The use of anisotropic texturing for control of directional friction

Abstract

This paper presents a study on the influence of anisotropically shaped texture arrays on friction behaviour of an oil lubricated sliding contact, especially on directional friction control based on the diverging and converging characteristics of the textures. Experiments have been conducted on a TE77 reciprocating cylinder-on-plate test rig, where steel rollers were used to slide against steel plate samples with or without textures. A mineral base oil was used to lubricate the contacts. Three geometries of dimples were designed and laser textured on the steel plate samples with varied 3-dimensional features, including Square Flat (SF), Square slope (SS) and Triangular Flat (TF) shapes representing the shape in x-y (top view) and x-z (side view) planes respectively. These shapes were chosen to vary the converging and diverging properties of the lubricated contacts depending on the sliding direction. Relatively large dimple sizes (side length ~ and depth ~ ) have been used in this study to enable observation of the effect and easy control of the texturing process. The texture density has been kept at as most literature suggested. The large dimple sizes resulted that the dimples were not be fully covered by the contact area, i.e. the dimple sides were bigger than the Hertzian contact width of the roller-flat contacts. This has eliminated the 'lift' or 'load bearing' effect discussed in most papers thus focuses on other effects investigated in this study. The results show that beneficial effects of the anisotropic textures present in all lubrication regimes including the boundary, mixed and hydrodynamic lubrications, especially under prevailing boundary lubrication conditions. Using high sampling rate for the friction data during the tests, it was able to study local friction effect due to individual dimple array especially at their leading and trailing edges. The results show that a local friction reduction is observed at the leading while an increase at the trailing edge. Overall directional friction effect of the anisotropic textures has been observed that the converging shape in both y-z plane and the x-y plane reduces friction. Furthermore, it was found that the triangular shape dimples have a greater local frictional response at each dimple array, while the sloped bottom square dimples have a more significant overall directional fricition effect.

A Novel Surface Texture Shape for Directional Friction Control

“…As mentioned in the discussion in paper [32], the directional friction properties of square sloped bottom dimples appears at the overall friction trends, and the local frictional response is almost the same with the square flat bottomed dimples. While, the triangular flat dimples have less influence on total friction trends, but cause a larger difference in local friction response.…”

“…7. It should be mentioned that the other three types of dimples in paper [32] have the same dimple side length with the current triangular sloped shape, although the TS3 shaped dimples are four times the depth of the other type dimples.…”

“…The actual contact length varies with the roller's motion, and the fluctuation of the real contact length and the friction with trend line subtracted could be plotted against roller displacement, see Fig. 8b, which adopted from paper [32].…”

“…As has been discussed in papers published by current authors [32,33], dimples parameters (dimple depth, dimple diameter, depth-to-diameter ratio, and the texture density, dimple profiles (flat, spherical and linearly varying bottoms) and dimple shapes (circular, elliptical, rectangular, triangular and chevron shapes) have been studied in the past decades. Researchers have been aware of the benefit of anisotropically textured surfaces through numerical simulation, that the converging shape in the x-y plane (triangular dimples) and in the x-z plane (sloped bottom dimples) could both be reasons for enhancing the lubrication effects [34,35].…”

“…The current research is a follow-up of a recently published work [32,33], aims at gaining a better understanding of the mechanisms that friction reduction effects and directional effects produced by dimpled surfaces at reciprocating speeds of the lubricated line contacts. To show whether these factors cause a significant impact, three samples with novel textured designs (triangular top-view shapes and wedge bottom shapes) were created by ultrafast laser surface texturing technique.…”