Superhard tools cost many times a carbide insert and pay back many times over, but only on the right material. Diamond is for non-ferrous and abrasive materials, CBN is for hard ferrous ones, and soft steel belongs to neither. This page is the short selection map, with the reasons behind it.
| Workpiece | First choice | Why |
|---|---|---|
| Aluminium, low silicon | PCD, fine to medium grain | Carbide works too; PCD wins on volume, finish and burr-free edges. |
| Aluminium, over ~5% silicon (pistons, blocks, heads) | PCD, coarse grain; CVD diamond for hypereutectic alloys | Silicon grains are abrasive and wear carbide fast; diamond resists them. |
| Copper, brass, bronze, magnesium | PCD | Non-ferrous, so diamond is stable; long life and a sharp edge. |
| CFRP, GFRP and filled plastics | PCD or CVD diamond | Fibres are very abrasive; binderless CVD diamond lasts longest on the hardest fibres. |
| Graphite, green ceramics, presintered carbide | CVD diamond or PCD | Pure abrasion, no chemistry problem; the most wear-resistant diamond wins. |
| Hardened steel, 45 to 65 HRC | CBN | Hard turning and finishing instead of grinding. Low-content CBN for continuous cuts, high-content for interrupted. |
| Grey cast iron | CBN (high content) at high speed | Very high speeds are possible; ceramics are the other option. |
| Powder-metal (sintered) steels | CBN | Hard particles and porosity; CBN holds the edge. |
| Chilled and white cast iron, hard facing | CBN | Too hard for carbide to last. |
| Nickel superalloys (Inconel 718) | CBN in selected finishing cuts | Case by case; ceramics and carbide remain the default in most shops. |
| Soft steel and unhardened stainless | None of the three | Carbide or cermet. Diamond reacts with iron, and CBN needs hardness and heat to work. |
Diamond is carbon. At the temperature of a cutting edge in steel or cast iron, carbon diffuses into the iron and the diamond structure breaks down. That is why PCD and CVD diamond are reserved for aluminium, copper alloys, composites, graphite and other non-ferrous materials, and why a diamond tool in steel wears in minutes.
PCD is diamond grit sintered with a metallic binder (usually cobalt) onto a carbide backing. The binder gives toughness and lets the edge be ground, eroded or laser-cut sharp, and it brazes easily. CVD thick-film diamond is grown as pure diamond with no binder: more wear resistant and more heat stable, but more brittle. Choose PCD for most aluminium and copper work, CVD diamond for the most abrasive cases such as hypereutectic aluminium, CFRP and graphite. Grain size matters inside PCD too: fine grain for a sharp edge and finish, coarse grain for abrasion resistance.
CBN is the second hardest material and stays stable with iron, so it takes over where diamond cannot go: hardened steel, cast iron, sintered steel. Low-content grades with a ceramic binder are more chemically stable and suit continuous hard turning. High-content grades are tougher and suit interrupted cuts, cast iron and powder metal. CBN needs a hard workpiece to work properly; in soft steel it wears faster than carbide.
Small batches, unstable setups, heavy interruptions and soft materials. A superhard tool pays back through tool life and cycle time, so the decision is cost per part, not the price of the insert. Run the numbers before you switch.
Laser machining has changed what is possible in PCD and CVD diamond: chip breakers, edge preparations and profile tools that were hard to grind can now be cut directly. If chip control is the reason a shop gave up on diamond, it is worth a second look.
Example values from the cited sources, not recommendations for your machine. Start from your tool maker's data.
No. At cutting temperatures the carbon in diamond reacts with iron and the tool wears out very quickly. For hardened steel use CBN; for soft steel use carbide or cermet.
It is more wear resistant and more heat stable because it has no binder, but it is more brittle. It wins on the most abrasive materials; PCD is the more forgiving choice for most aluminium and copper work.
A high-content CBN grade, because it is tougher. Low-content grades with ceramic binder are better for continuous cuts where chemical wear dominates.
Often yes in high-volume work, because it allows very high cutting speeds. Ceramics are the other option; ductile iron is a case-by-case decision.
Which grade, which edge and what the equivalent is in another brand: the advisor answers from catalog data across 15 brands, brand-neutral.