Diamond and CBN are not competing grades of the same material. One is carbon, the other is cubic boron nitride, and the choice is decided by what you are grinding: if the workpiece contains iron, CBN usually wins; if it does not, diamond usually wins. Buying the wrong one is expensive in a way that does not show up on the invoice.
The Short Answer
Look at the workpiece, not the price list. Diamond reacts with iron at grinding and sintering temperatures. CBN does not. That single difference decides most of the cases you will meet in a tool shop, a glass plant or a carbide grinding room.
If your material is hardened steel, high-speed steel, tool steel, superalloys or grey cast iron, the abrasive you want is CBN. If your material is cemented carbide, ceramic, glass, sapphire, stone, graphite, non-ferrous metal or a composite, the abrasive you want is diamond.
Why Iron Changes The Decision
Diamond is carbon in a metastable crystal form. At elevated temperature, in contact with iron, cobalt or nickel, it does not simply wear down. It transforms. The surface graphitises, the crystal loses its cutting edges, and the wear rate rises far faster than the pressure would suggest.
From the outside this looks like “the wheel wore out too fast” or “the diamond is soft this time”. It is neither. The abrasive was chemically wrong for the workpiece. No amount of grade juggling fixes a carbon-to-iron reaction.
CBN is chemically inert against iron group metals at the temperatures these processes reach. That is why CBN dominates grinding of hardened steel, and why it is the standard answer for HSS tool re-sharpening.
Hardness Is Not The Whole Story
On paper diamond is roughly twice as hard as CBN. That number gets quoted in every comparison, and it is misleading on its own.
Hardness tells you how the abrasive starts. Retention tells you how long it stays in the bond, and thermal stability tells you whether it is still an abrasive five minutes into the cycle. Diamond wins on hardness. CBN wins on chemical stability against ferrous work. A tool that loses its abrasive in the first minute never gets to use the hardness advantage.
Diamond also conducts heat far better than CBN. On thermal-sensitive work such as thin glass or sapphire, moving heat away from the contact zone is part of the job, and that points back to diamond.
CBN In Practice
The typical purchase in our plant is CBN micron powder or coated CBN grit for ferrous work. The grade is ordered as a size band, the same way diamond is, and the coating question is the same: nickel or titanium on the surface depending on the bond.
CBN is more expensive per carat than diamond. On hardened steel it is still the cheaper abrasive per finished part, because it lasts. On a non-ferrous workpiece it is simply wasted money.
One practical note: CBN does not like water-based coolant in some resin bond systems, and it is more sensitive to bond choice than diamond. If you are moving from diamond to CBN for a new steel job, treat the bond as a new variable rather than assuming the old recipe carries over.
Where Diamond Still Wins On Metal
There are metal jobs where diamond is used deliberately. Very low speed, very light load, no sustained temperature rise, and short contact time: diamond can survive on steel. It is a compromise, not a first choice, and it is normally made when the same tool must handle a mix of materials.
Cemented carbide is the classic case that confuses people. Carbide contains cobalt, which is a metal, yet diamond is the right abrasive. The carbon reaction is driven by iron, not by cobalt in the small fractions present at the surface, and carbide grinding is a diamond process worldwide.
What To Send With An Enquiry
Four things decide the recommendation faster than any catalogue: workpiece material and hardness, the process step (rough, semi-finish, finish), the bond system, and the target surface finish or removal rate. Add the machine type if you have it, because spindle speed changes the thermal picture.
With those four, the abrasive family is decided before size is discussed. Without them, a supplier can only guess, and the guess is usually “diamond, because that is what we sell most of”.
Next Step
Ferrous work, hardened steel and HSS: ask for CBN micron powder and coated CBN grit. Non-ferrous and non-metallic work: start from HDG monocrystalline micron powder or HDY W50 to W1 for fine work, and from MBD high-strength grit for coarse bonded tools.
Then match the bond: resin and vitrified bond wheels, electroplated tools, or saw blades and core drills.