Aspects of the crystal structure of diamond, and its associated defects, have been considered with reference to the effect such characteristics might have on its mechanical properties. Also, established resolved shear stress models, which account for anisotropy in conventional Knoop indentation hardness of all single crystals, have been reviewed. Particular attention has been given to the role of microplasticity and the nature of crack formation in the deformed zone formed beneath the indenter. It is then shown that a similar approach can be applied to the case where a cone, made from a softer material, replaces the conventional rigid indenter. By using different materials covering a range of hardness, impressions can be formed beneath which there is a controlled density and depth of dislocations. In this work, the 'soft' indenter technique has been extended to high temperatures and applied to study the plasticity of various types of natural and synthetic diamond. Consequently, the effect of temperature on the critical resolved shear stress of synthetic type Ib, and natural type Ia and type IIa has been established. Above a critical threshold temperature for the onset of plasticity, time dependent growth of the impression volume occurs whilst the mean contact pressure is decreasing. It is shown that geometrical similarity, i.e. the ratio of the impression size to dislocated volume, is maintained whilst the critical mean pressure continues to be exceeded during this process of 'impression creep'. Activation energies of about 2.9 eV and 1.2 eV were determined, from rates of volume change, for natural (both type I and II) and synthetic type Ib respectively. Whilst no significant differences were observed between 98.9% 12C (natural abundance) and 99.9% 12C (isotopically enriched) synthetic diamonds, their behaviour was most like that of a type IIb diamond. Finally, by studying type la diamonds with a nitrogen concentration ranging from 14 - 750 ppm, evidence is obtained which supports the suggestion that this element reduces the intrinsic resistance to dislocation movement and encourages the initiation of cracks in the diamond structure.

Investigations concerning the microstructure and mechanical properties, composition and chemical bonding of alumina coatings have been performed. Alumina coatings have been deposited by both ionized reactive magnetron sputtering (IMS) and conventional reactive magnetron sputtering (CMS) in an argon/oxygen discharge onto stainless steel coated silicon substrates. X-ray diffraction (XRD) was used for the phase analysis, and nanoindentation was used to evaluate the mechanical properties. Substrate temperature during deposition was <500°C, which is the technologically interesting temperature range to coat temperature sensitive substrates such as tool steels. Formation of the x-phase was observed at 472°C. At substrate temperatures <472°C evidence for the formation of the amorphous alumina phase was found. Films containing a mixture of K and θ-alumina phases was grown at 430°C. The crystalline film hardness was 22+-1 GPa, which is equivalent to values reported for alumina films deposited by Chemical Vapor Deposition (CVD). Films grown at the same temperature by conventional magnetron sputtering were X-ray amorphous, and the hardness was found to be strong function of the substrate temperature. Furthermore, a novel, very high rate reactive magnetron sputtering deposition process for alumina hard coatings at substrate temperature ≤250°C has been developed. Utilizing pulsed D.C. power to sputter A1+A1Ox off the target surface and partial pressure control of the reactive gas to maintain a certain partial pressure value (accuracy of better than 0.005 mTorr), fully dense, transparent alumina coatings could be produced at 76% of the metal deposition rate. The coatings have an elastic modulus of,140 GPa, a hardness of 12 GPa, a chemical composition close to stoichiometric, and a refractive index of 1.65.

The use of water as a hydraulic fluid in a pump necessitates the use of conformal contacts to reduce the high rates of wear and leakage losses that result from the low viscosity and lubricity of water. Swashplate type axial piston pumps are ideal in this respect because they incorporate such conformal contacts. Furthermore, the development of such a pump for use with water, especially sea-water, critically relies on the correct selection and application of materials. The purpose of this research work is firstly to examine the contact conditions within an axial piston pump for a range of sleeved and lined components manufactured from a variety of different materials. The use of finite element analysis with gap elements is a useful way of determining the contact pressure distribution between conformally contacting components. It is shown that this method gives excellent agreement with available analytical methods for the two-dimensional cylindrical and axisymmetric spherical cases, and thus can be extended to layered components. Extension to three dimensions, when the contact cannot be accounted for by plane strain or plane stress conditions, is also possible, allowing a much more representative analysis of the contact conditions within an axial piston pump. No single combination of materials is identified as being the most suitable, rather, the method enables the consequences of choosing materials for their tribological characteristics to be examined. Once the contact conditions are known within the pump it is then possible to more accurately design the pump components. However, conventional deterministic methods are not appropriate for designing ceramic components, due to the inherent scatter of limiting defects, and statistical methods are necessary. Thus the second part of this research work is aimed at reviewing and examining the different probabilistic design methods with the long-term view of determining which, if any, are best suited to the design of ceramic components in this particular application. It is conduded that no single method adequately predicts the probability of failure of ceramic specimens with more complex stress distributions than four-point flexure bars.

