Titanium diboride which is mainly covalent bonded, has high melting point ($2980^\circ$C) and high hardness (2500 - 3000 kg/$mm^2$). Also it has a good thermal and electrical conductivity. It is wet by molten metals, resists molten metals and halides. Above superior properties of $TiB_2$ make it possible that $TiB_2$ - based ceramics is used as structural materials, such as light weight armor materials, Hall - Heroult cells, and crucibles for metal melting, etc. But it is difficult to obtain fully dense body, although $TiB_2$ powder is routinely hot - pressed at $1800^\circ$C or even lower temperature, with densification aids. Furthermore, the use of $TiB_2$ - based ceramics is restricted because of intrinsic brittle character of $TiB_2$, like other ceramics.
In present work, the effect of the addition of $B_4C$ and Fe on the pressureless sintering behavior was studied under vacuum atmosphere. And also, the toughening mechanism of $B_4C$ - $TiB_2$ particulate composite was studied with hot - pressed $B_4C$ - $TiB_2$ specimens.
Pressureless sintering was performed at 1950 - $2050^\circ$C under vacuum atmosphere with vibratory - milled $TiB_2$ powder (mean particle size = 1.64 $\mu$m). In this case, it was revealed that the appropriate sintering temperature was 2000$^\circ$C for pressureless sintering under vacuum atmosphere. The grain size of $TiB_2$ was reduced and sintered density was increased by the addition of Fe. But when the large amount of Fe (10 wt%) was added, the sintered density was decrease. In $B_4C$ - $TiB_2$ composite system, $B_4C$ grain acted as a grain - growth inhibitor of $TiB_2$ and increased the density of sintered composites. The maximum sintered density (=97%) was obtained when 2 wt% Fe and 10 wt% $B_4C$ were added.
Densities up to 99% of theoretical were achieved by hot-pressing of $TiB_2$ - $B_4$C composite at $1700^\circ$C for 1h using 1 vol% Fe as a sintering aids. And these specimens were used for evaluating the toughening mechanism of these particulate composite. The microstructure consisted of dispersed $B_4C$ particles in a fine - grained $TiB_2$ matrix. Addition of $B_4C$ particles increased the fracture toughness of the composite (to 7.86MPam$\frac{1}{2}$ at 15 vol% $B_4C$) and yielded the high fracture strength (to 710MPa at 15 vol% $B_4C$). Microstructural observations indicated that the improved strength is resulted from higher density, smaller grain size, and intergranular fracture. But abnormally grown $B_4C$ grains decreased the fracture strength when 20 and 30 vol% $B_4C$ were added. On the matter of fracture toughness, it was considered that the toughening was ascribed to crack deflection caused by residual strain field which was generated during cooling process because of differences in thermal expansion coefficients and/or elastic moudi between $TiB_2$ and $B_4C$.
