Boron carbide, $B_4C$, is one of the interesting members of nonoxide covalent ceramics, because it has high melting point, high elastic modulus, high hardness, chemical inertness and low density. Thus it as been available for wear components and light weight ceramic armor. Also it has been used for reactor control rods in nuclear power engineering because of its high neutron absorption capability. But pressureless sintering of boron carbide is restricted by its high covalent bonding character. And the use of boron carbide as structural ceramics was restricted by its low fracture toughness.
In order to improve these problems, two researches were done in the present work. In the first part, the sintering behavior of boron carbide with addition of alumina as sintering aid was studied at the temperature range of $2075\,^\circ\!C \sim 2150\,^\circ\!C$ in flowing Ar atm. The addition of alumina upto 3 wt\% was effective on the increase of the sintered density. But further increase of alumina addition resulted in density decrease compared to 3 wt\% alumina addition. Maximum sintered density, 95.7\% of theoretical, was obtained with 3 wt\% alumina addition. AES and XRD analysis on the reaction couple showed that only $B_{12}C_2Al$ phase was formed by the reaction between boron carbide and alumina. It is suggested that substitution of Al in boron carbide lattice resulted in lattice defects and promote the densification of boron carbide. Microstructural investigation revealed uniform grain structure at the composition of $B_4C$-3 wt\% $Al_2O_3$. But abnormal grain growth(or exaggerated grain growth) was observed in the specimens with 5 wt\% and 10 wt\% alumina additions. It is considered that the decrease of sintered density attributed partially to this exaggerated grain growth. Also, it is suggested that gas phase evolution during sintering caused by the reaction between boron carbide and alumina resulted in density decrease.
In the last part of this work, toughening behavior of boron carbide by the addition of $TiB_2$(upto 30 vol\%) as second phase particles was studied. Dense bodies with sintered density above 97\% were obtained by hot pressing at 1900$^\circ$C, 35 MPa for 1h. Microhardness of $B_4C-TiB_2$ system revealed the value of about 3300 kg/$mm^2$ with no significant variations through all the composition ranges. Fracutre toughness of the composites was increased with the addition of $TiB_2$. Maximum toughness of 6.25 MPa $m^{1/2}$ was obtained at the composition of $B_4C$-15 $TiB_2$-0.5Fe. The variation of fracture toughness with the content of $TiB_2$ was similar to that of median crack deflection angle. The toughness enhancement of boron carbide by reinforcement of $TiB_2$ particles was attributed to the crack deflection caused by residual stress field which was generated during cooling process because of the differences in the thermal expansion coefficient and/or elastic moduli between $B_4C$ and $TiB_2$.
Further increase of $TiB_2$ content resulted in decrease of fracture toughness. But even at the composition of $B_4C$-30 $TiB_2$-0.5 Fe, toughness (5.3 MPa $m^{1/2}$) was higher than of monolithic armor grade $B_4C$ (4.67 MPa $m^{1/2}$). Significant superposition of residual stress was showed in the composition of $B_4C$-30 $TiB_2$-0.5 Fe, compared to $B_4C$-5 $TiB_2$-0.5 Fe by the calculation of the stress distribution between nearest two particles. Thus it is thought that the toughness decrease revealed in this composition range was attributed to superposition of stress field.
