TEM study on UHTCs and composites

Research on UHTCs has generally involved two major fields: the improvement of mechanical properties and the oxidation resistance. High performances are achieved upon careful tailoring of the microstructure, obtained through an aware control of the sintering additives, secondary phases and densification process. Recent advancements in high temperature strength have been achieved through addition of W-based compounds, thanks to the formation of highly refractory secondary phases. Another vivid issue related to UHTCs concerns strategies to improve their low fracture toughness. In order to increase their intrinsic brittleness, long and short fibers are added to ZrB2. However, cautious choice of the sintering parameters must be paid, owing to the tendency of the fibers to react with the matrix at increasing temperatures, change their characteristic structure and thus lose their properties. The addition of fibers or sintering additive notably alters the oxidation behavior of monolithic ZrB2, through the formation of new crystalline phases and change of the viscosity of the outermost silico-boride glass. In this work, a series of UHTCs possessing outstanding high temperature mechanical strength, or improved fracture toughness, or exceptional oxidation resistance will be reviewed. In particular, the microstructure evolution upon sintering or oxidation will be studied by SEM and TEM and correlated to the specific thermo-mechanical performances.