Hot-Pressing of Niobium Boride

Abstract

Hot-pressing was carried out with NbB₂ powder in graphite die under the conditions of 1650° to 2240°C and 50 to 900kg/cm², in the atmosphere (CO+N₂) or in vacuum. The microstructures, densities and Rockwell hardness of the compacts were investigated.
The microstructures were mainly dependent on hot-pressing temperature, and to less extent on hot-pressing pressure. The grain growth was observed in the compacts hot-pressed above 1740°C. In the compacts hot-pressed above 2000°C, the grain growth was intensive and a number of microcracks transversing the grain boundaries were observed.
The density can be expressed as a function of hot-pressing temperature and the logarithm of hot-pressing pressure as follows:
ρ(g/cm³)=-3.58+0.00336T(°C)+1.19logP(kg/cm²)
The Rockwell hardness increased with increasing densities of the compacts. However, the compacts hot-pressed at higher temperatures and lower pressures showed rather higher hardness than those of the same densities hot-pressed at lower temperatures and higher pressures.
The compacts hot-pressed in vacuum had higher densities than those hot-pressed in the atmosphere at the same conditions. On the other hand, the compacts hot-pressed in vacuum showed much lower hardness compared with those of the same densities hot-pressed in the atmosphere.