Study on Modeling and Design of Surface Texturing under Mixed Lubrication


Student thesis: Doctoral Thesis

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  • Hui ZHANG


Awarding Institution
Award date4 Oct 2016


Surface texturing is known as arrangement of pits, grooves or bulge structures in micro- or nano-scale, which are manufactured on mechanical sliding surface by physical or chemical methods. Studies in recent decades suggest that surface texturing is a feasible approach for improving the tribological performance of friction pairs and widely utilized in engineering fields. However, current practice on the texture arrangements was mainly based on experiential trial and error method or human experience. Design guidelines for surface texturing, especially in mixed lubrication regime, are rather lacking. Hence, effective designs of texturing to accomplishing optimal tribological behaviors seem hardly to be found in literature. This thesis mainly aims to develop an analytical numerical model for concave texturing under mixed lubrication condition, which facilitates the following study of texture shapes and distributions.
With the use of average flow Reynolds equation and the K-E elasto-plastic model, respectively, the mixed lubrication model modified in the current study allowed prediction of the hydrodynamic pressure and the contact pressure. The validity experiments were also conducted to verify the model. Predictions by the model permitted the obtainment of Stribeck curve for a circular surface texturing. The simulated results demonstrated the reduction of friction and wear for surface texturing. And the main mechanism for such effect may be attributed to the hydrodynamic lift/up-thrust, which increases the mating gap and reduces probability of asperity contact. The tribological influences of concave textures in circular, rectangular and triangular shapes with various sliding directions were discussed. And it turned out that triangular texture in a specific sliding direction yielded the minimum friction coefficient. Influences of geometrical parameters of the circular dimple, i.e. depth, distance and diameter, were also studied, which facilitated the obtainment of a set of optimal geometrical parameters to minimize the friction coefficient. Moreover, the Reynolds equation was further developed and extended to address the boundary slip condition with variable slip length. The developed and extended Reynolds equation was used to study the boundary slip behavior on a DOCTORIAL DISSERTATION OF CITY UNIVERSITY OF HONG KONG typical journal bearing.
Incorporation of GA method with the average flow Reynolds equation permits to evolve the design of irregular geometries of concave textures, which facilitates the near optimal tribological performances in unidirectional and reciprocating sliding motions. It allowed the obtainment of several preferable concave surface texture shapes, typically as: (i) bullet and fish shapes for unidirectional sliding; and (ii) elliptical and fusiform shapes for reciprocating sliding. These shapes gave more superior tribological performance over their circular counterpart. Analysis of the contour map for their individually hydrodynamic pressure distributions identified the increase of hydrodynamic lift to be the main mechanisms for their corresponding tribological improvement. Experimental tests, to compare the tribological performance of the optimized texture shapes with that of the circular texture shape, implied that the tribological advantages of low friction coefficient for the optimized texture shapes can be magnified in light load and high speed operating conditions.
Pin-on-disk experimental tests, in rotary and reciprocal sliding motions, with five texture layouts of Bravais lattice were performed to study the influence of surface texture distributions on the tribological behaviors. In rotary sliding motion, the hexagonal distributed textures yielded the lowest friction coefficients. In reciprocating sliding motion, oblique distributed texture layout exhibited the lowest friction coefficients under low load and high frequency conditions. Tests illustrated that the order of rotational symmetry and the coverage ratio of textures in the direction perpendicular to sliding were the major parameters affecting the tribological performances of these five texture layouts. These two influential parameters may be used to derive the possible mechanisms dominating the experimental results.
This research has filled the gap on modeling and design of surface texturing through proposing corresponding design theories and mothods for the determination of their geometrical parameters, shape and layout, which can provide theoretical support for design of surface texturing on mechanical friction pairs.