Friction Reduction Effect Caused by Microcontact and Load Dispersion on the Moth-Eye Structure

Abstract

Friction reduction is important from the viewpoint of energy problems and other issues. Frictional forces are known to vary depending on the material property, surface texture, and measurement scale. However, the effect of submicron-sized moth-eye structures prepared of robust plastic deformation materials on dry friction under high-load conditions has not been investigated in detail. To investigate this, a copper moth-eye structure is fabricated via electroforming for experimental measurements. Results from the friction tests reveal that real contact area increase is suppressed, as the friction coefficient of the moth-eye structure decreases exponentially with increasing load. Further friction simulation demonstrates nanoscale contact between the structure's tip and indenter, indicating that the real contact area increase requires deformation of the moth-eye structure itself (microcontact). However, the contact pressure on the surface is reduced by dispersing the load to the sides and bottom of the moth-eye structure. Therefore, the suppression of real contact area increase can be attributed to the deformation suppression facilitated by load dispersion. These findings expand the possibilities for friction design with surface textures because they reveal the role of robustness due to submicron-scale surface microstructure in reducing friction between plastic deformation materials.