What a diamond compact is
A diamond compact is a solid body made by fusing many small diamond crystals into one piece. Unlike a single gemstone, it is polycrystalline: it consists of countless tiny grains oriented in random directions. This structure is the basis of the polycrystalline diamond cutters used on drilling bits and the polycrystalline inserts used on cutting tools. Because the grains point every which way, there is no single easy cleavage direction, and the body is generally tougher than a large single crystal of similar hardness.
Making such a body is not simple. Diamond does not melt in the ordinary sense, and loose diamond powder will not stay together by being pressed. Something has to bond the grains, and that something is called the binder phase.
Why a binder is needed
Joining diamond grains directly to each other requires extreme pressure and temperature, in the range handled by HPHT presses. Under those conditions, a small quantity of a second material in contact with the diamond helps the process. In the most common commercial route this is a metal from the iron group, usually cobalt, mixed with or infiltrated into the diamond powder. The metal softens or melts, flows into the spaces between grains, helps dissolve and redeposit carbon at the contact points, and so promotes direct diamond-to-diamond bonding. When the process is complete, the compact is a network of bonded diamond with the metal remaining in the gaps.
The consolidation is a close relative of the sintering process used for ceramics, although the pressures involved are far higher.
Other kinds of binder
Cobalt is not the only option. Several alternatives exist:
- Silicon-based binders. Molten silicon can be introduced and react with carbon to form silicon carbide, which bonds the diamond grains. The result is sometimes called a silicon carbide-bonded diamond material.
- Carbonate or other non-metallic binders. Experimental and specialized routes use these, mainly in research and in niche products.
- Leached compacts. A cobalt-bonded compact can have most of its metal removed by acid treatment, leaving a porous but more heat-tolerant structure.
Each choice shifts the balance among toughness, heat tolerance, cost and the ease of attaching the compact to a tool body.
What the binder limits
The binder is both helpful and a weak point.
Temperature. Cobalt is a catalyst for converting diamond into graphite at elevated temperature. As a compact gets hot during cutting, the metal can accelerate that conversion at grain surfaces. Cobalt also expands more than diamond does, so heating puts the grain boundaries under stress and can start micro-cracks. As a result, cobalt-bonded compacts have a practical temperature ceiling well below the point where pure diamond would be affected. Published limits vary with grade and conditions, and should be taken from specific data rather than assumed.
Thermal conductivity. The metal occupies some of the volume and interrupts the diamond network, so a compact conducts heat less well than a perfect diamond of the same size, though it is still usually much better than the tungsten carbide on which it is mounted.
Chemical behavior. Metal binders can be attacked by some chemicals, which matters in aggressive environments.
Hardness and wear. The metal is softer than diamond, so a higher binder content tends to give a slightly less wear-resistant body, while a lower content gives a harder one that can be more brittle.
Thermally stable variants
To address the temperature limit, manufacturers produce thermally stable compacts. In one approach, cobalt is leached out of the surface layer or throughout the body. In another, the binder is chosen to be non-catalytic, such as silicon carbide. These variants can tolerate heat better, but they may be less tough or harder to attach, so they suit particular uses rather than replacing standard compacts everywhere.
The substrate behind the compact
Most cutting compacts are a thin diamond layer sintered onto a support of cemented carbide. The support provides toughness and a surface that can be brazed into a tool. It also shrinks and expands differently from the diamond layer, so the interface between them is a source of residual stress. Cracking or delamination at that interface is a known failure mode, and design of the layer thickness and shape aims to reduce it.
Choosing between types
For interrupted cuts and impact, a tougher, cobalt-bonded compact is usually favored. For hotter, more abrasive work where thermal stability matters, a leached or silicon carbide-bonded type may suit better. Neither is suited to cutting steel, where diamond tends to react with the iron. Actual selection depends on the application and the supplier’s data for each grade, and this site does not recommend specific products.