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PhD Thesis – Abstract

The appealing properties of ceramics include retention of strength and hardness at high temperatures, chemical inertness and low density. However, monolithic ceramics fail in a brittle, unpredictable manner making them unsuitable for many applications where reliability is a key requirement. It has been recognised that continuous fibre reinforced ceramic composites (CFCCs) can show pseudo-plastic, non-brittle fracture behaviour, whilst maintaining the attractive high temperature properties of monolithic ceramic materials. Such materials offer the prospect of improved thrust-to-weight ratios, greater efficiencies and reduction of hazardous emissions in gas turbine aero-engine applications. Currently available non-oxide based CFCCs usually have an interphase between the fibre reinforcement and the surrounding matrix. The interphase consists of graphitic carbon or boron nitride which is prone to oxidation. This effect is particularly severe when matrix cracks are present and under cyclic conditions. In contrast, an all-oxide based CFCC would be insensitive to damage by oxidation, even at high temperatures and after matrix cracking. Conventional all-oxide composites also rely on a fibre coating, or interphase, to promote crack deflection and frictional sliding along the fibre/matrix interface. A more recent approach involves the use of a controlled amount of fine-scale porosity. In this case the crack deflection takes place within the porous matrix, obviating the need for an interphase, and thereby providing opportunities for easier manufacturing and reduced cost. The main objectives of the work presented in this thesis have been to identify concepts for CFCCs stable at high temperatures for long lifetimes in oxidising environments and to develop and characterise such materials. Paper I reviews commercially available oxide fibres and a literature review of chemically stable weak interphases in all-oxide composites is presented. Papers II, III, IV, V and VI deals with development, processing, characterisation and evaluation of a large diameter sapphire fibre reinforced alumina composite with a zirconia interphase. Development of two different interphase concepts is reported in Paper II: a double carbon and zirconia interphase and a porous zirconia interphase. Flexural testing was used to characterise the materials both before and after ageing at elevated temperatures. The concept utilising the porous zirconia interphase was further explored, the manufacturing process was scaled up and results from extensive tensile testing at various temperatures and after ageing are described in Paper IV. Combustor rig tests of a flat composite tile are also reported. Paper VI contains a more detailed study of the room temperature mechanical behaviour and the results were linked to measurements of interfacial properties. The problem of formation of harmful emissions in gas turbine combustors is reviewed in Paper III, followed by a discussion of different techniques to lower the emissions and implications on material requirements. An alternative processing route was explored in Paper V: hot isostatic pressing was used instead of more conventional hot pressing. Papers VII, VIII, IX and X explores various aspects of porous oxide matrix composites reinforced with small diameter polycrystalline oxide fibres. Paper VII reports on a pre-preg based processing method to make porous mullite matrix composites reinforced with alumina or aluminosilicate fibre weaves. Stress-strain behaviour and notch sensitivity were studied in flexural testing. Based on the same processing technique, thin-walled tubes were manufactured and characterised in burst tests as reported in Paper VIII. A review of literature on mechanical properties and processing of porous matrix all-oxide composites is given in Paper IX. Paper X reports on an alternative processing route: phosphate bonding is used to strengthen the mullite matrix, which allows lower processing temperatures.