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Format:
Online
Author:
Malina, Evan W.
Dept./Program:
Mechanical Engineering
Year:
2011
Degree:
MS
Abstract:
Graphene, an atomically-thin layer of hexagonally bonded carbon atoms, is the strongest material ever tested. The unusual electrical and mechanical properties of graphene are particularly useful for next-generation transparent touch screens, flexible electronic displays, and photovoltaics. As such applications arise, it is critically important to characterize the resistance of this material under impact and defonnation by nanoscale contact. The objective of this thesis is to study the physics of defonnation in graphene sheets on a flat substrate under nanoindentation, as a function of number of graphene layers and applied force. In this work, the nanoindentation behavior of single and few layer graphene sheets was investigated by using atomic force microscopy (AFM). Graphene was created by mechanical exfoliation and deposited on a flat SiO₂ substrate. The system of graphene on SiO₂ simulates many of graphene's applications, but its characterization by nanoindentation is not fully understood.
Here, it was found that the deformation of the atomically-thin film remains purely elastic during nanoindentation, while the amorphous substrate defonns plastically. Also, both modulus of elasticity and contact stifthess were found to increase by 18% when few layer graphene sheets were added to a SiO₂ substrate. However, no pronounced change in nanohardness was observed in the substrate with and without the addition of graphene. Furthermore, three modes of deformation were observed including purely elastic deformation, plastic deformation and an abnormal force-depth step mechanism. Each of these mechanisms was analyzed in detail using force-displacement curves and AFM images, and a deformation mechanism map, as a function of number of graphene layers and contact force, was developed. In addition to nanomechanical experiments, computer simulations by finite element analysis (FEA) were conducted in order to better understand the nanonindentation process and underlying deformation mechanisms in this system.