Role of Porosity in the Effect of Microcracking on the Thermal Conductivity of Brittle Ceramic Composites
Fracture-mechanical principles were used to predict the role of porosity in the formation of microcracks in brittle ceramic composites containing a dispersed phase with a coefficient of thermal expansion less than that of the matrix. It is shown that microcracking will occur only if the amount of porosity lies within a region Pi<P<Pa, and that all the microcracks will be of equal size. Using this criterion, the effect of the microcracks and pores on the thermal conductivity is examined. The expected behavior includes a sharp drop in the relative thermal conductivity and diffusivity at Pi, followed by a porosity-independent value between Pi and Pa. These conclusions were confirmed by measurements of the thermal diffusivity of composites containing silicon carbide dispersed in a matrix of magnesium oxide, beryllium oxide, and aluminum oxide, each with a range of compositions and amounts of porosity.
KeywordsThermal Conductivity Stress Intensity Factor Thermal Diffusivity Silicon Carbide Crack Size
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