Influência da radiação no comportamento de atrito de filmes finos de dióxido de titânio: fototribologia

Abstract

In dry sliding of two surfaces, the friction force depends on the pair of materials and the contact conditions. If the material and operating conditions remain unchanged, the friction force is constant. Techniques for adjusting frictional forces are widely known, but they are all irreversible processes. Recent works analyze the influence of external electric and magnetic fields and, in particular, radiation on the tribological properties of materials. However, very few studies published so far bring the analysis of the contribution of radiation acting as a fourth body (in addition to the two bodies that are in relative motion, there is a third body that would be a lubricant or the medium) in the friction between two surfaces, making this area practically unexplored and, consequently, very attractive. In this work we report the active control of friction forces in TiO2 thin films under light illumination of specific wavelength within the ultraviolet spectrum. We conclude that this is a reversible phenomenon, stable and can be adjusted/controlled with the light wavelength. The analysis of atomic force microscopy signals by wavelet spectrograms reveals different dissipative mechanisms acting in the darkness and under UV. Ab initio simulations on UV light-exposed TiO2 show changes in electron density that may result in a lower atomic orbital overlapping on the surface, which leads to friction reduction of up to 60 %. In addition, we verified that the light intensity (that has a linear ratio with the current applied to the LED) affects the friction force reduction, that is, the greater the light intensity incident on the sample, the greater the friction reduction. [resumo fornecido pelo autor]