Relationship between microstructure and mechanical properties of fired Ecuadorean clay

Hardness and fracture toughness measurements are reported for individual phases and interfacial composites present in fired specimens of an Ecuadorian clay mineral. Each investigation used a half disc from strength tests previously done and reported elsewhere. The aim was to rationalise trends in the macroscopic strength and density to quantify contributions from the important components of the microstructure. This was achieved through application of an etching and gold coating procedure prior to microindentation testing. The coating technique improved the clarity of the indents and made the diagonal and radial crack measurements more accurate. Two peaks in the strength versus firing temperature curve are shown to arise from opposite trends in the toughness of the quartz grains and the matrix phase, together with a no-linear build up of the volume of interfacial composite material which confines any cracks initiated in the residual quartz crystals at high firing temperatures.