Date of Award

2009

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical, Automotive, and Materials Engineering

First Advisor

Alpas, Ahmet (Mechanical, Automotive & Materials Engineering)

Keywords

Engineering, Materials Science.

Rights

CC BY-NC-ND 4.0

Abstract

This study aimed to develop new wear-resistant materials with superhydrophobic surfaces and low friction by combining the high strength of nanocrystalline (NC) materials with the biological surface textures. This goal was accomplished in three stages. First, the tribological properties of electrodeposited NC Ni and NC Co were studied, and the role of the oxide-rich tribolayers in reducing the friction and wear rates was delineated. For the NC Ni, micromechanisms of wear in different testing environments were characterized, and the sliding-speed sensitivity of friction and wear rate was investigated. A modified Archard equation was proposed to predict the wear rates of NC Ni as a function of grain size and sliding speed. Additionally, it was found that the high-temperature wear resistance of NC Ni could be improved by using SiC nanoparticles as reinforcements. In the second stage, NC Ni replicas of the surface textures of a lotus leaf and a snake skin were fabricated through replication and electrodeposition. The NC Ni snake skin replica displayed anisotropic frictional properties, due to the asymmetric shape of the protrusions at the scales' ridges. The NC Ni lotus leaf replica featured a high density of microscale conical protuberances that prompted a 30% lower peak coefficient of friction (COF) compared to a smooth surface, due to a smaller real area of contact. In the third stage, the surface texture of the NC Ni lotus leaf replica was modified using a short-duration electrodeposition process that increased the radius of the protuberance tips, followed by a perfluoropolyether (PFPE) solution treatment that reduced the surface energy and resulted in a multi-level surface roughness consisting of a nanoscale surface texture superimposed on microscale protuberances. The produced surfaces had a high water contact angle of 156░, similar to that of the natural lotus leaf, and had a 60% lower steady-state COF.

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