Controlling high-speed dry friction through the geometry of micro-patterned asperities

Abstract

Advances in manufacturing technologies have provided means to create surface textures with regular patterns of uniform asperities, leading to the potential for improved control of friction. In order to design surface topologies that induce desirable tribological effects, an understanding of the influences of the geometric features of asperities on measures of frictional resistance is required. Dynamic elastic-plastic finite element modeling methods, which included material damage and failure, were used to study the interactions of directly modeled 100 micron rib-like asperities on two deformable aluminum blocks. The relationships between the mechanics of the deformation and failure of five unique asperity geometries, the coefficients of static and kinetic friction, and the system energy stored and dissipated were studied under dry, high-load rate conditions, where motion was initiated in under 1 ms and acceleration approached 100 kG. Influences of the geometric features of the asperities were explored using semi-circular, triangular, and square-shaped cross-sectional profiles and evaluated for complex geometries created by combinations of these basic shapes. Static coefficients of friction were found to vary more than two-fold with asperity geometry based on the contact area normal direction. The study found that it was also possible to maintain the static friction coefficient but more than triple the force to initiation motion simply by changing the asperity shape. While kinetic friction coefficients were less influenced by asperity shape for the high-speed conditions studied, the geometric characteristics directed the way an asperity deformed under load and the extent of the material failure during sliding. A more than four-fold variation in energy stored within the system and over an order of magnitude variation in energy dissipated by the system was found for the geometries examined. This study demonstrates the importance of understanding the mechanical behavior of the asperity when designing surface textures to tailor dry, high-speed friction.