Zirconium diboride and hafnium diboride were fabricated by hot-pressing at 180OoC and 120,000 psi. Bend strengths were measured on the fully dense materials from 25' to 140OoC in an argon atmosphere. These diboride compounds do not exhibit any gross plastic flow in the temperature range studied. The bend strengths go through a maximum between 700' and 1000°C and vary from 39,000 to 68,000 psi for HfB2 and 30,000 to 56,000 psi for ZrBz. The maxima in strength correspond to maxima in the fraction of transgranular fracture. The bend strength and room-temperature elastic modulus measurements Were combined with available thermal conductivity and expansion data to calculate thermal stress resistance parameters. Under steady-state heat flow conditions, the calculated thermal stress resistance parameters of the borides are higher than those calculated for other refractory compounds.
The fabrication temperature was the principal variable in a kinetic study of the densification of hafnium diboride in high-pressure hot-pressing.Densification studies for conventional hot-pressing were reviewed and correlated with the high-pressure hot-pressing results. The consolidation of HfBz in the open pore region during highpressure hot-pressing is attributed to particle rearrangement caused by grain boundary sliding and fragmentation. The final stage of densification (relative density >go%) was analyzed in terms of the Nabarro-Herring vacancy creep model. An activation energy of 22,900 cal/mole was obtained for the rate-controlling step in the creep process.
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