Amorphous hydrogenated carbon (a-C:H), diamond & diamond-like (DLC) thin films are some of the many terms used when referring to the generic group of coatings based on hard carbon. They are an emerging technological area within the surface coating discipline and are being increasingly used to improve the efficiency of a wide range of engineering components. In addition, the unique and extreme characteristics of these films result in unequalled material properties, such that in many cases a wide range of new and superior performance devices have only recently begun to be realised. This study focuses on hydrogenated & non-hydrogenated diamond-like thin films deposited by various plasma based, hybrid and beam deposition techniques. The wear resistant and low friction properties of these films are of great importance in many of the potential application areas and has attracted particular interest in recent years. Therefore the major thrust of this research has been on the tribological aspect, particularly in relation to other advanced ceramic coatings, and to highlight the applicability of endurance wear tests used to evaluate diamond-like films. The main findings have been:- a} That carbon can be deposited by several techniques in a hard amorphous phase, the properties of which depend heavily upon the conditions, substrate choice and method of deposition. For a particular technique, material properties can be made to be repeatable by a good understanding of deposition process control. b} The use of plasma based hybrid PVD and beam methods have resulted in a considerably improved structural performance of the films over those produced by the direct evaporation of graphite. The introduction of a hydrocarbon gas into the plasma at the synthesis stage has also been shown to provide further improvements in the physical properties which has correspondingly led to an enhancement in the tribological behaviour. The levels of hydrogen, whether in an unbonded or bonded form, included in the film after deposition has been demonstrated to affect the mechanical and optical properties of the considerably. c) The wear resistant and frictional performance of these coatings has been shown to be variable, depending upon the method and conditions of deposition as well as test parameters such as humidity, surface roughness, film structure, adhesive strength and oxide/impurity formation. In some cases the tribological performance was found to be excellent. The presence of the diamond-like carbon coating has been shown to be beneficial in reducing wear between contacting bodies experiencing relative movement by encouraging the formation of a carbon transfer layer on the surface of the counterface material which acts as a zone of low shear and provides a physical barrier to tribo-chemical interactions. Under certain conditions, such tribo-chemical interactions can occur readily at the interface, facilitating the formation of strong interfacial bonding and increased wear. d) The inclusion of metallic elements into the carbon matrix has been shown to enhance the wear resistant properties of the film to only a small extent, although at the expense of a deterioration in the friction coefficient. The most beneficial effect of doping carbon films with metal species has been the improved resistance to thermal degradation. e) Thin intermediate layers of titanium nitride have also been shown to produce a remarkable improvement in both wear resistance and frictional performance of the diamond-like carbon films to an extent which appears to be related to the level of stoichiometry of the titanium nitride. The main mechanism behind this increased performance appears to be due largely to an enhancement in adhesive strength at the diamond-like carbon/titanium nitride junction, with an increase in load support being provided as a secondary benefit. f) A critical assessment of the available techniques and methodology available for testing hard carbon films has been made and in some cases methods have been found to be either entirely inappropriate or appropriate only when suitable precautionary measures have been taken. These difficulties largely stem from the exacting demands of thin, hard layers of diamond-like carbon due to its unique and extreme mechanical, electrical and optical properties.

Acoustic arrays have been widely studied and can be used for a variety of applications. Existing acoustic array systems have mainly utilised linear and planar geometries. Such geometries have limited scan angles and can suffer from beam broadening and large grating lobes. This thesis presents a new acoustic array topology, the random spherical volumetric array. The system developed consists of 64 omni-directional microphones arranged at pseudo random locations within a spherical volume. The spherical array geometry provides the potential for full elevation and azimuth scan coverage. The projected aperture of a spherical array is direction independent, with the beamwidth and gain of the direction response being constant for all scan angles. The non-periodic element arrangement eliminates the possibility of large grating lobes and results in an array pattern with an average sidelobe level that is inversely proportional to the number of array elements. These properties enable wide-angle beam steering over a very large frequency bandwidth. The potential of acoustic volumetric arrays is examined and the results of theoretical and experimental investigations are presented. Holographic techniques have been implemented on the experimental system to produce images of sound sources and of reflections in the test environment. The concept of a synthetic volumetric array is introduced in which original synthetic aperture and multiple frequency techniques can be successfully used to reduce the average sidelobe level of the random spherical volumetric array. Initially, the acoustic random spherical volumetric array was envisaged as an inexpensive test-bed for microwave and radar system algorithm development. Since that time it has been found that application areas for the random spherical volumetric array also include covert surveillance operations, acoustic imaging and auditorium characterisation. Development of the system could allow security forces to monitor large crowds and riot situations; help in the detection of sniper location; and assist designers to build better auditoriums by highlighting areas of high reflection and reverberation.

The aim of this work has been to use the soft impressor technique to investigate the plastic deformation of single crystal diamond and in particular to determine the effect that single substitutional nitrogen has on plasticity. Traditionally hardness tests in the form of Vickers or Knoop rigid indenters have been used to investigate the mechanical properties of materials which cannot be fabricated into tensile or three point bend test specimens. The high stress concentrations created by these types of test introduce a large degree of brittle failure in ultra-hard, covalently bonded materials. The soft impressor technique, on the other hand, allows large pressures to be applied without large stress concentrations. The result is that plastic deformation can be more readily induced into super hard materials such as diamond. This work has shown that not only can diamond be readily plastically deformed but that traces of nitrogen impurities within the lattice have a significant effect on the conditions necessary to produce dislocations. For this work, several different soft impressors were used to produce a range of pressures in the temperature range 800° to 1400°C. A selection of synthetic (HPHT) diamonds with various nitrogen concentrations were impressed and compared with impressions placed in natural type IIa specimens containing no nitrogen but heavily dislocated. Numerous analytical techniques were used to determine the level of deformation produced and gain a better understanding of the effect of nitrogen related defects. The first two chapters of this thesis review, first plasticity and then diamond, with reference to those properties/characteristics relevant to this topic. The third chapter discusses the principle of the soft impressor technique and the methodologies used. In the fourth chapter, models by which single crystal diamond plastically deforms are introduced, together with results that have extended the brittle-ductile transition schematic produced by Brookes, EJ. (1992). Results on the effect of dwell time and the phenomenon of impression creep are also presented. The fifth chapter identifies the predominant defects associated with substitutional nitrogen in HPHT diamond and presents profiles of impressions for diamonds with different 'grown-in' defect levels. The results are discussed and conclusions are made, in conjunction with suggestions for further work in chapter 6.