Microstructure and properties of ZrB2-based ceramics

Microstructure and properties of zirconium diboride based ceramics, sintered without sintering aids and with the addition of 4 wt% of nickel, are evaluated. Although the two powder compositions have been densified by hot pressing, only the presence of Ni allows the achievement of high final densities (98%), while pure ZrB2 presents a residual porosity of about 13%. All the same, this latter material has a rather constant flexnral strength (300-350 MPa) up to 1400°C, while the doped material exhibits an improved strength in the temperature range 600-800°C and a strong strength degradation above 1000°C. Phenomenologies governing this behaviour are discussed. The Ni-doped material reveals the presence of NJ-rich grain boundary phases mainly located at triple points and of spherical shaped particles of glassy phase constituted by boron oxide and metal cations. At high temperatures, these features favour the surface and subsurface degradation because of oxidation reaction, that propagates toward the bulk material through the grain boundaries. Other mechanical (hardness, toughness, Young's modulus) and physical (thermal expansion, electrical resistivity) properties are discussed in terms of the microstructural characteristics and composition.