Microstructural Modeling and Design Optimization of Adaptive Thin-Film Nanocomposite Coatings For Durability and Wear

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Title: Microstructural Modeling and Design Optimization of Adaptive Thin-Film Nanocomposite Coatings For Durability and Wear
Author: Pearson, James Deon
Advisors: Dr. Mohammed Zikry, Committee Chair
Dr. Kara Peters, Committee Member
Dr. Yong Zhu, Committee Member
Dr. Jeffrey Eischen, Committee Member
Abstract: Adaptive thin-film nanocomposite coatings comprised of crystalline ductile phases of gold and molybdenum disulfide, and brittle phases of diamond like carbon (DLC) and ytrria stabilized zirconia (YSZ) have been investigated by specialized microstructurally-based finite-element techniques. A new microstructural computational technique for efficiently creating models of nanocomposite coatings with control over composition, grain size, spacing and morphologies has been developed to account for length scales that range from nanometers to millimeters for efficient computations. The continuum mechanics model at the nanometer scale was verified with molecular dynamic models for nanocrystalline diamond. Using this new method, the interrelated effects of microstructural characteristics such as grain shapes and sizes, matrix thicknesses, local material behavior due to interfacial stresses and strains, varying amorphous and crystalline compositions, and transfer film adhesion and thickness on coating behavior have been investigated. A mechanistic model to account for experimentally observed transfer film adhesion modes and changes in thickness was also developed. One of the major objectives of this work is to determine optimal crystalline and amorphous compositions and behavior related to wear and durability over a wide range of thermo-mechanical conditions. The computational predictions, consistent with experimental observations, indicate specific interfacial regions between DLC and ductile metal inclusions are critical regions of stress and strain accumulation that can be precursors to material failure and wear. The predicted results underscore a competition between the effects of superior tribological properties associated with MoS2 and maintaining manageable stress levels that would not exceed the coating strength. Varying the composition results in tradeoffs between lubrication, toughness, and strength, and the effects of critical stresses and strains can be controlled for desired behavior. The analysis also indicates that coating strength increases at a higher rate than the internal coating stresses with decreasing grain size. For transfer films, the present study underscores the beneficial material effects of increasing transfer film thickness and reducing transfer film extrusion due to increased thickness.
Date: 2009-02-17
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/5789

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