Friction and Adhesion Forces at a Nanoscopic Contact Between Titanium Dioxide Thin Film Surfaces

Abstract

Magnetron sputtering deposition of titanium dioxide (TiO2) thin films on commercial AFM probes (silicon nitride) and silicon wafer substrates was employed to fabricate surfaces with controlled wettability. The wettability of the TiO2 covered AFM probe and sample surfaces was improved by UV light irradiation, as indicated by macroscopic measurements of the water contact angle. Then, the friction and adhesion forces at a single-asperity contact between TiO2 surfaces were measured in air at constant humidity before and after UV light irradiation. An important increase of adhesive and friction forces was observed when the wettability of the AFM tip and sample surfaces was increased. The increase of the friction force cannot be explained solely by the increase of the adhesive force. A friction force associated to the energy dissipation in formation and destruction of an ice-like structure of water at the sliding tip-sample contact is considered in order to explain the increased friction force observed at the contact of the hydrophilic TiO2 surfaces. Infrared adsorption spectra of a thin water layer captured between two superhydrophilic TiO2 surfaces suggested a solid-like hydrogen bonding structure of the water adsorbed on the superhydrophilic surfaces. Formation of such water structure increases the shear stress of the water at nanoscopic contacts and causes a corresponding increase of the contact friction force.