A close relative of diamond
Cubic boron nitride, often shortened to CBN, is a synthetic compound of boron and nitrogen. Its atoms are arranged in the same cubic pattern as diamond, with boron and nitrogen alternating where diamond has carbon. Boron, carbon and nitrogen sit next to each other on the periodic table, which is why the two crystals look so alike. That similarity gives CBN strong, directional covalent bonds and a very high hardness.
CBN does not occur in nature in any useful amount. It is made by subjecting hexagonal boron nitride, a soft, graphite-like white powder, to very high pressure and temperature, usually with a catalyst. The equipment is similar in principle to the HPHT presses used for diamond. The result is a grit that can be used loose or bonded into tools, or sintered into solid inserts.
How the two compare
Diamond is harder than CBN, and by a clear margin. For an abrasive, though, hardness is only part of the picture. Three differences explain why both materials exist side by side.
Chemical stability with iron. Diamond is pure carbon, and at the temperatures reached in cutting, carbon tends to dissolve into iron and react with it. CBN does not behave this way to nearly the same degree, so it stays intact against steel. The same chemistry limits diamond in ferrous metal machining generally.
Heat in air. Diamond begins to oxidize in air at temperatures that are within reach of heavy grinding. CBN generally holds up to higher temperatures before oxidizing. That gives it more margin when cutting produces a lot of heat, as it does with hard, strong alloys.
Thermal conductivity. Diamond conducts heat extremely well, better than any other common bulk material, which is the reason for its exceptional thermal conductivity. CBN conducts heat well for a ceramic, but not nearly as well as diamond. This favors diamond when the aim is to pull heat away from the cutting edge on non-ferrous work.
Where CBN is chosen
CBN is used mostly on ferrous materials, and especially on hard ones:
- Hardened tool steels and bearing steels, where the workpiece is hard enough that ordinary abrasives wear quickly.
- Nickel- and cobalt-based superalloys used in hot engine sections.
- Cast iron, including hard white iron, where diamond would wear quickly.
Typical product forms include CBN grinding wheels, honing stones and polycrystalline CBN inserts. The inserts allow hard turning: cutting hardened steel with a single-point tool, which can replace a grinding step in some operations. Whether that is practical depends on the part, the machine rigidity and the tolerance needed.
CBN grinding wheels are commonly built with vitrified, resin, metal or electroplated systems, and the choice follows the same logic as for diamond tools, with the various bond types suiting different jobs.
Where diamond is still preferred
Diamond tends to be the better choice for non-ferrous metals such as aluminum, for composites, glass, stone, ceramics and cemented carbide. Machining aluminum and other non-ferrous metals is a typical area where diamond tools perform well and CBN offers no advantage. A simple rule of thumb is often quoted: diamond for non-ferrous and non-metallic work, CBN for ferrous work. It is a useful guide, not a law, and real selection depends on speed, coolant, workpiece grade and cost.
Toughness and grit properties
Both materials are brittle in the sense that matters for abrasives, but grit manufacturers can influence behavior. CBN grits come in several types, from blocky, strong crystals to more friable ones that fracture and re-sharpen during use. Choosing between them affects how aggressively a wheel cuts and how often it needs dressing. This is another case where fracture toughness is not simply good or bad: some fracture is useful when it exposes fresh cutting edges.
Cost and practical points
Superabrasive wheels cost far more than conventional alumina or silicon carbide wheels, but they last much longer and hold form better, so the economics are often judged on cost per part rather than price per wheel. The framework for this thinking is total cost of ownership. Cooling and machine condition also matter. Many CBN processes use abundant coolant, which keeps temperatures down and flushes away chips, and the choice of machining coolant is part of getting good life from the wheel.
Summary of the differences
| Feature | Diamond | CBN |
|---|---|---|
| Hardness | Highest known | Second highest of the common materials |
| Reaction with iron at high temperature | Significant | Low |
| Heat tolerance in air | Lower | Higher |
| Heat conduction | Exceptional | Good for a ceramic |
| Typical use | Non-ferrous and non-metallic | Hardened steel, cast iron, superalloys |