Date of Award

12-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical Engineering

First Advisor

Tushar Borkar

Second Advisor

Majid Rashidi

Third Advisor

Petru Fodor

Abstract

Carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) with superior mechanical, thermal, chemical, and electrical properties are appealing reinforcements for the fabrication of lightweight, high-strength, and wear-resistant metal matrix composites with outstanding mechanical and tribological performance. The nickel-carbon nanotube composites (Ni-CNT) and nickel-graphene nano-platelet composites (Ni-GNP) were processed using two separate ball milling methods, namely dry ball milling (DM) and solution ball milling (SBM), and then sintered using the spark plasma sintering (SPS) technique. The influence of the premixing process, milling time, reinforcement morphology, and concentration on matrix grain size, microstructure, dispersion in the nickel matrix, mechanical properties, and tribological performance of these composites was studied and compared to SPS processed monolithic nickel. The influence of different structural morphologies of CNTs and GNPs on the wear performance and coefficient of friction of these composites was studied using ball-on-disc tribological testing. After the addition of CNTs/GNPs to the nickel matrix, the experimental findings show significant grain refinement and improvement in the microhardness of these composites. The uniform distribution of reinforcement inside the nickel matrix, the finer grain size, and the strong nickel-reinforcement interfacial bonding, which efficiently transfers stress during tensile deformation, are principally responsible for this improvement in these nanocomposites. Various strengthening mechanisms related to CNT-metal matrix composites have been extensively discussed. Using micromechanical models, we have tried to quantify the contribution of these strengthening mechanisms. Moreover, compared to the pure nickel sample, the CNTs and GNPs successfully generated a lubricant layer, improved wear resistance, and reduced friction coefficient during the sliding wear test. When the CNT/GNP-reinforced composites were loaded with higher reinforcement concentrations, wear resistance and CoF uncertainty increased. Furthermore, the influence of different forms of reinforcement with varied size, surface area, and structure on their dispersion and the effect on the performances were addressed. For additional study, a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were employed to examine the fracture surface and wear tracks.

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