Pressureless sintering and mechanical properties of boron carbide - titanium boride particulate ceramic composites.

摘要

The unique combination of extremely high hardness and low density makes boron carbide an attractive structural ceramic material. So far, the application range of boron carbide has been greatly limited by its brittleness, low strength, poor sinterability, and the need of very high sintering temperatures.; The primary objectives of this study are to develop a new pressureless sintering technique for boron carbide ceramics at reduced sintering temperatures, and to increase the strength and fracture toughness of the material.; It is shown that the boron carbide ceramics with the addition of TiO 2 and carbon can be sintered without external pressure at temperatures of 2060--2130°C to 97.5--99.0% of its theoretical density. The resultant material contained TiB2 particles formed through the in-situ reaction between the stoichiometric amounts of TiO2 , carbon and boron carbide. The presence of fine TiB2 particles provides the material with high driving force for sintering and allows a significant reduction of sintering temperature. The optimum sintering cycles were developed for the specimens containing various amounts of TiB2. The role of the in-situ formed TiB2 particles in the densification process and microstructural evolution of the B4C-TiB2 composite was. examined.; It was found that the presence of the TiB2 phase has a major positive effect on the mechanical properties of boron carbide ceramics. Maximum values of bend strength (over 520 MPa), fracture toughness (4.7 MPa x m1/2) and hardness (32 GPa) were observed in the specimens containing 15 vol. % TiB2. The strength and fracture toughness were found to be controlled by thermal mismatch stress generated by the TiB 2 particles as well as by the microcracking of the particle-matrix interface.