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Composition-structure-property relationships were studied in glasses of the type used to produce glass lonomer dental cements. These materials are currently being developed as a bone cement in joint replacement surgery. Initially, a simple quaternary (25i02-Al203-CaO-CaF2) glass was produced and was found to undergo minimal fluorine loss. This results in a glass whose composition is reproducible between batches. This composition, however, was too reactive to form a glass ionomer cement. A glass-ceramic approach was pursued to deactivate the glass and following heat treatments, cements were produced whose properties varied depending upon the precise heat treatment used. The heat treatments above the glass transition temperature resulted in amorphous phase separation, probably by a spinodal decomposition mechanism, followed by a two stage crystallization of calcium fluoride (fluorite) and anorthite. The anorthite phase crystallized from fluorite nuclei and, in effect, crystallization occurred by a bulk nucleation mechanism. The glass will also undergo significant crystallization to fluorite below the glass transition temperature, with surface nucleation the dominant mechanism. This explains the deactivation of ionomer glasses used commercially to control the setting properties of dental glass ionomer cements. The structural role of fluorine in these glasses was also investigated. It seems that fluorine is present in the glass network, rather than existing as a discreet entity bonded to a modifying cation. There is evidence that the fluoride ions are bonded only to aluminium sites and act to disrupt network connectivity. A commercial glass was examined and was found to be clearly phase separated. This glass was found to crystallize to apatite and mullite. Apatite is the mineral phase of bone and enamel and this may explain the excellent biocornpatability of glass ionomer cements. Glasses containing sodium and phosphate ions have also been produced. The sodium containing glasses were all found to crystallize from the melt to calcium fluoride. The phosphate containing glasses formed cements without any subsequent modification, and following a simple heat treatment, all of the phosphate containing glasses were found again to crystallize initially to an apatite phase. Selected compositions additionally crystallized to mullite. These glass-ceramics were tougher than the base glasses and this is attributed to the presence of interlocking needles of the two phases. Some of the glasses were easily castable to produce optically clear glasses, and in addition the glasses underwent bulk nucleation via prior amorphous phase separation, to give needles of apatite and mullite. These glass-ceramics thus show tremendous potential for use as a bloactive bone substitute material or as a castable crown.

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The adhesion and subsequent failure of a commercially available photocured resin applied to various ceramic substrates, used for electronics applications, has been investigated. The adhesive system was studied under both ambient and hostile, (water immersion at 50°C) conditions. The environmental stability of the system was investigated using two different mechanical tests, (simple butt-joints and a modified Boeing wedge test). In each case, the precise locus of failure was determined by a combination of surface analytical (XPS and ToF-SIMS) and microscopy techniques, (SEM). On the basis of these results, adhesion and failure mechanisms are proposed for the system. The presence of defects within the adhesive was found to initiate failure under ambient, (dry) conditions. Prolonged immersion in water, (for at least 2 weeks), was found to result in near-interfacial failure with a thin, (of the order of nanometres), overlayer of polymer remaining on the surface of the adherend. Subtle differences between this polymer overlayer and the bulk resin were observed. A model is proposed in which one of the minor resin components aggregates at the inorganic surface, creating a localised region which is extremely hydrophillic in nature. This makes the system particularly vulnerable to attack by water. In order to test this model, a reformulated batch of resin underwent the same environmental tests. The modified resin displayed a modest improvement in its ability to withstand hostile conditions, (prolonged immersion in water at 50°C) and the polymer residue remaining on the inorganic substrate was identical to that of the bulk polymer. This project has provided a unique opportunity to study the adhesion of a specific class of adhesives to bulk ceramic oxides, rather than to oxides of metals. One of the major benefits of which, was therefore the absence of any effects due to corrosion of the substrate. The project was also unusual in that it effectively went "full-circle", with the resin undergoing reformulation on the basis of the results obtained from the standard resin. The reformulated product was found to have superior wet adhesion, but poor mechanical properties.

