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Microscopic ice friction Kietzig, Anne-Marie

Abstract

Microscopic ice friction was studied systematically across all to ice friction relevant friction regimes using several metallic interfaces. A rheometer with a newly designed fixture for friction measurements was used in these studies. The investigations focus on the influence of material properties, such as surface wettability, roughness, surface structure, surface nanopatterning, and thermal conductivity. Using a femtosecond laser process certain dual scale roughness structures were created to mimic the lotus leaf on the surface of inherently hydrophilic metal alloys. After laser irradiation the samples show initially superhydrophilic behavior with complete wetting of the structured surface. However, over time these surfaces become hydrophobic to superhydrophobic. The change in wetting behavior correlates with the amount of carbon found on the structured surface. The explanation for the time dependency of the surface wettability lies in the combined effect of surface morphology and surface chemistry. With regard to ice friction this controlled lotus-like roughness significantly increases the coefficient of friction at low sliding speeds and temperatures well below the ice melting point. However, at temperatures close to the melting point and relatively higher speeds, roughness and hydrophobicity significantly decrease ice friction. This decrease in friction is mainly due to the suppression of capillary bridges. The influence of surface structure on ice friction was also investigated isolated from the effect of surface roughness. It is shown that grooves oriented in the sliding direction also significantly decrease friction in the low velocity range compared to scratches and grooves randomly distributed over a surface. The isolated effect of thermal conductivity on ice friction is investigated by thermally insulating the slider and the friction fixture with fiberglass. A decrease of the friction coefficient in the boundary friction regime and an earlier onset of the mixed friction regime in terms of sliding velocity are reported. Furthermore, the dependence of the ice friction coefficient on sliding velocity is compared for different sliding materials. It was concluded that the influence of thermal conductivity decreases with increasing sliding velocity.

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