaces were quantified and compared. The hydrophilic and hydrophobic surfaces were modeled with an OH-terminated native oxide layer on silicon and a HF-etched silicon terminated with hydrogen, respectively. The silicon substrate allows the use of attenuated total reflection infrared (ATR-IR) spectroscopy for quantitative measurement of adsorbed water without interferences from the gas phase water. On the hydrophilic Si–OH surface, the average thickness of the strongly hydrogen-bonded water layer increases up to ∼2 molecular layers as relative humidity (RH) increases, beyond which the weakly hydrogen-bonded structure is dominant. On the hydrophobic Si–H surface, the adsorbed water layer consists predominantly of the weakly hydrogen-bonded structure and its average thickness remains less than a monolayer even at RH = 90%. The differences in the thickness and structure of adsorbed water layers on hydrophilic versus hydrophobic surfaces found from ATR-IR measurements provide critical insights needed for better understanding of various physical processes affected by water adsorption in ambient conditions.