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No description available.

No description available.

Sliding wear of metal-on-ceramic, ceramic-on-metal, and ceramic-on-ceramic have been investigated using a tri-pin-on-disc machine. A technique has been developed for thin foil preparation for transmission electron microscopic examination perpendicular to the wear surface. The role of transformation toughening in the wear behaviour of zirconia ceramics has been investigated. In addition, the role of high strain deformation in a steel surface has been evaluated. The wear factor of 316L stainless steel pins worn against a zirconia disc was found to decrease as the load was increased, believed to be associated with metal oxide formation. TEM of the stainless steel revealed a worn surface which consisted of a mechanical mixture of metal oxide and heavily deformed metal. Deformation of the metal had occurred by shear banding with a microstructure similar to that observed in rolled specimens, although the texture formed was a wire texture rather than a rolling texture. The crystallite size was found to decrease towards the surface, demonstrating that the shear stress was a maximum at the surface. The shear bands at the surface had always been formed by the passage of the last asperity indicating that contact was plastic over the load range 6-60N/pin. The majority of wear occurred by transfer resulting from plastic overload, although a contribution to the material loss was made by metal extruded off the end of the pin as a result of the high strains. The depth of deformation correlated closely with the wear volume. The wear of the zirconia discs was found to be dominated by metal transfer. With Mg-PSZ, transformation occurred cooperatively in crystallographically determined bands. Microcrack coalescence led to preferential wear in these bands. However, with a Y-TZPdisc transformation appeared to have been responsible for widespread surface fracture. The wear of zirconia pins against a bearing steel disc gave limited metal transfer. Very little transformation of tetragonal to monoclinic was observed. However, milder forms of the transformation related wear mechanism did occur. Zirconia had formed a solid solution with the iron oxide, leading to the conclusion that the wear mechanism was tribochemically based. TZP worn against a ZTA disc showed evidence of very high temperature rises at the interface. The surface layer was amorphous and contained a mixture of alumina and zirconia suggesting that melting had occurred at the interface during sliding. At a depth of O.5pm. the surface consisted of heavily elongated tetragonal grains, with a low dislocation density, indicating a strain of at least 1.7. At a depth of 2-4pm a layer of monoclinic was found. There was evidence that the stresses imposed by friction extended to at least 8-10pm from the surface. TZP containing 20vol% SiC whiskers gave exceptionally low wear rates when worn against a ZTA disc. The greater wear resistance is believed to be a result of the improved load bearing capacity and of the higher thermal conductivity. It is clear that the poor thermal conductivity of zirconia dominates its tribological behaviour. Temperature generation was high enough to substantially reduce the driving force for transformation of the tetragonal to monoclinic, with a high enough temperature for plastic' deformation where a low thermal conductivity counterface was used. Where transformation occurred, its effect was to increase the wear rate.

Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Ceramic and Materials Science and Engineering." Includes bibliographical references.

The aim of the current study is to develop an efficient model which can predict low-energy compatible microstructures in ferroelectric bulks and film devices and their dynamic behaviour. The results are expected to assist in the interpretation of microstructure observations and provide a knowledge of the possible domain arrangements that can be used to design future materials with optimum performance. Several recent models of ferroelectric crystals assume low energy domain configurations. They are mainly based on the idea of fine phase mixtures and average compatibility, and can require intensive computation resulting in complex domain configurations which rarely occur in nature. In this research, criteria for the exact compatibility of domain structure in the form of a periodic multi-rank laminate are developed. Exactly compatible structure is expected to be energetically favourable and does not require the concept of a fine mixture to eliminate incompatibilities. The resulting method is a rapid and systematic procedure for finding exactly compatible microstructures. This is then used to explore minimum rank compatible microstructure in various crystal systems and devices. The results reveal routes in polarization and strain spaces along which microstructure can continuously evolve, including poling paths for ferro- electric single crystals. Also, the method is capable to generate all possible exactly compatible laminate configurations for given boundary conditions. It is found that simple configurations are often energetically favourable in conditions where previous approaches would predict more complex domain patterns. Laminate domain patterns in ferroelectrics are classified and corre- lated with observations of domains in single crystals, showing good agreement. The evolution of microstructures under applied mechanical and electrical loads is studied. A variational method, which minimises the overall energy of the crystal is developed. A new concept of transitional “pivot states” is introduced which allows the model to capture the feature that the microstructure in ferroelectric crystal switches between possible domain patterns that are energetically favourable, rather than assuming one particular domain pattern throughout. This model is applied to study the hysteresis responses of barium titanate (BaTiO3) single crystals subjected to a variety of loads. The results have good agreement with experimental data in the literature. The relationship between domain patterns and ferroelectric hysteresis responses is discussed.

The phenomenal growth rate for the use of engineering ceramics is attributed to successful scientific responses to industrial demand. These materials are replacing metal and its alloys in diverse applications from cutting tools and heat engine components to integrated circuits. Joining technology plays a vital role in this changing and evolving technology as success and failure comes with breaking new barriers. It is important to improve existing techniques and to develop new techniques that reliably join simple shape components to form complex assemblies or join dissimilar materials such as metal to ceramic. Joining of ceramics is not simple due to their high chemical stability and low coefficient of thermal expansion (CTE). Joining between metal and ceramic is usually carried out at elevated temperatures and upon cooling thermal residual stresses are induced that lead to joint failure or poor strength. Most metal-ceramic joints cannot be used over 500°C primarily due to the low melting temperature of the interlayer. This investigation was concerned with the successful joining for higher temperature applications (above 500°C) of two dissimilar high temperature oxidation and corrosion resistant materials, Fecralloy and silicon nitride. The primary focus was on the effects of process conditions upon the microstructure and mechanical properties of the joint and to also study/identify the joining mechanism. Two novel techniques were employed to join successfully the metal to ceramic. The first was by use of a thin Cu foil that did not remain after joining. Joining occurs by a process that results in partial melting of the Fecralloy interface, where Fe, Cr, Al and Cu reactively infiltrate into the silicon nitride. This liquid mixture causes partial dissolution of the silicon nitride interface, where Si and N diffuse into the Fecralloy. A thin reaction product layer was formed at the silicon nitride interface and our results suggested that this was AIN. The free surface Si and porosity of the silicon nitride along with the eutectic temperatures above 1100°C are all vital for this joining process. The highest average shear strength of a Fecralloy-silicon nitride joint produced by the method was 67.5 MPa. The second route was that of a powder metallurgy one, where cold pressed Ni-Al (1:1 molar) compacts were used to join successfully the Fecralloy to silicon nitride. The formation of NiAl from its constituents is highly exothermic and this is initiated between 500-650°C. The high temperature reached causes partial melting of the Fecralloy interface and dissolution/reactive wetting at the silicon nitride interface. Mostly Fe infiltrates the NiAl improving room temperature ductility, fracture toughness and yield strength. Molten Al from the interlayer reacts and wets the silicon nitride interface with small amount of infiltration and no reaction product forming. The reaction synthesis of NiAl was studied using DTA and TGA, where the effects of Ni particle size and heating rate were investigated. This joining process is highly dependant upon process conditions, the most important of which are applied pressure, heating rate and Ni/A1 particle size. The highest average shear strength attained was 94.30 MPa and this is attributed to good interfacial bonding, high pressure, moderate process temperature and dwell time. The exothermic formation of the NiAl interlayer that is densified and monophase was paramount for this joining process. The Bansal-Doremus kinetic model for evaluating the kinetic parameters from non-isothermal DTA data was shown to be valid. The results obtained were identical to those by other authors who used a different model and approach.