Date of Award

2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Dr. V. Stoilov

Second Advisor

Dr. A.T. Alpas

Keywords

Applied sciences, Adhesion, Friction control, Micropatterning, Surface texturing

Rights

CC BY-NC-ND 4.0

Abstract

Recently, surface texturing has gained momentum as a way to control the friction which is involved in various applications and systems, such as components of internal combustion engines, dies and punches of the metal forming processes and Micro-electrical-mechanical Systems (MEMS). This dissertation demonstrates that under dry sliding, there is a specific significant surface texturing parameter at which the coefficient of friction should be at a minimum. This dissertation met this objective through an extensive study of the relevant literature on surface texturing and friction, analysing the friction mechanisms involved in dry sliding, and highlighting the key factors that control friction as the real area of contact and material properties. An analytical proof is derived demonstrating that a minimum friction force exists if the two components of the friction force, adhesion and mechanical deformation, are differentiated with respect to the real contact area. In addition, numerical simulations and experimental work were performed to test this hypothesis. In the two and three dimensional finite element models, normal and sliding contact between a rigid indenter and elastic-plastic surfaces, which are textured by circular and hexagonal dimples of different sizes and densities, are simulated and analysed. Circular craters of different sizes and densities, are fabricated using laser ablation on hardened tool steel samples, while the hexagonal dimples are fabricated using photo-lithography. The dimples are arranged in adjacent equilateral triangles layout. Coefficients of friction were measured using a scratch tester under dry sliding conditions and constant load. In addition, adhesion forces were estimated using an Atomic Force Microscope (AFM). The adhesion force is found to be exponentially decreasing with the increase of the spatial texture density. The dimensionless quantity, spatial texture density (D/L) was identified as the most significant texturing parameter. It is equal to the ratio of the size of the crater (D) to the distance between the centres of two consecutive craters (L). A minimum coefficient of friction exits at values of spatial texture densities that lie between 0.25 and 0.5.

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