Doctor of Philosophy (PhD)
Engineering Science (Interdepartmental Program)
Nanoscale multilayer coatings have high potential for numerous engineering applications because they can exhibit enhanced properties due to nanoscale effects and combine different properties from individual components. At present, scale effects on the mechanical behavior of multilayers are not well understood. Three multilayer nanocomposite systems, namely Al/Al2O3, Ti/TiN, and Cr/a-C, have been synthesized by using a dual-gun e-beam physical vapor deposition, to investigate the effect of layer thickness, the nature of components and their microstructures on the mechanical behavior. The deposited Al and Ti nanolayers were found to have polycrystalline fcc and hcp structure, respectively, the Cr and TiN layers had fine columnar bcc and fcc structure, respectively, and the Al2O3 and C layers were amorphous. Nanoscale effects were observed in all three systems with the metal layer thickness affecting significantly the mechanical behavior. The hardness response of the present systems can be described as a function of the metal layer thickness by a Hall-Petch relationship. A critical Al layer thickness of 40 nm, below which there was no further hardness enhancement, was found for the Al/Al2O3 multilayers. The critical Al layer thickness could be predicted by previous theoretical models. A hardness increase was observed down to a Ti layer thickness of 5 nm for the Ti/TiN system. The strengthening of the Ti/TiN multilayers was consistent with the macroyield maps based on a confined layer slip model. Hardness in the Cr/a-C system showed a continuous increase down to a Cr layer thickness of 20 nm. The fracture toughness of the monolithic ceramic phase was significantly improved by introducing a metal/ceramic multilayered structure. The wear behavior of the present multilayers was mainly controlled by the ceramic phase. The Cr/a-C multilayers achieved a low friction coefficient (~0.1) and low wear rate (~10-5 mm3/N m). The present research shows that properties can be tailored by appropriate selection of layer thickness and nature of individual components.
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Qi, Zuqiang, "Processing, microstructure and mechanical behavior of nanocomposite multilayers" (2004). LSU Doctoral Dissertations. 918.
Efstathios I. Meletis