Phase composition control of hot pressed boron carbide ceramics

Abstract:

Boron carbide ceramic has been known to possess ultrahigh hardness and good thermal conductivity. However, these properties greatly depend on the phase composition of boron carbide, that is, the B/C ratio. As the combined B/C ratio in boron carbide deviates from the stoichiometry of B₄C, all these properties decrease dramatically. In some standards and industrial applications, the ratio of total boron and total carbon which may be easy to control has been concerned, rather than the combined B/C ratio. Few investigations have been conducted on the control of the boron-rich phases and other impurities in B₄C ceramics. In this paper, the phase compositions of several commercial boron cabide powders were confirmed through the detailed analysis of their X-ray diffraction patterns, from which the diffraction peaks of the boron-rich phases were separated and calibrated from the overlapping peaks. The final phase compositions of hot pressed boron carbide ceramics and the transformation of boron-rich phases to B₄C under different sintering temperatures were then investigated. The results show that the commercial boron carbide powder is usually a mixture of B₄C and boron-rich phases such as B_6.5C as well as other impurities. The results of X-ray diffraction and Raman spectroscopy show that the boron-rich phases react with free carbon to form B₄C as the sintering temperature increases. A boron carbide ceramic with pure B₄C phase was prepared using a commercial boron carbide powder by hot pressing at 2100^_oC and 40MPa for 60min.

Key words:boron carbide; hot pressing sintering; boron-rich phases; B/C ratio