Hot Pressed Boron Nitride – An Advanced Machinable Ceramic
Just like carbon, boron nitride also has various crystalline forms. Common machinable boron nitride is in hexagonal form. The hot pressed boron nitride is made by hot pressing the hexagonal boron nitride powder. Thus, the material is also called hBN.
Due to the graphite-like structure, boron
nitride has excellent thermal stability, unless it reacts with water or oxygen
in air. In an inert atmosphere, hBN could withstand 2800 degree C according to
Wikipedia but there are applications of more than 3000C in an inert atmosphere.
Chemical resistance for boron nitride is remarkable. Boron nitride doesn’t react with common acids and the
wettability with metal is very low. However, this material does react with
molten alkali, e.g. LiOH, KOH, NaOH-Na2CO3,
The thermal conductivity of hexagonal boron
nitride ceramic is rather high. Usually, common hot pressed boron nitride could
obtain 50 W/mK, higher than most of sintered ceramics. It is also one of the
reasons boron nitride has better thermal shock resistance than other high
density ceramics, and could be used to seal graphite heating elements.
Just like isostatic graphite, hexagonal
boron nitride is soft and machinable. However, unlike graphite, boron nitride
is an electric insulator and has low wettability with metal.
In general, boron nitride would be a great
material for high temperature vacuum environment applications such as melting
high purity special alloys and rare metal. Compared with alumina and other
ceramics, delivery lead time for proto type boron nitride parts are much
shorter due to the machinable low hardness. However, as boron nitride material
is much more expensive than alumina, hBN parts in large quantity usually costs more than alumina.