The Friction Properties of Firebrat Scales

Dynamics of Sliding Friction between Laser-Induced Periodic Surface Structures (LIPSS) on Stainless Steel and PMMA Microspheres

In this work, we investigated the sliding friction measured between poly(methyl methacrylate) (PMMA) colloidal probes with two different diameters D (1.5 and 15 μm) and laser-induced periodic surface structures (LIPSS) on stainless steel with periodicities Λ of 0.42 and 0.9 μm, when the probes are elastically driven along two directions, perpendicular and parallel to the LIPSS. The time evolution of the friction shows the characteristic features of a reverse stick-slip mechanism recently reported on periodic gratings. The morphologies of colloidal probes and modified steel surfaces are geometrically convoluted in the atomic force microscopy (AFM) topographies simultaneously recorded with the friction measurements. The LIPSS periodicity is only revealed with smaller probes (D = 1.5 μm) and when Λ takes the largest value of 0.9 μm. The average value of the friction force is found to be proportional to the normal load, with a coefficient of friction μ varying between 0.23 and 0.54. The values of μ are rather independent of the direction of motion, and they reach their maximum when the small probe is scanned on the LIPSS with the larger periodicity. The friction is also found to decrease with increasing velocity in all cases, which is attributed to the corresponding decrease of the viscoelastic contact time. These results can be used to model the sliding contacts formed by a set of spherical asperities of different sizes driven on a rough solid surface.

Effect of the Microstructures on Vulcanized Rubber Frictions

Vulcanized rubber is widely used in a wide range of applications because of its flexibility, durability, sealing properties, and high degree of friction. However, this high degree of friction can also become an issue, as it leads to the wearing and breakage of parts. In this report, we investigated the effects of the vulcanized rubber microstructures on friction force by using simple, anisotropic microstructures. The line and space master microstructures were prepared from a photoresist, and the structures were transferred to PDMS, PSt, and then Ni. After surface modification of the Ni microstructures by TEOS, the vulcanized rubber microstructures were fabricated by a simple hot press process with the TEOS-coated Ni microstructure molds. The structural parameters of the vulcanized rubber line and space microstructures were found to be successfully varied by elongation, and the structural deformations were also investigated by FEM simulations. Measurements of the frictional force of the vulcanized rubber microstructures revealed the friction coefficient was reduced by the surface microstructures and was affected by the directions of the contact because of the microstructure anisotropy. The reason for of these results can be explained by the changes in the contact area and hysteresis friction. These results suggest that the friction coefficients of vulcanized rubbers can be reduced by the simple surface microstructures that are applicable to a wide range of fields.