Strengthening of metals using a graphene monolayer

Abstract

A practical route to exploiting graphene’s supreme properties for a variety of applications is to incorporate graphene layers in composite materials. Harnessing the high stiffness, intrinsic strength as well as transport properties of graphene in its composites requires the combination of high-quality graphene having low defect density, and the precise control of the interfacial interactions between the graphene and the matrix. These requirements equally hold for polymer and metal matrices, and enable the use of graphene in applications ranging from tough thin films for use in flexible electronics to the design of advanced aerospace structures. My dissertation addresses the synthesis, understanding and control of these composites and their mechanical properties probed from the nano- to the microscales. To this end, a model system of ultrathin metal films coated with graphene monolayer via chemical vapor deposition (CVD) is designed and used to study as-grown graphene’s contributions in graphene-metal composite thin films. Due to the thinness of the metal layer - typically less than 300 nm - individual or few graphene layers have a strong contribution on the composite thin film’s mechanics. To create the most ideal interface between the metal and the graphene, CVD synthesis is used to grow the graphene wrapping around the surface of the films. A highly dynamic CVD synthesis route is developed to achieve high-quality graphene monolayer growth on ultrathin metal films while avoiding solid-state dewetting instability which takes place at the extremely high synthesis temperatures. We study how the competition between temperature-driven segregation and precipitation of carbon radicals governs the graphene’s nucleation and growth kinetics on ultrathin metal catalysts. The result of the dynamic recipe is repeatable growth of graphene monolayers with ultralow defect density as confirmed by Raman spectroscopy. Precise mechanical characterization of ultrathin films is carried using various nanoindentation modalities including indentation of supported and freestanding thin films. CVD grown graphene-metal thin film composites exhibit unusual increase in the elastic modulus, strength and toughness. For example, there is 35 % and 57 % increases in the Young’s modulus and tensile strength in graphene-palladium thin film composites compared to those for a bare palladium film having a thickness of 66 nm. Notably, this enhancement exhibits scale effects, where the composite modulus increase varies with the thickness, and is highest for the thinnest metal thicknesses. My work demonstrates that the inherent strong interfaces between graphene and strongly interacting metals like Ni and Pd after synthesis could lead to the manufacturing of composites with significantly higher performances. I also observed increase in toughness and qualitatively different modes of crack propagation owing to the addition of the high stiffness graphene shield on the metal surface during synthesis. Raman spectroscopy and electron imaging of surface reconstructions confirm the high interfacial stresses due to the combination of the lattice mismatch between the graphene and the metals and the kinetics of growth. The findings of this dissertation promote graphene-based thin film composites for flexible electronic devices, and enable fundamental studies of exploiting strain engineering at the graphene-metal interface for electronics, chemistry and mechanics. Furthermore, the results of this dissertation are broadly relevant to the design of bulk graphene-based composite materials.

Kaihao Zhang

My research interests include metal additive manufacturing, cabron nanomaterials, and composite materials.