Super Hard Tool Material: What It Is and When It Pays Off

• 1
  What Counts as Super Hard Tool Material
• 2
  How the Edge Actually Fails
• 3
  Where Super Hard Tool Material Fits in a Job Shop
• 4
  Cutting Parameters and Fixturing Rules
• 5
  How to Verify the Result Before Shipping
• 6
  Grade Selection at a Glance

Super hard tool material means a cutting edge with hardness well above cemented carbide. Two families dominate: polycrystalline diamond (PCD) at roughly 8,000–10,000 HV, and polycrystalline cubic boron nitride (PCBN) at roughly 3,000–4,500 HV. Cemented carbide sits near 1,500–2,000 HV, which is why these grades hold an edge on work that chews through carbide in minutes.

The hardness comes from the same dense covalent bonding that makes diamond and cubic boron nitride so stiff. That matters on the shop floor for two reasons. A harder edge resists abrasive wear at high cutting speed, and it keeps its geometry longer, so the last part in a run cuts close to the first. Neither property helps if the edge chips, and that is where binder content and edge preparation decide the outcome.

Both grades are made by sintering fine powder under high pressure and high temperature, often 5–7 GPa and 1,300–1,600 °C for diamond, with a metal or ceramic binder holding the grains together. The binder is the weak link. It gives the tool toughness and electrical conductivity for EDM shaping, but it also limits the maximum working temperature. PCD starts to graphitize in air above roughly 700 °C; PCBN holds up past 1,200 °C.

That single temperature gap explains most grade selection. If the cutting zone runs hot because the workpiece is hard and the speed is high, diamond is the wrong answer no matter how hard it is. If the workpiece is non-ferrous and the zone stays cooler, diamond usually wins on wear life by a wide margin.

• 1
  PCDDiamond grains plus metal binder; for aluminium, copper, composites and graphite.
• 2
  PCBNCubic boron nitride grains plus ceramic or metal binder; for hardened steel and cast iron.
• 3
  CVD diamondBinder-free film grown on a substrate; sharp edges, low cobalt content, no grain pull-out.

A super hard edge does not wear the way a carbide edge wears. Carbide dulls gradually. Diamond and PCBN usually fail through one of four mechanisms, and each one points at a different fix.

Abrasive wear is the slow, expected mode. Hard inclusions in the workpiece, such as sand in castings or silicon particles in aluminium, scratch the edge. This is the mode you want, because it is predictable. Choose the finest grain size that still gives enough edge strength, and tool life becomes something you can schedule instead of something you discover.

Chemical wear is temperature driven. Diamond and iron have a strong chemical affinity, so cutting steel with PCD produces rapid diffusion of carbon into the chip. The edge disappears in a way that looks like wear but is actually a reaction. This is why PCD is not used on steel and why PCBN exists as a separate family.

Chipping and spalling come from mechanical shock. Interrupted cuts, hard scale on a casting skin, or a part that is not rigidly fixtured will knock grains out of the binder. A 20–30 μm edge hone plus a small chamfer usually stops the chipping at a cost of a few percent of cutting force.

Thermal fatigue is the mode that surprises people. In interrupted cutting, the edge cycles between hot and cool hundreds of times per minute. Diamond cracks along its cleavage planes under this cycling. Flood coolant helps only if it is applied consistently; intermittent coolant is worse than none at all.

• 1
  Abrasive wearSlow and predictable; reduce grain size and control speed.
• 2
  Chemical wearTemperature driven; keep diamond away from steel.
• 3
  ChippingMechanical shock; add an edge hone or chamfer.
• 4
  Thermal fatigueHeat cycling; use steady coolant or none.

A super hard insert costs more than a carbide insert, sometimes ten to twenty times more. The economics work when the tool survives long enough or produces a finish that carbide cannot. Those two conditions cover most of the real applications.

Aluminium and copper alloys with high silicon content are the classic PCD case. Hypereutectic aluminium with 16–20% silicon will destroy a carbide edge in a few parts. PCD runs at 500–2,000 m/min and holds Ra 0.4 μm or better on a bored surface. On a mirror-finish bore, PCD often removes the need for a separate honing step.

