Boron carbide ($B_4C$) is a very attractive materials in various fields of engineering because of its high hardness, high wear resistance and low density. It is difficult to obtain sintered bodies of very low porosity, although $B_4C$ powder is routinely hot pressed at about 2100℃ or somewhat lower temperature with densification aids. Hot pressing, however, is relatively expensive and is limited to simple shapes. Thus, it is necessary to find a pressureless sintering process.
In present dissertation, the pressureless sintering behavior and accompanying microstructural change of boron carbide- based ceramics were examined by using titanium diboride and iron as sintering aids. The selection of iron and titanium diboride was based on the following considerations: Iron has a relatively low wetting angle and reacts with boron carbide to form iron boride which is expected to exist as a liquid phase at the sintering temperature. Titanium diboride is a suitable additive since dense $TiB_2$ is known to have high wear and corrosion resistance. It was also examined the possible role of sintering aids and correlations between microstructure and mechanical properties of the boron carbide-based ceramics. Fracture toughnss was measured by indentation method and with chevron notched three point bend specimens.
Boron carbide-based ceramics (contains $TiB_2$ up to 50 wt.%) with densities exceeding 95% of theoretical were obtained by pressureless sintering with the addition of 1 wt.% Fe. Boron carbide with 10 wt.% $TiB_2$ and 1 wt.% Fe showed the highest density (97% of theoretical) at the sintering temperature of 2175℃. Thin foil energy dispersive X-ray spectroscopy analysis and microstructural observations in TEM confirmed that an iron-rich crystalline phase exist both at triple junctions and at grain boundaries. It was suggested that the iron-rich phase forms a liquid phase and contributed to the densification of the $B_4C$-based ceramics. The specimens that contained only a small amount of $TiB_2$ (a grain growth inhibitor) and/or a high amount of Fe (a grain growth promoter) showed exaggerated grain growth particularly at high sintering temperature, resulting in decreased flexural strength. Strength decays observed in $B_4C$-based ceramics were attributed to exaggerated grain growth which could be retarded by limiting the amount of Fe added or by increasing the $TiB_2$ added.
Values about 400 MPa for room temperature flexural strength was maintained up to 1200℃ . Flexural strength values at room and elevated temperature in this work agreed favourably with the literature data obtained fromm hot-pressed $B_4C$. It was observed that fracture toughness of $B_4C-TiB_2$ ceramics is independent of amount of $TiB_2$ added contrary to the case of another particulate composite materials. It was considered that crack deflection may not occurred for the similarity of thermal expansion coefficient between $TiB_2$ and $B_4C$ grains
With chevron-notched specimens, fracture toughness was determined from maximum load with no need for precracking. Fracture toughness of sintered $B_4C-TiB_2$ was determined with three point bend specimens with chevron notches. With various notch configurations, stree intensity factor coefficient formulas of the speciman obtained by use of Bluhm's slice model were presented. Stress intensity factor coefficient formulas have been verified by comparing experimental $K_{IC}$ datas of sintered alumina with different north length. Fracture toughness of $B_4C$-based ceramics at room and elevated temperature was determined by chevron notched specimen. Fracture toughness, $3.8 MPam^{\frac{1}{2}}$, was independent of temperature below 1000℃ within experimental error. The temperature dependence of fracture toughness revealed similar trend with hot-pressed $B_4C$. Above 1000℃ increase in fracture toughness was observed and thin oxide layer was formed on surfaces. It is considered that the increase in toughness is the result of the increase in crack growth resistance due to the oxidized layer.
