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Title:Synthesis and indentation of boride and carbide coated carbon nanotube composite microstructures
Author(s):Sandin, Carly Renee
Advisor(s):Tawfick, Sameh
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):carbon nanotubes
carbon nanotube composites
hafnium diboride
polymer derived ceramics
Abstract:The goal of this thesis is to investigate the synthesis and mechanical behavior of novel refractory coatings for applications subjected to extreme conditions. Starting from aligned carbon nanotube (CNT) forests as scaffolds, composite foams are fabricated by infiltration with refractory materials to achieve desirable mechanical properties such as high stiffness and strength. Similar to naturally occurring materials such as bone and teeth, these properties are dictated by the composition and nanoarchitecture of the material structure. CNTs grown into vertically aligned pillars by chemical vapor deposition (CVD) mimic the microstructure of these natural foams through their porous nanostructure. The CNT pillars are then coated using two methods: (i) static CVD with a hafnium diboride precursor (Hf[BH4]4) leading to coating thicknesses ranging from 3nm to 50 nm; and (ii) through cycles of elasto-capillary imbibition and pyrolysis of a silicon oxycarbide (SiOC) polymer derived ceramic leading to coating thickness from 8 nm to full infiltration. Both coatings enable the infiltration of CNT pillars and lead to a significant increase in stiffness and strength. Nanoindentation tests using a flat punch were performed on the CNT pillars to measure their Young’s modulus and compressive strength. By varying the CNT coating thickness, a trend develops for the Young’s modulus as a function of coating thickness for the CNT pillars where E ~ ρ^1.698 for hafnium diboride coated pillars. The maximum stiffness and strength was 56.49 GPa and 1.94 GPa for the fully infiltrated HfB2 composite and 3.80 GPa and 13.87 MPa for the SiOC pillars. We also identify the different regimes of deformation for the pillars to better understand the coating processes and the material behavior. These results can enable new applications of CNTs in extreme environments where high temperature resistance and high mechanical resilience are needed, such as hypersonic vehicles.
Issue Date:2016-12-09
Rights Information:© 2016 Carly Renee Sandin
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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