ACOUSTIC MICROSCOPY OF CERAMIC-FIBER COMPOSITES .2. GLASS CERAMIC-MATRIX COMPOSITES

Two silicon-carbide-fibre composites, one with a magnesium-aluminosilicate matrix and the other with a calcium-aluminosilicate matrix, were studied using acoustic microscopy. Contrast was found between the fibres and the matrix, and there was, in addition, a distinctive contrast associated with the boundary between them. In some cases the contrast at the boundary was characteristic of a well-bonded interface, and in others it was characteristic of a crack; strong contrast was also found from cracks in both the matrix and the fibres. It was possible to obtain a good quantitative fit between observation and theoretical calculations of boundary contrast. Radial cracks and radial variations of the microstructure in the fibres were studied. From measurements of fringes in the areas of contrast and estimates of Poisson's ratio, values of the shear modulus and Young's modulus could be deduced; independent values from nano-indentation measurements were in good agreement. Some of the calcium-aluminosilicate composites were studied after straining below and beyond the elastic limit; and the appearance of cracks was related to a theoretical model, yielding a value for the interfacial strength within the range of reported values.