Micromechanical characterization of metallic glass — crystalline nanocomposite coatings

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2018
Abboud, Mohammad
Amorphous/crystalline nanolayers provide an effective model system to study the mechanical behavior and size effects of metallic glasses and crystalline metals in confined geometries. They also provide an advantageous structure for improving the ductility of amorphous metals while maintaining their outstanding strength. Combination of high strength and ductility make these nanocomposites promising materials as wear resistant coatings. The structure-property relationship in Amorphous/Crystalline nanolayers containing HCP crystalline layers were investigated. CuTi/Ti and CuZr/Zr nanolayers were prepared by magnetron sputtering with layer thicknesses in the range 10–100 nm. The mechanical properties of these nanolayers were investigated using nanoindentation technique, while the wear resistance of CuZr/Zr was investigated using nanoscratch technique. X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and High-Resolution Transmission Electron Microscopy (HRTEM) were used to characterize the structure and composition of the films. The hardness of the CuTi/Ti and CuZr/Zr nanolayers were close to those of the monolithic CuTi and CuZr, respectively. The hardness remained virtually the same for different layer thicknesses as opposed to CuTi/Cu amorphous/FCC crystalline nanolayers which exhibit increasing strength with decreasing layer thickness. The results can be explained by the confined layer slip model that predicts an effective flow stress for HCP crystalline layers higher than those of the amorphous layers. As a result, the strength and size-effects are governed by the mechanical behavior of the softer amorphous layer. The scratch resistance was the highest for the monolithic CuZr, and diminished with decreasing layer thickness for nanolayered coatings, although hardness and elastic modulus were independent of layer thickness. The nanocomposite with layer thickness of 10 nm did not show any sign of failure in spite of compressive strain exceeding 80%. Low shear strength of the CuZr-Zr interface and strain hardening of Zr layers can explain the layer thickness dependent wear resistance and outstanding damage tolerance observed. Layered metallic glass/crystalline nanocomposites combine high hardness and resistance to fracture, providing a new design space for the development of effective wear resistant coatings.

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Citation Formats
M. Abboud, “Micromechanical characterization of metallic glass — crystalline nanocomposite coatings,” M.S. - Master of Science, Middle East Technical University, 2018.