A comparative overview of molybdenum disilicide composites
MoSi2-based composite ultimate stress values will have to be determined by tensile testing, because of the plasticity which occurs at elevated temperatures. MoSi 2-based composite yield stress values are comparable with or exceed structural ceramic ultimate stress values at 1200 °C. At 1500 °C, structural ceramic ultimate stress levels are higher than the present MoSi2-based composite yield stress levels. However, one cannot use a structural ceramic at its ultimate stress, since it will have a 100% probability of catastrophic fracture. One has to employ a factor of safety of at least 2, or perhaps more, depending on the Weibull modulus of the structural ceramic material. Since MoSi2-based composites are at present in their infancy, it is very likely that, with further materials development, the MoSi2-based composite yield stress levels at 1500 °C can be increased by an additional factor of 2 and perhaps more at 1500 °C. The plot of per cent ultimate strain (measured in bending) vs. temperature is particularly important (Fig.7(c)). Structural ceramics have almost no ductility at high temperatures. By way of contrast, MoSi2-based composites exhibit significant ductility above the MoSi 2 BDTT (900-1000 °C). The ductility of MoSi 2- based composites is at least an order of magnitude greater than that of the structural ceramics at high temperatures. This ductility means that MoSi2-based composites will have much higher fracture toughness and resistance to catastrophic fracture at elevated temperatures than the structural ceramics. Thus, components made from MoSiz-based composites will be more reliable than components made from structural ceramics.
Author:A. K. Vasudfvan,J. J. Petrovic