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Geological processes give natural clays a different structure to that of clays that are reconstituted in the laboratory. In soft clays, this structure often breaks down under loading; this is called destructuration. This project aimed to develop a model to predict destructuration in soft natural clays. An understanding of the main characteristics of the behaviour of these clays detained was from data reported in the literature. Existing frameworks that describe the behaviour of these clays were reviewed, and basic concepts proposed to model structured soils. The Sensitivity framework (Cotecchia & Chandler, 2000) uses sensitivity as a parameter that can represent structure in both volumetric and stress space before significant destructuration takes place. Study of the behaviour of three soft clays with low to medium sensitivities; Sibari, Bothkennar and Pisa clays, demonstrated that sensitivity changes in parallel with destructuration during both volumetric compression and undrained shearing such that there is a single expression that directly relates change in normalised sensitivity to change in damage strain, where the increment of damage strain is the magnitude of the vector of plastic strain increment. This destructuration law was used to extend an existing model, the Three-surface kinematic hardening (3-SKH) model which was developed by Stallebrass & Taylor (1997) for reconstituted clays. The new model requires only three new parameters to represent structure and its degradation that can each be derived from data from a single isotropic compression test. They are: the initial sensitivity, which represents the initial degree of structure in the natural clay; the ultimate sensitivity, which represents the stable elements of structure in the clay; and the parameter k, which controls the rate of destructuration with plastic strains. The other parameters used are the same as in the 3-SKH model and are derived from data from tests on the corresponding reconstituted clay. The model was evaluated against data from tests on Bothkennar and Pisa clay. Qualitatively, the model could predict the important features of behaviour observed in these clays. Quantitatively, results of analyses showed that determining initial sensitivity in a consistent way by using the Sensitivity framework leads to predicted values of undrained shear strength within 10 to 20% of the experimental values. Typically destructuration was correctly predicted in analyses simulating volumetric compression, but it was over-predicted by about 15 to 25% in analyses simulating undrained tests. This could be improved in some cases by using an ultimate sensitivity greater than unity in analyses simulating tests on specimens that are likely to have stable elements of structure arising from fabric. Structural anisotropy seemed to influence the behaviour of Pisa clay, and a model including structural anisotropy may improve predictions on such soils. The main limitation of the current research is the difficulty in determining the initial stress state and sensitivity to be used in the analyses; improvement of this should be the prime aim of further work.

A systematic study of the properties of ion-exchanged float glass has been carried out by a.c. impedance spectroscopy. The measurement of electrical parameters allows the ionic mobility within the surface region to be investigated separately from the bulk glass. The correlation of a.c. impedance measurements with electron probe micro analysis, infrared reflectance spectroscopy, differential scanning calorimetry and dynamic mechanical thermal analysis has lent some new insights into anomalous behaviour of ionic transport under these circumstances. A highly resistive layer is found to exist as a result of the ion-exchange process in float glass. This is not attributable to compressive stress alone nor does the mixed alkali effect (MAE) operate within this diffusion zone. Glasses of the same composition prepared by homogeneous mixed-melting, however, indicate a strong mixed alkali effect. Infrared reflectance spectroscopic measurements clearly illustrate a more uniform distribution of non-bridging oxygen ions (and therefore some structural relaxation) as a result of ion-exchange. This also holds true for the comparison of ion-exchanged glass and mixed-melted glass. This result clearly shows that a different structure is generated depending on whether alkali cations are mixed homogeneously or via the ion-exchange process. It is proposed that some type of foreign ion repulsion effect (FIRE) operates when the larger cation is substituted into the glass below T_g. The repulsion of such foreign cations, and their search to find their own new sites causes their immobilisation (and a slow ion-exchange process) via the break up of conduction pathways, and thus the conductivity continues to decrease without any recovery, as more K⁺ ions are introduced. In contrast, only a weak mixed alkali effect is apparent in melt-grown lithium-alumino-silicate compositions and no high resistance (or cation immobilisation effect) is found in the ion-exchanged alumino-silicate system.

The superior qualities of ceramics such as high hardness, chemical stability and wear resistance make them promising tribological materials for machine elements, for example, pumps, bearings and heat engines. Ceramics are hard and brittle, therefore, machining such materials is time-consuming, difficult and expensive. A low cost machining process that can remove material rapidly while maintaining a good surface finish is required. The general aim of the studies presented is to find a correlation between the wear mechanisms and surface modifications induced by interactions of tribochemical nature and to identify the most effective combination of parameters involved in producing high material removal rates of ceramics. This study concentrates on silicon nitride, as this material has most potential for use in industry. The influence of lubricant chemistry on the friction and wear of silicon nitride is investigated using a ball-on -plate tribotester and a modified four-ball machine. A wide variety of liquid lubricants and additives supplied by Castrol International are tested to evaluate the role of the chemical characteristics of the lubricant on the friction and wear of silicon nitride. Surface and chemical analysis results reveal that by using different chemistries of liquids, the material removal rate and the surface finish of the silicon nitride can be significantly altered. The highest material removal rate is obtained when using the ester base fluid T80884 + 0.3% triethanol amine. This gives an increase by four fold when compared to the material removal rate obtained with the industrial reference slurry Kemet. The topography of the silicon nitride ball after the grinding test is found to be very smooth indicating that the predominant mechanism of material removal rate could be due to a tribochemical reaction occurring at the contact interface.