This PhD project is set out to develop a high precision ceramic fabrication approach suitable for mass production, and to meet the needs of microengine application. A group of new processes have been developed and the results are characterized for fabrication of high precision ceramic oxides and composite microcomponents using soft lithography and colloidal powder processing. The materials chosen in the research are alumina, yttria stabilised zirconia and their composite for their excellent properties at high temperature.

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Petrological and neutron activation analysis of bricks and tiles from York and neighbouring sites with discussion of the analytical and documentary evidence for their production in York up until AD 1750.

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A low-clay version of fine translucent china was designed to have desired properties and acceptable behaviour during manufacture. Low-clay contents of 10 to ISw/o were employed to reduce the deterious effects of preferential clay particle alignment and its adverse effect on colour. For example, alignment of clay particles in cast wares causes anisotropic drying and firing shrinkages and these cause distortion. Replacing clay with a pre fired body with low Fe and Ti contents, as in the present case, allowed a very white material to be produced. The prefired body was made with calcium carbonate, aluminium trihydroxide, quartz and a small fluxing addition of an hydrated magnesIUm carbonate. Desification of a whiteware is enhanced by increasing volume fraction of the viscous liquid and reducing by its viscosity. Both of these also enhance sagging. Consequently, compositional change cannot on its own lead to the favourable combination of high density, required to give translucency, with little sagging, that will allow wares to be fired without significant distortion. It was found by trail and error that use of finer particles reduced the sagging occurring in the densification heat treatment and enhanced densification. This finding allowed the body to be designed so that it densified without sagging excessively. After establishing this important result, an iterative approach was employed to produce a ware that was very white, translucent, had the required thermal expansion coefficient for glaze fitting, shrank acceptably during the first firing and did not sag during the second firing that stimulated glost firing. The finding on sagging was applied to an anorthite/mullite porcelain body that was under development. This body was also made with a low-clay content and the same prefired body. This whitware has potential to replace bone china and hard porcelain for use in the servere service conditions of hotels and restaurants. It has a higher fracture toughness than hard porcelain but has the same scratch resistant glaze and is more resistant to thermal shock. The sagging of the anorthite/mullite porcelain was substantially reduced while the body was densified. This was achieved by using a combination of finer particles and a reduction in the liquid-phase content that developed during firing.

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The high oxidation resistance of ceramics based on silicon nitride has contributed to their potential for use in high-temperature structural engineering applications. The oxidation behaviour and its effect on mechanical properties of a range of hot-pressed ceramic alloys of composition Si6-z Alz 0z N8-z (1 < z < 4) has been investigated at temperatures between 1200° and 1400°C. The mechanism of oxidation has been identified by determination of the kinetics of oxidation and the nature of the oxide scales using X- ray diffractometry, and scanning and transmission electron microscopy. The rate-controlling process in the oxidation of these ceramics is that of the inward diffusion of oxygen to the ceramic/oxide interface where the oxidation reaction takes place. In the early stages of oxidation the composition of the oxide scale and the rate of oxidation is modified by the diffusion of grain-boundary segregated additive and impurity ions such as Mg and Ca into the scale. The redistribution of these elements on oxidation creates a ceramic of more stabilised structure and of higher oxidation resistance. By heat-treatment of large blocks of ceramic the mechanism of this redistribution has been identified as that of diffusion into the oxide scale with negligible dissolution into the β' crystal phase of the ceramic. The out-diffusion of elements is accompanied by the removal of grain boundary phases by the local growth of the β' phase. Local enhancement of the rate of oxidation by the out-diffusion of the elements Mg and Ca towards the scale gives rise to a low creep rupture stress for the ceramic via the formation of intergranular cracks which grow at sub-critical stresses. This has been determined by tensile creep rupture tests which have been performed on two ceramics of composition Si5A10N7 at temperatures between 1275° and 1325°C.

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