Polycrystalline Diamond (PCD)
Polycrystalline diamond — PCD — is the hardest cutting-tool material there is, and the superhard answer for the work that carbide cannot survive: high-silicon aluminium, abrasive non-ferrous alloys, composites and similar materials that wear out a carbide edge in minutes. Diamond cuts these at high speed with a sharpness and finish no other tool matches, and an edge life many times that of carbide. Its one great limitation is chemical rather than mechanical: it cannot machine steel, which is why it pairs with cubic boron nitride as the other half of the superhard family.
Diamond made into a tool
Diamond is carbon in its hardest crystal form, and nature’s single crystals are too brittle, too small and too expensive for general cutting. PCD is diamond engineered around that problem. Millions of microscopic synthetic-diamond grains are grown together under high pressure and high temperature, and the random orientation of the crystals means there is no natural cleavage plane for a crack to run along — the polycrystalline mass is far tougher than any single crystal. The diamond layer is bonded to a cemented-carbide substrate that carries it and gives the tool its strength, and the composite blank is cut and ground into cutting edges, finished with diamond wheels, since only diamond can grind diamond. A PCD edge therefore combines the extreme hardness and abrasion resistance of diamond with enough toughness to be a practical tool.
PCD tooling is built like other superhard tools: solid diamond composite where the whole edge region is PCD, or — far more common — a small PCD tip brazed onto a steel or carbide tool body so that the diamond is only where the cut happens. Grades follow the grain size: fine grain for the finest finish and edge, coarse grain for the most abrasive roughing, with the general-purpose grades in between.
Why diamond cannot cut steel
PCD’s one hard boundary is chemistry. Diamond is carbon, and carbon is strongly attracted to iron at temperature. When a diamond edge cuts steel or cast iron, the two react at the hot contact — the diamond’s carbon is drawn into the iron, the edge breaks down, and the tool is gone almost immediately. The reaction begins at surprisingly modest temperatures for a cutting edge, so it is not a matter of running cooler; steel simply cannot be machined with diamond-based tooling. That is the entire reason the steel superhard material — cubic boron nitride — exists, and why the two materials divide the work between them by what is being cut.
Where PCD wins
Given a workpiece that does not contain iron, PCD is the extreme performer. Its hardness resists the abrasive wear that destroys other tools, and its keen, low-friction edge cuts cleanly and holds size for remarkably long runs. The classic job is high-silicon aluminium: as the silicon content of an aluminium alloy rises, the alloy becomes harder to machine and far more abrasive, and the point comes where carbide edges fail quickly while a PCD edge machines thousands of parts between changes. The same advantage applies across the non-ferrous and non-metallic world — aluminium and magnesium parts, copper alloys, carbon-fibre composites, glass-reinforced plastics, ceramics, and even wood and precious metals. On these materials PCD runs at high cutting speeds, produces an excellent finish, and often outlasts carbide by an order of magnitude or more. High-volume automotive aluminium — engine blocks, pistons, intake components — and aerospace composite machining are the industries that keep the PCD toolmakers busy.
PCD shares the superhard temperament. It is brittle, so it needs a rigid machine, rigid workholding and a stable cut; interrupted cutting, chatter, scale or a flexing setup will chip the edge, and carbide is the better tool where the operation cannot be made stable. It is used mainly for finishing and semi-finishing with light stock removal, because diamond is too costly and too brittle to rough away large volumes in an unstable cut. Coolant is used generously or not at all — a thin edge alternately heated and quenched by coolant is prone to thermal cracking. And grain and geometry are chosen to the job: a coarse grade and a stronger edge for abrasive roughing, a fine grade and careful edge preparation for mirror finishes.
PCD and CBN: the two halves of superhard
The choice between the two superhard materials is made by the workpiece, not by the operation. Ferrous work — steel and cast iron, hard or soft — belongs to cubic boron nitride, the only superhard material chemically stable to iron. Non-ferrous metals, composites and other iron-free materials belong to PCD, which is harder and gives the longer life wherever its chemistry allows it. Both are expensive, brittle, high-speed finishing tools that pay only when volume and rigidity let their edge life be used, and both sit at the top of the tool-material ladder above carbide, ceramic and the steel tools. Getting the material class right is the first step; a CBN grade run on aluminium wears oddly, and PCD on steel fails at once.
When a shop chooses PCD
A shop reaches for PCD when three conditions come together: the workpiece is non-ferrous or non-metallic and abrasive enough to punish carbide; the volume is high enough that tool life and cycle time, not insert price, decide the cost per part; and the machine and setup are rigid and stable enough to protect a brittle edge. Where the metal is a plain, non-abrasive aluminium or a soft non-ferrous alloy, carbide and even the high-speed steels may do the job more cheaply; where the run is short, carbide is the sensible tool. It is always a volume and material decision, and it is made the same way the tool ladder is read — from high-speed steel up through carbide and ceramic to the two superhard siblings, cubic boron nitride for steel and PCD for everything else.
PCD’s edge life and finish are only realised where the machine holds the cut stiffly — the subject of machine structure and rigidity — and its speeds and feeds are set within the same framework as every tool in CNC machining.