Hardened steel above 45 HRC is the classic PCBN case. Turning a 58 HRC bearing race or a hardened shaft with PCBN replaces a grinding operation. Depth of cut is typically 0.1–0.3 mm and speed 100–250 m/min. The surface comes off the lathe at Ra 0.4–0.8 μm, which is often inside the drawing tolerance for a bearing seat.

Composites and graphite are a third case. Carbon fibre and graphite are abrasive and can be machined with PCD at high speed, though edge geometry matters more than grade here. A sharp positive rake and a polished flute keep the fibres from pulling.

There is also a class of work where super hard tools are simply the wrong call. Soft low-carbon steel, general-purpose aluminium, and short prototype runs are all better served by coated carbide. If the run is a handful of parts and the tolerance is loose, the tool cost never comes back. We quote both options so the choice is visible before the chips start flying.

• 1
  Good fitHigh-silicon aluminium, hardened steel above 45 HRC, graphite, composites.
• 2
  Poor fitSoft steels, loose tolerance, very short runs.
• 3
  Better served by carbideGeneral prototyping and mixed-material job shop work.

Super hard tools run fast, but they do not tolerate poor setups. Speeds and feeds follow a narrow window, and the window shifts with the workpiece hardness.

For PCBN turning of hardened steel, start near 150 m/min and 0.1 mm depth of cut, then raise speed until the chip turns light straw or blue. A dull grey chip means the edge is rubbing rather than cutting. Feed per revolution usually lands between 0.05 mm and 0.15 mm. Lower feed with a large nose radius polishes the surface; higher feed with a small radius breaks the chip but leaves a coarser finish.

For PCD milling aluminium, surface speed runs 500–2,000 m/min and feed per tooth 0.05–0.2 mm. Chip load per tooth matters more than spindle rpm. If the load drops below about 0.03 mm per tooth, the edge rubs, generates heat, and wears quickly. That is the most common mistake we see on PCD milling jobs.

Rigidity is not optional. A super hard edge transfers cutting force into the part and fixture, and any movement shows up as chipping. For parts we machine on our 5-axis centres, we check overhang first. A tool held in a long holder at 4× diameter overhang will chip a PCD edge even at conservative parameters.

Coolant choice is simple. For PCBN hard turning, dry cutting is common and works well because the chip carries the heat. For PCD on aluminium, high-pressure through-tool coolant clears the chips and stabilizes the edge temperature. Never use water-based coolant on PCD cutting steel; the reaction is the same as dry cutting, just faster.

• 1
  PCBN hard turning150 m/min, 0.1 mm depth, 0.05–0.15 mm/rev.
• 2
  PCD aluminium500–2,000 m/min, 0.05–0.2 mm per tooth, never below 0.03 mm.
• 3
  RigidityKeep overhang under 3× diameter; shorten holders where possible.

A super hard tool changes what you can measure, not just what you can cut. Because the edge holds its geometry, the process is stable enough to inspect on the machine and trust the reading.

For hard-turned surfaces at Ra 0.4–0.8 μm, a portable surface tester with a 0.8 mm cut-off gives a reliable Ra value. Measure in three places around a bore or across a face; a single reading hides taper and chatter. If the spread between readings is more than 0.2 μm, the setup is moving, not the tool wearing.

Dimensional checks follow the drawing. Hard turning routinely holds ±0.005 mm on diameter for parts up to 200 mm, and roundness under 2 μm is achievable with a rigid setup. We inspect 100% of parts before shipment and can supply reports on request.

Edge condition is worth a look before the next run. Under a 20× loupe, a healthy PCD edge shows uniform flank wear and no cratering. Micro-chipping at the nose means the edge hone is too light or the feed is too high. Cratering on the rake face means the temperature is too high and the grade or speed needs to change.

If a part needs both a hard-turned seat and a milled feature, sequence matters. Turn the hard surface last so the finishing tool is not damaged by a subsequent interrupted cut. This one sequencing rule prevents more chipped edges than any parameter change.

• 1
  SurfaceThree readings minimum; spread over 0.2 μm points at the setup.
• 2
  Dimension±0.005 mm on diameter, roundness under 2 μm with rigid fixturing.
• 3
  Edge20× loupe check for micro-chipping and cratering before the next run.