Copper-boron nitride interaction in hot-pressed ceramics
- PDF / 333,507 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 95 Downloads / 189 Views
MATERIALS RESEARCH
Welcome
Comments
Help
Copper-boron nitride interaction in hot-pressed ceramics Milan Hub´acˇ eka) and Tadao Sato National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, 305 Japan
Masanori Ueki Advanced Technology Research Labs, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki, 211 Japan (Received 3 September 1995; accepted 7 February 1996)
Ceramics were prepared by hot pressing of chemical mixtures of turbostratic boron nitride with copper. A positive effect of copper on boron nitride crystallization and densification has been found. Preferred orientation of boron nitride grains has been revealed in such ceramics, being opposite to general experience: the c-axis of boron nitride crystallites was arranged preferably within a plane perpendicular to the direction of uniaxial pressure. Such an arrangement is interpreted as a result of the uniaxial compression of the boron nitride grains extensively growing within the c crystallographic axis.
I. INTRODUCTION
Hot pressing is the most suitable process to prepare boron nitride ceramics, owing to the strong covalent character of boron-nitrogen bonds within macromolecular layers; formation of a liquid phase during sintering is beyond expectation. Sinterability of starting boron nitride powder was found to be influenced by various factors. For example, the better the crystallinity is, the worse the sinterability is.1 High purity is another factor negatively affecting the process of densification during hot pressing. Especially, powders in which oxygencontaining phases present as a consequence of an imperfect reductive nitriding of starting oxide materials, show the best sinterability.2 Crystallization of starting turbostratic powder (having a high degree of layer disordering and a certain amount of oxygen) was found as a process of thermal decomposition of boron oxynitride into hexagonal boron nitride and boric oxide.3 Crystallization and densification then were found as simultaneous processes occurring during hot pressing of boron nitride powder.4 Preferred orientation of boron nitride grains in uniaxially pressed ceramics is a logical result of a strong layered character of hexagonal boron nitride crystals. In such materials, boron nitride lamellar grains are oriented preferably with the c-axis parallel to the pressure axis.5 In our previous study, we examined the effect of metal addition on crystallinity of boron nitride after direct synthesis of boron with nitrogen. It was found that copper, chosen as a model metal due to its chemical stability to boron nitride, brings significantly the a)
Author to whom correspondence should be addressed, presently with Advanced Technology Research Labs, Nippon Steel Corporation, 1618 Ida, Nakahara-ku, Kawasaki, 211 Japan. J. Mater. Res., Vol. 12, No. 1, Jan 1997
http://journals.cambridge.org
Downloaded: 31 Mar 2015
arrangement of boron nitride layers in hexagonal ordered sequence with interlayer spacing as short as 0.3328 nm. Crystal growth was observed as well in heat-treated
Data Loading...
