Interfacial properties of fibre reinforced ceramic matrix composites
A review of ceramic matrix composites development over the past thirty years is presented, with emphasis placed on their application in gas turbine engine components. The fracture mechanics of brittle solids are outlined and the toughening mechanisms operating within continuous fibre reinforced ceramic matrix composites are discussed. The importance of the fibre-matrix interface in governing the overall mechanical properties of a composite is highlighted with respect to the micromechanical properties of interface debond fracture surface energy OJ and frictional shear stress t. Current techniques for measuring OJ and t are listed, together with their inherent disadvantages. The requirement for a micro-indentation system that can be used to measure interfacial properties via individual fibre pushing experiments across a wide range of composite systems, is discussed. The development of a unique Scanning Electron Microscope (SEM) based microindentation system is described in detail. It enables dynamic, high magnification imaging of the indentor tip and specimen contact point, and continuously records applied load and tip displacement throughout the indentation cycle. A piezoelectric load cell, coupled to a specifically developed amplifier, enables load resolution of 2mN measured up to the maximum possible of 20N. Novel capacitance displacement gauge design gives a resolution of lOnm over a l00pm range. The instrument has been used successfully to measure the interface micromechanical properties across a wide range of silicon carbide fibre reinforced glass and glass ceramic matrix composites. This data has been correlated with interface structural information obtained via Transmission Electron Microscopy (TEM) and SEM. Effects of oxidation, fatigue testing and interface pre-synthesis via fibre coating, have been measured. Fibres with diameters ranging from 7JJ;mto 150pm have been tested to demonstrate the versatility of the device for interfacial property measurement across the full range of modem ceramic matrix composites. Successful attempts have been made to correlate changes in the interfacial OJ and t to changes in overall composite mechanical behaviour. Theoretical requirements for values of G, that introduce toughness to composites have been discussed and compared to those determined by experiment. Variation of t and its effect on matrix micro-cracking and the tough!brittle property transition of a composite has been measured. Other applications that exploit the instrument's high resolution and imaging capability have been demonstrated. They include hardness and modulus measurement of individual phases in heterogeneous materials, and direct observation of controlled crack growth in ceramic composites. Ideas for the development of the instrument into a more versatile SEM based mechanical test facility are proposed.