Densification mechanisms and properties of boride-based composites

UHTCs possess outstanding physical and engineering properties, such as high hardness and strength, low electrical resistivity and good chemical inertness. The extremely high melting point and the low self-diffusion coefficient make the full densification of these materials very difficult: temperatures above 2000°C and the application of pressure are necessary conditions. However these processing parameters lead to coarse microstructures with trapped porosity, all features that prevent the full UHTC potential to be achieved. The composite approach via the introduction of secondary phases, such as metals, nitrides, carbides or silicides, has become a successful strategy to improve the densification and thermo-mechanical properties of UHTCs. In general the addition of these secondary phases does decrease the sintering temperature, but may generate residual secondary phases that are detrimental for high temperature applications. In this talk, an overview of the most important additives for UHTCs will be provided and possible densification mechanisms will be illustrated. Relevant mechanical properties, such as hardness, room- and high-temperature flexural strength will be reported.