The response of samples of unidirectional and cross-ply Nicalon fibre-reinforced calcium aluminosilicate (CAS) to a variety of thermal regimes has been examined using microscopy techniques and retained mechanical property measurements. The degree of matrix damage has been investigated by observation and measurement of cracking features and the results used in simple models in order to relate the occurrence of matrix cracking to stress distributions in the laminates. Thermal shock induced matrix crack damage was first seen to appear on the end faces of the unidirectional [0]16 laminate at a temperature differential of 400 °C and in the transverse plies, parallel to the longitudinal fibre direction, in the [0/90]3s cross-ply composite at a temperature differential of 350 °C. At more severe thermal shocks the next damage in both laminates was cracking in the matrix perpendicular to the fibre direction. The density of matrix cracking was seen to increase, initially, with increasing severity of thermal shock, but then to be less extensive at the highest temperature differentials (800 °C) used in this study. Crack density data for the unidirectional material at increasingly severe thermal shocks were compared with literature data for cracking under quasi-static loading using a simple thermal shock analysis incorporating a stress reduction factor. The effect of matrix cracking on retained mechanical properties has been examined by means of three-point flexure testing and values for Young's modulus, onset of non-linear behaviour and retained strength of the composites have been determined. Multiple thermal shock tests indicated that thermal treatment of previously cracked samples accelerated the rate of deterioration in the retained properties of the composite. It was proposed that the response of the samples to changes in the duration and severity of thermal treatments was consistent with interfacial modifications that have been reported to occur in this composite system at elevated temperatures. The suitability of applying a modified ACK model to predict critical temperature differentials for matrix cracking in the unidirectional laminate and longitudinal plies in the cross-ply composite has been tested. This approach combined applied thermal stresses, calculated using the simple thermal shock formula, with residual stresses, obtained from the model proposed by Powell et at. (1993). This method was found to be valid for the unidirectional material providing that some of the key parameters were determined independently. The use of a tunnelling crack model to predict thermal shock induced matrix cracking in the transverse plies of the cross-ply composite was less successful. This was partially attributed to the observed cracking patterns generated in the cross-ply material by flexure tests not conforming to those expected from stress calculations or reported from tensile tests.

This work is concerned with the processing and properties of chromiuni reinforced alumina ceramics with the Cr particles in both the micro- and nano-scale ranges. The influence of processing and microstructure on the mechanical properties has been studied. Al2O3-20vol%Cr micro-composites have been fabricated using both sintering and hot pressing techniques. Sintering environment has a crucial influence on the microstructural development of the pressureless sintered Al2O3-Cr composites. It was found that too little or too much oxygen is detrimental to Al2O3/Cr interfacial bonding. Attempts have been made to improve the Al2O3/Cr interfacial bonding by sintering in a graphite powder bed in order to control the oxygen partial pressure. The fracture toughness of the composite with strengthened interfaces was the highest of all the sintered samples. However, the improvement is limited by the brittle fracture of Cr. This may be caused by the high carbon content associated with Cr particles in the composite. The ductility of Cr was higher in the hot-pressed Al2O3-Cr samples. The possibility of further toughening Al2O3 by Cr80Ni20 and Cr20Ni80 alloys with higher ductility was explored. It was shown that 20 wt% of Ni present in the alloying phase did not change the ductility, but when the Ni content increased to 80 wt% the crystal structure changed to fc.c., giving a inherently ductile metal. However, the large thermal mismatch between Al2O3 and Ni/Cr alloys led to a high density of microcracks at the interfaces. The composites with different metallic phases had similar' fracture toughness values as measured by double cantilever beam testing. Among the Al2O3-CrxNi1-x composites, the highest fracture toughness, 5.8 MPa m1/2, was achieved by the hot pressed Al2O3-Cr composite. This value is comparable to values measured for other alumina-metal systems. The poor bonding at the alumina/metal interface is the main limitation to toughening in these composites. Thus, it may not be possible to have a strongly bonded and ductile reinforcement. The pressureless sintered Al2O3-Cr composites with different particle sizes showed different thermal shock behaviour. The composite with fine Cr particles exhibited a thermal shock behaviour which is typical of engineering ceramics, but with an improved critical temperature difference compared to sintered Al2O3. The specimen with a larger Cr particle size showed gradual strength degradation with increasing temperature difference. The increased fracture toughness, low initial strength and low Young's modulus of the composite are the primary reasons for the greater strength retention following quenching. Although Al2O3 was toughened by Cr and Cr/Ni alloys, the strength of the micro-composites was not improved as the metal particles acted as large flaws. In order to reduce the flaw size nano-composites were investigated. Al2O3-5vol% Cr nanocomposites were fabricated using a chemical method. Optimisation of the processing procedure led to a desirable microstructure and significantly increased strength. Among the nanocomposites, the highest strength, 736+/-29 MPa, was achieved by hot pressing at 1450°C. The improved strength of the nanocomposites is the consequence of the microstructure refinement by homogeneously distributed nano-sized Cr particles. The nanocomposites are slightly tougher than the parent Al2O3, although the values of the fracture toughness are lower than those for the 20vol% micro-scale particle toughened Al2O3. Thus, a small degree of toughening and significant strengthening have been achieved by Al2O3-Cr nanocomposites.

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