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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The mechanical behaviour of woven fabric-based continuous silicon carbide fibre reinforced Pyrex (SiC/Pyrex) and calcium alminosilicate (SiC/CAS) matrix composites under quasi-static and cyclic tensile loading has been investigated. Both a plain weave and a satin weave architecture were examined for each material type. Under quasi-static loading for all systems except the low temperature processed Pyrex system (which failed prematurely) a linear elastic region was observed up to an applied strain of 0.04-0.06%. Above this strain (the matrix microcracking threshold) a reduction in the composite modulus was seen. The reduction in composite stiffness is attributed to matrix microcracking, and the morphology of matrix microcracking was examined and quantified using an edge replication technique. In all systems the matrix microcrack density was seen to increase approximately linearly with increasing strain up to failure. The corresponding reduction in the composite modulus at failure was 40-50%. Associated with the damage there is hysteretic behaviour and an increasing residual strain. The strain to failure of the satin weave composites was higher than the plain weave composites. In the cyclic fatigue tests the number of cycles to failure decreased with increasing peak stress level. A progressive reduction in the composite modulus was seen with cycles even when the applied strain was below the matrix microcracking strain threshold. It is likely that at strains below this threshold there is non-interacting matrix microcracking which does not initially affect the composite modulus. However, on continued tensile fatigue cycling these microcracks grow through a possible sub-critical crack growth mechanism reducing the laminate modulus. A modified shear lag model was used to model the reduction in composite stiffness as a function of the measured matrix crack density. The woven composite was converted to an equivalent cross-ply sub-laminate on to which the matrix microcracks were superimposed. A model allowing for the presence of microcracks in both the matrix and transverse plies gave the best agreement between the experimental and predicted reduction in modulus.

This Thesis is primarily concerned with the effects of abrasive machining (diamond grinding) and diamond indentation on the fracture properties of a range of silicon nitride materials. Test specimens machined to surface finishes representative of those found on Aero Gas Turbine components were produced for Modulus of Rupture (MOR) testing, and variations in the fracture strengths were assessed. Optical and Scanning Electron Microscopy (SEM) were performed as a means of identifying the nature of the defects found within these materials. Having determined the dependence of strength and reliability on the machined surface finish, attempts were made to palliate the machining damage by thermal annealing and Nitrogen Ion Implantation. X-ray diffraction residual stress measurements were performed in order to quantify the magnitude of the near surface stresses in both the "as machined" and annealed conditions.* Diamond indentation techniques (Vickers and Knoop) were employed in order to determine the hardness of the materials studied and to quantify the extent of the Indentation Size Effect (ISE). These studies were then extended to the point of indentation fracture as a means of assessing the materials fracture toughness (KIC) and the nature of the crack systems beneath the indentation. *Residual stress measurements were carried out on a sub contract basis at the CEGB Central Laboratories by P E J Flewitt and D Lonsdale, their help throughout this work is gratefully acknowledged.

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