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The relationships between fabrication variables, microstructure and selected properties of carbon bonded magnesia-graphite refractory composite materials have been investigated. A novel optical microscope method of characterizing the morphology of flake graphites was developed and used to determine distributions of length and thickness and average aspect ratios for the four graphite samples used in the study. The compaction behaviour of magnesia alone and in combination with the flake graphites has been studied in some detail and the microstructures of the products elucidated. It is shown that the amount of magnesia of small particle size plays a significant role in determining the graphite-graphite contact area in the structure. An irreversible volume expansion is observed on firing composites, the magnitude of which can be related to the microstructure and the graphite content. A phenolic resin binder restricts this expansion. It is shown that the carbon binder does not bond to the graphite phase and only weakly, if at all, to the magnesia. Consequently the strengths and moduli are low and show only a small variation with graphite type. The effect of adding graphite to carbon-bonded magnesia is to lower the strength slightly, but increasing the graphite content from 20-30% causes a small increase in strength. Increasing the amount of carbon bond from pitch has little effect on strength at levels of 5-15% whereas over the range 5-13% the resin binder has a more pronounced effect. The most significant factor affecting the strength and modulus of fired composites is the amount of silicon or aluminium, added as oxidation inhibitors, which react to form carbide and nitride phases. Finally, a brief study of slag penetration shows that this can be reduced by decreasing the amount of oxide fines in the composite because of the changes in microstructure that, result.

The potential application of nitride based ceramics in structural applications is dependent on their performance at elevated temperatures. To meet these demands the microstructure of a syalon ceramic has been refined. Improved processing techniques, by the use of high purity powders and "balanced" compositions, have been shown to improve the degree of inter granular crystallisation. The key aspects are the reduction of impurity levels, particularly calcium, and the use of high nitrogen compositions such that full crystallisation is achieved. The achievement of complete crystallisation results in the elimination of subcritical crack growth during fracture and high temperature creep processes dominated by non-cavitational grain boundary diffusional processes. The mechanism for oxidation has been identified by determination of the kinetics, surface and sub-surface reactions. A temperature limit of l300 oC has been identified by the reversion of YAG to a eutectic liquid by reaction with the Si02-rich oxidation layer. In this regime oxidation rates are determined by the YAG reversion and cation out-diffusion to the oxide layer. Oxidation effects have been shown to be responsible for the onset of sub-critical crack growth at temperatures above 1300oC, where crack extension results directly from YAG reversion effects. Enhanced high temperature performance above and beyond l300oC by surface transformation and surface coating was investigated. Surface microstructures based upon BI and Si2N20 have been shown to increase oxidation resistance upto 1375oC but are difficult to form without substantial surface degradation. Si3N4 and SiC coatings deposited by chemical vapour deposition (CVD) were found to have similar effects, increasing oxidation resistance beyond 1300oC.

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The present work has been concerned with a study of the structure of germanium and silicon dichalcogenide glasses. Initially, the aim of this research was to obtain direct structural evidence of reversible photostructural changes in bulk germanium chalcogenide glasses. However, having confirmed the existence of such changes using EXAFS and neutron scattering techniques, it became obvious that a more comprehensive understanding of the as-quenched, annealed glass structure is required before a valid interpretation of the data can be made; in particular, the extent of medium-range order in these systems is of interest. Although limited to few examples at the moment, the potenital of both structural modelling and NMR studies for solving such problems has been investigated in a variety of inorganic glass systems, and these approaches can now be extended to those glasses exhibiting photostructural changes. Modelling studies of the structure of a-SiSe<SUB>2</SUB> have shown that the total correlation function T(r) is sensitive to different structural features in the generated glass structure. The present studies heavily favour a structure based on chains of edge-sharing (SiSe<SUB>4</SUB>) tetrahedra. A degree of spatial correlation is required between the chains, such as can only be obtained by short lengths of parallel chains 'pinned' by corner-sharing tetrahedra. <SUP>29</SUP>Si NMR studies of vitreous silica have shown that water, incorporated into the SiO<SUB>2</SUB> network as OH, acts as a major source of spin-lattice relaxation in this system. Numerical methods of self-calibration ('phasing') and removal of data truncation and lineshape apodization effects have been investigated, thereby enabling quantitative information concerning the Si-O-Si bond-angle distribution in silica to be obtained. Although these algorithms are presented with reference to NMR spectroscopy, their extension to other branches of spectroscopy is obvious. NMR has also been used to probe defect states in a-Se. <SUP>77</SUP>Se spin-lattice relaxation time data provide evidence of a defect state (perhaps a VAP) in equilibrium with C<SUB>1<SUP>0</SUB></SUP> centres in the glass.

This thesis is concerned with the microstructures and wear characteristics of eight ceramic tool materials which have been, or are currently, commercially available as indexable inserts for lathe tools. Two of the materials are alumina-based and the other six are derived from silicon nitride. The opening chapters of the thesis outline the essential features of metal cutting, the properties required of a good tool material, the development of ceramic tools and possible wear behaviour as discussed in the literature. Subsequent chapters describe the experimental procedures adopted in this work and both present and discuss the results obtained. The eight ceramics have been microstructurally characterised by the use of X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. Machining tests have been performed using five common workpiece materials, spanning a range of nickel and iron contents: mild steel, stainless steel, two nickel-based superalloys and commercial purity nickel. The two alumina-based materials were found to wear in a different, less severe manner to the silicon nitride-based tools. The mechanical response to surface contacts was established using identation techniques to give hardness (as a function of contact size and temperature) and fracture toughness (as a function of temperature) to test the correlation between these properties and wear behaviour, but this proved to be unfruitful. Since other classes of tools can be subject to dissolution/diffusion wear and little is known about the compatability of these ceramics with molten metals, a second type of investigation was instigated. The involved immersing pieces of ceramic in samples of molten workpiece materials, followed by cooling, sectioning and examining in the scanning electron microscope. Vast differences in the behaviour of the materials were observed and results from these tests correlated well with machining data, indicating the importance of high temperature stability with respect to the hot workpiece. Drawing on the experimental observations made during the project, the thesis concludes with suggestions for improving and optimising ceramic tool materials for turning purposes.

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