TaB2-based ceramics: microstructure, mechanical properties and oxidation resistance

Among ultra-high temperature ceramics, major attention has been devoted to zirconium and hafnium borides and carbides. Tantalum composites remain a less explored class of ceramics. In this contribution, TaB2-based ceramics were hot pressed with addition of 5-10 vol% MoSi2. Temperatures in the range of 1680-1780°C led to relative density around 90-95%. The microstructure was studied through X-ray diffraction, scanning and transmission electron microscopy and the results enabled the rebuilding of the densification mechanisms occurring upon sintering. The hardness was about 18 GPa, the fracture toughness 4.6 MPam1/2 and the room temperature flexural strength was around 630 MPa, but abruptly decreased to 220 MPa at 1200°C. The composite containing 10 vol% of MoSi2 was tested in a bottom-up furnace in the temperature range 1200-1700°C for 30 minutes. The microstructure appeared covered by a SiO2 layer, but the bulk remained unaltered up to 1600°C. At 1700°C the specimen vaporized. Nanoindentation was employed on the oxidized cross section to detect eventual mechanical properties modification associated to chemical/microstructural change.