Thesis (M.S. in materials science and engineering)--Washington State University, August 2009. / Title from PDF title page (viewed on Aug. 10, 2009). "School of Mechanical and Materials Engineering." Includes bibliographical references.

Thesis (Ph.D)--Chemical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Teja, Amyn S.; Committee Member: Abdel-Khalik, Said I.; Committee Member: Meredith, Carson; Committee Member: Nair, Sankar; Committee Member: Skandan, Ganesh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Ph. D.)--University of Washington, 1995. / Vita. Includes bibliographical references (leaves [168]-171).

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains ix, 98 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 87-91).

Thesis (Ph. D.)--Washington State University, May 2007. / Includes bibliographical references (p. 130-136).

Niobium is well known for its excellent corrosion resistance based on the formation of a stable passive oxide layer, which protects the metal against corrosion in most aqueous media and makes it an interesting candidate for corrosion resistant coating applications. However, deposition of Nb films is restricted to few technologies and difficulties arise from the toxic nature of the electrolytes employed in electrodeposition of Nb, the high reactivity of the metal with residual gases in vacuum plasma spraying and its high melting point (Tm = 2500 0C) in PVD deposition. The present thesis describes the development of corrosion resistant Nb coatings on stainless steel and brass substrates by the combined steered arc/unbalanced magnetron sputtering technique. Evaluation of the corrosion behaviour is performed by potentiodynamic polarisation measurements in 3% NaCl. It is shown that corrosion resistant Nb coatings, with passivation characteristics similar to that of bulk Nb, can be produced on stainless steel substrates by unbalanced magnetron sputtering at a low deposition temperature (T = 250 0C) under reduced ion bombardment. However, the ion etching pre-treatment of the substrate prior to deposition has a significant influence on the corrosion resistance of the coating/substrate system. The employed polarisation measurements reveal that a fully passive and protective behaviour could only be achieved if Nb ions from the cathodic arc source are chosen as the etching species. In contrast, coatings deposited after Cr ion etching from the arc source and inert Ar ion etching, utilising a glow discharge, exhibit localised breakdown and pitting of the substrate. Cross sectional TEM imaging and STEM-EDX analyses reveal that bombardment of the stainless steel substrate by the multiply charged Nb ions generates a compositionally intermixed, very fine crystalline or "amorphised" interface layer, with a thickness of ~3-8nm, depending on the Nb ion energy. It is proposed that this layer acts as an additional barrier against corrosion due to (i) the structural homogeneity achieved by amorphisation and (ii) chemical stabilisation due to the introduction of Nb in the near surface region. The energy of the bombarding Nb ions, i.e. the substrate bias voltage during the etching stage, was found to further influence the corrosion resistance. Best results are achieved with "medium" bias voltages in the range of -600V to -800V, which is believed to be due to an optimum combination of structural and chemical protection mechanisms. The fully passive corrosion behaviour could not be observed in the case of brass substrates. However, the PVD coating systems on brass and on stainless steel are superior, in the employed polarisation measurements, to commercially produced, electroplated Cr, Ni and Ni/Cr coatings on the same substrate materials. Other coating properties investigated in the present study include microstructure, hardness, crystallographic orientation and residual film stresses.

