Densification of Transition Metal Borides

The densification of single-phase and pure ceramics of transition metal diborides is complicated by two characteristics of these compounds, the high melting point and the comparatively high vapor pressure of the constituents. As a rule, sintering temperatures exceeding 70% of the absolute melting temperature have to be applied. 

Densities above 95% have been achieved by axial hot-pressing at pressures exceeding 20 MPa and temperatures above 1800°C. The microstructures consist typically of particles of 20 qm in size. Another problem is related to the hexagonal layered structure of the AlB2-type borides. Because of the strong anisotropic behavior of the physical properties, especially of the coefficients of thermal expansion, the 

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