Materials Glossary
CBN is the second-hardest material known, right behind diamond. It's made the same basic way as polycrystalline diamond — boron nitride powder sintered under extreme heat and pressure — but it has one crucial difference: it doesn't chemically attack iron the way diamond does. That single fact is why CBN exists as its own tool material instead of everyone just using diamond everywhere.
CBN is a synthetic superabrasive, first produced at General Electric labs in 1957 by transforming hexagonal boron nitride into a cubic crystal lattice under extreme heat and pressure — the same high-pressure-high-temperature (HPHT) sintering concept used to make polycrystalline diamond (PCD). Fine CBN crystals are sintered together, often with a ceramic or metallic binder, into a compact that's brazed or bonded onto a carbide insert body. The result is a cutting edge that approaches diamond in raw hardness but behaves completely differently once it's actually cutting steel.
Diamond is carbon, and at the temperatures generated in a cut, carbon has a strong chemical affinity for iron — it reacts with the workpiece and effectively dissolves into it, a failure mode called diffusion wear. That makes diamond a poor choice on any ferrous material. CBN uses boron instead of carbon as its base element, and boron does not share that affinity for iron. CBN stays chemically stable against steel and cast iron even at high cutting temperatures, which is precisely why it exists as a separate material from PCD rather than everyone simply using diamond on everything.
CBN's core job is hard part turning: cutting hardened steel, cast iron, and powder-metal parts that are typically too hard for tungsten carbide to hold an edge on — commonly in the 45–65 HRC range, well above what a standard carbide grade is designed to handle. This lets shops turn hardened gears, bearing races, and transmission components directly, in some cases replacing a grinding operation entirely. Because it is a premium, application-specific material, CBN inserts cost significantly more than carbide and are reserved for jobs where carbide genuinely cannot do the work — not used as a general-purpose upgrade.
Most of the work in this shop still belongs to standard tungsten carbide. CBN is the specialist option for the hardened-steel and cast-iron jobs carbide can't touch.
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