Phototribology: Control of Friction by Light

Abstract

In dry sliding, the coefficient of friction depends on the material pair and contact conditions. If the material and operating conditions remain unchanged, the coefficient of friction is constant. Obviously, we can tune friction by surface treatments, but it is a nonreversible process. Here, we report active control of friction forces on TiO2 thin films under UV light. It is reversible and stable and can be tuned/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show a lower atomic orbital overlapping on the surface, which leads to a friction reduction of up to 60%. We suggest that photocontrol of friction is due to the modification of atomic orbital interactions from both surfaces at the sliding interface.

Nanotribology of Hydrogenated Amorphous Silicon: Sliding-Dependent Friction and Implications for Nanoelectromechanical Systems

“…The illumination setup used was previously presented and validated for phototribological studies. 46 A wavelength different from the one used on the equipment was chosen to avoid any interferences during the measurement, while still having an energy higher than the band gap of the sample. Figure 7 shows the results for the different setups when additional light was added in distinct moments of scanning.…”

“…To examine if such a behavior may constitute an artifact, an independent material was tested under the same conditions. TiO 2 films were chosen because they were recently analyzed using the same apparatus and repeated scanning (in both dark and illuminated conditions), with relevant phototribological effects when illuminated with a wavelength within the band gap (in the UV region) . As also reported in the Experimental Section, using a well-studied sample in the same equipment and conditions as comparison helps the identification of the peculiarities of a newly evaluated sample without adding new or nondesirable variables.…”

“…For the nanotribological evaluation, bidirectional sliding tests were performed using an atomic force microscope (SPM-9700Shimadzu) in lateral force microscopy (LFM) mode, equipped with a previously calibrated pure silicon probe (PPPLFMRNanosensors) of nominal ending radius <10 nm on a rectangular-shaped cantilever. The calibration followed a methodology previously proposed and widely applied in different works. , The cantilever’s properties were also followed during the experiments to keep track and avoid any changes or damage to the tip, the phenomena that would possibly influence the measurements. The temperature and relative humidity were maintained at (18 ± 2) °C and (49 ± 1)%, respectively, during the measurements and registered using a high-precision digital thermometer hygrometer.…”

“…The calibration followed a methodology previously proposed 11 and widely applied in different works. 5,46 The cantilever's properties were also followed during the experiments to keep track and avoid any changes or damage to the tip, the phenomena that would possibly influence the measurements. The temperature and relative humidity were maintained at (18 ± 2) °C and (49 ± 1)%, respectively, during the measurements and registered using a highprecision digital thermometer hygrometer.…”

“…Recently, a new field of nanotribology has emerged: phototribology . It was possible to reversibly change the frictional behavior of a semiconductor (titanium dioxideTiO 2 ) using its photoactivity.…”