CUBIC BORON NITRIDE (CBN)
Next to diamond, cubic boron nitride (CBN) is the hardest material presently available. Introduced in 1962, it is also used as an abrasive. Cubic boron nitride is made by bonding a 0.5 - 1 mm layer of polycrystalline cubic boron nitride to a carbide substrate by sintering under pressure. While the carbide provides shock resistance, the CBN layer provides very high wear resistance and cutting-edge strength. Cubic boron nitride tools are also made in small sizes without a substrate.
At elevated temperatures, CBN is chemically inert to iron and nickel, and its resistance to oxidation is high. It is, therefore, particularly suitable for cutting hardened ferrous and high-temperature alloys.
Because CBN tools are brittle, stiffness of the machine tool and fixturing is important in order to avoid vibration and chatter. Furthermore, in order to avoid cracking due to thermal shock, machining should generally be performed dry - i.e., cutting fluids should be avoided, particularly in interrupted cutting operations such as milling.
