BORON NITRIDE FACTS The synthesis of hexagonal boron nitride powder is achieved by nitridation or ammonalysis of boric oxide at elevated temperature. Cubic boron nitride is formed by high pressure, high temperature treatment of hexagonal BN. Hexagonal boron nitride (h-BN) is theequivalent in structure of graphite . Like graphite its plate like microstructure and layered lattice structure give it good lubricating properties. h-BN is resistant to sintering and is usually formed by hot pressing. Cubic boron nitride (C-BN) has the same structure as diamond and its properties mirror those of diamond. Indeed C-BN is the second hardest material next to diamond. C-BN was first synthesised in 1957, but it is only in the last 15 years that commercial production of C-BN has developed. Key Properties Hexagonal Boron Nitride (h-BN) •h-BN has excellent lubricating properties •In the hot pressed state, h-BN is readily machinable using conventional metal cutting techniques, hence complex shaped components can be fashioned from hot pressed billet. •Providing oxidation of the surface can be prevented, h-BN is not wetted by most molten metals, glasses and salts and hence has a high resistance to chemical attack. •High dielectric breakdown strength •High volume resistivity •Good chemical inertness Cubic Boron Nitride (C-BN) •C-BN is the second hardest material known, inferior only to diamond •High thermal conductivity •Excellent wear resistance •Good chemical inertness Applications Electrical insulators The combination of high dielectric breakdown strength and volume resistivity lead to h-BN being used as an electrical insulator however its’ tendency to oxidise at high temperatures often restrict its use to vacuum and inert atmosphere operation. Crucibles and reaction vessles Its chemical inertness leads to application as thermocouple protection sheaths, crucibles and linings for reaction vessels though as above oxidation must be avoided. Moulds and evaporating boats h-BN is used in bulk form or as a coating for refractory moulds used in glass forming and in superplastic forming of titanium (see figure 2). It is also used as a constituent in composite materials e.g. TiB2/BN composites for metal evaporation boats, and Si3N4/BN for break rings in continuous casting of steel. Hot isostatic pressing Its refractoriness combined with the fact that it is not wetted by molten glass lead to h-BN being used in the production of hot isostatically pressed (HIP’ed) material, most notable ceramics. In this application preformed parts are coated in h-BN prior to glass encapsulation and HIP’ing. This protects the part being HIP’ed from actually coming into contact with the glass, which in turn makes it easier to remove after HIP’ing. Machine cutting tools and abrasives Cutting tools and abrasive components particularly for use with low carbon ferrous metals have been developed using C-BN. In this application the tools behave in a similar manner to polycrystalline diamond tools but can be used on iron and low carbon alloys without risk of reaction. Substrates for electronic devices C-BN is used for substrates for mounting high density and high power electronic components where the high thermal conductivity achieved allows efficient heat dissipation. Wear resistant coatings Due to its high hardness and excellent wear resistant properties, coatings of C-BN have been developed.
Posted on: Tue, 13 Aug 2013 16:56:16 +0000
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