PRESSURELESS SINTERING OF TITANIUM DIBORIDE WITH NICKEL, NICKEL BORIDE, AND IRON ADDITIVES

The effect of relatively small additions (1-5 wt%) of nickel, nickel b oride (NiB), and iron to promote the liquid-phase sintering of titaniu m diboride (TiB2) has been studied, Carbon also was added to some samp les, to reduce the amount of oxygen impurities in the TiB2 ceramics. G reen bodies that were formed by uniaxial pressing were sintered in a g raphite furnace at 13000-1700 degrees C, both under vacuum and in a 50 0 mbar argon atmosphere, and high densities (>94% of theoretical densi ty) were obtained at temperatures greater than or equal to 1500 degree s C. The weight loss of the samples during sintering was shown to be d ependent on the densification rate and the final density and was not g overned only by the thermodynamics of the system, Significant exaggera ted grain growth was observed in samples with nickel, NiB, and iron du ring sintering at 1700 degrees C, The exaggerated grain growth was obs erved to be closely related to the oxygen content of the samples and t o temperature, The addition of carbon strongly reduced the density and the oxygen content and, thereby, inhibited grain growth, We have prop osed that the exaggerated grain growth is enhanced by surface diffusio n in a titanium-oxide-rich layer on the TiB2 grains.