The oxidation behaviour of TiAlN based hard coatings with the addition of Cr and/or Y was investigated using scanning electron microscopy, scanning/transmission electron microscopy, energy dispersive X-ray analysis, thermogravimetry and X-ray diffraction. The coatings were deposited using the combined cathodic arc/unbalanced magnetron deposition technique. The main practical application of these films is dry high speed cutting in difficult to cut materials such as AISI A2 steel. Especially in the case of TiAlCrYN coating with an oxynitride and Cr-enriched overcoat, extensive research on the oxidation behaviour was performed and described here. Heat treatments in air between 600-1000°C for different duration were carried out. The Ti[0.44]Al[0.54]Cr[0.02]N coating was used as the starting point for the investigations. The effect of heat on the composition of the interface region was investigated. This region is of utmost importance for the adhesion of the film. In the case of TiAICrN the interface stability was not guaranteed because of diffusion of the substrate elements Cr and Fe to the coating surface after annealing for 1h at 900°C. In comparison, the diffusion of substrate elements Cr and Fe in a ~2.3 mum thick coating of Ti[0.43]Al[0.52]Cr[0.03]Y[0.02]N and of Ti[0.34]Al[0.62]Cr[0.03]Y[0.01]N with overcoat, reached only a distance of ~600nm into the coating. This was achieved by the diffusion of Y to the grain boundaries. Y probably reacted at the same time with inward diffusing O. The diffusion of Y to the boundaries was observed after heat treatment for 1h at 900°C or 10h at 800°C.Ti[0.26]Al[0.26]N/Cr[0.48]N was the coating with the least oxide layer growth after oxidation for 1h at 900°C. An oxide layer thickness of only ~100nm was measured. For the TiAICrYN coating with overcoat an oxide layer of 230nm and for TiAICrYN of 430nm formed after 1h at 900°C. TiAlCrN in comparison formed an oxide layer of ~800nm after 1 h at 900°C.The oxide layers formed after 1h at 900°C consisted mainly of an Al[2]O[3] and TiO[2] bi-layer in the case of TiAlCrN and TiAlCrYN. The addition of a Cr-rich oxynitride overcoat led to the formation of a mixture out of Al[2]O[3], Cr[2]O[3] and TiO[2] in the oxide layer. In the case of TiAlN/CrN, a solid solution consisting of Cr[2]O[3] and Al[2]O[3] was observed. In general a stress relief after heat treatment was observed. At the same time the formation of voids along the column boundaries was identified. This was explained with the relaxation and diffusion of defects created during the deposition process. The effect of different substrate materials on the oxidation behaviour was also investigated. It was found that the formation of substrate oxides on the coating surface is very much dependent on the onset point of oxidation of the substrate material itself. The oxidation of substrate material occurred mainly through growth defects and pinholes. In cases where cracks formed during heat treatment of the coating, the formation of oxides out of substrate elements were observed in cracks connecting the substrate with the coating surface. Changing the bias voltage altered the formation of cracks. This research emphasises the importance of Y in the oxidation mechanism of TiAlN based hard coatings. Y blocks the diffusion path along the column boundaries and thus stowed down the diffusion and oxidation process. At the same time the addition of Cr can increase the oxidation resistance considerably, which was observed in the TiAlCrYN coating with and without overcoat.

No description available.

The current state of the art with regards to the production of metallic foams is reviewed, with melt-based processes identified as the most promising for cost-effective large-scale production. The potential for metal carbonates as an alternative to currently-used titanium hydride foaming agents is explored, with calcium carbonate identified as the most suitable. The influence of a range of material and processing parameters on the stability of metallic foams in the molten state is discussed, and current methods of controlling melt viscosity and surface tension are reviewed. Characteristic features of the compressive deformation of metallic foams are described in the context of use as an impact-absorbing material, with a review of work in the literature linking the bulk mechanical properties to details of the cell structure. Calcium carbonate is found to be a highly effective foaming agent for aluminium. The foams obtained have notably finer cell structures than can be achieved in foams produced with titanium hydride, coupled with enhanced stability in the molten state. This is attributed to the presence of a thin continuous surface film of metallic oxide that counteracts the effect of surface tension. This film, combined with the finer cell structure of the calcium carbonate-based foams, is found to significantly reduce the rate of gravity drainage of the melt. The formation of the thin oxide film during foaming gives rise to a number of artefacts on the cell surface, including stretch marks and tear bands. A range of chemical and surface analysis techniques are used to identify the chemical composition and thickness of the oxide film. The distribution of refractory particles in the cell faces, which are commonly employed to stabilise molten foam structures, is found to be highly non-uniform in foams which undergo significant gravity drainage of liquid metal during the foaming process. Experiments in which the concentration of particles is varied demonstrate the importance of their effect on the melt viscosity in addition to their known role as a surface stabilising phase. The effect of alloy content and foaming gas on the stability of standing molten foams is also investigated in the context of other foaming processes. The formation of an oxide film on the surface of the cells is shown thermodynamically to be a necessary step in the production of low-density aluminium foams with a calcium carbonate foaming agent. A temperature-dependent upper limit on porosity is observed. It is established that this is the result of inhibition of the calcium carbonate decomposition reaction by its products as the thickness of the surface oxide film increases. The effect of varying cell size, porosity and chemical composition on the thickness of the surface oxide film is derived. The rate of thermal decomposition of calcium carbonate is found to be dominated by the partial pressure of carbon dioxide, with particle size and small impurity contents having only a small effect. Compressive mechanical properties of the foams produced are compared with those of foams produced with a titanium hydride foaming agent and theoretical predictions. A reduced cell size apparently minimises the influence of point defects on the properties of specimens of finite dimensions. A significant difference in the shape of the stress-strain curves of calcium carbonate- and titanium hydride-based foams is noted, with the latter marked by extensive serrations. This difference is demonstrated to be independent of differences in cell size. Microstructural analysis of foams in various stages of failure suggests that this is due to differences in the distribution of refractory particles in the two foams, which is in turn a consequence of the reduced extent of gravity drainage of liquid metal in the calcium carbonate-based foams.