Growing Appeal of PCD: Turning Performance

Some PCD background

PCD stands for “polycrystalline diamond,” and the name itself reveals a lot about what the material actually is. Structurally, sintered tungsten carbide and PCD are quite similar. Both are composites — materials made from two parts that retain some beneficial properties of each. In the case of carbide, millions of very hard grains of tungsten carbide are held together in a metallic binder, almost always cobalt. In simple terms, carbide owes its hardness to the tungsten carbide grains, and its toughness to the metallic binder.

PCD is essentially the same, except that grains of tungsten carbide are replaced with millions of tiny diamonds. Diamonds being much harder than tungsten carbide, the result is a material with a hardness that far exceeds any other cutting tool material.

But simply creating a harder cutting tool material doesn’t mean that it should be used in every possible application. Knowing when to use PCD cutting tools requires an understanding of how PCD can be tuned for optimal performance in different applications.

First, manufacturers can control the average size of their diamond grains. Smaller grains will limit plastic deformation and promote more intimate linking between individual grains (each grain will be touching more neighbors), ultimately making the material harder. Like carbide, PCD made from smaller grains will also be able to hold sharper cutting edges, so these materials are often used for finishing applications. PCD grades made from more coarse grains offer better wear resistance in materials like GFRP, CFRP, and high-silicon aluminum alloys.

Second, manufacturers can control the amount of metallic binder used in their PCD. Increasing the binder content will make PCD softer but tougher — more suitable for interrupted cutting — but this will also reduce PCD’s ability to hold a sharp cutting edge, which is essential for applications requiring fine finishes. Depending on how the PCD is manufactured, the binder might be mixed in with diamond grains as cobalt powder before sintering, or manufacturers might simply rely on the movement of liquid cobalt from nearby tungsten carbide when sintering temperatures reach the melting temperature of the cobalt.

Third, and less well-known, manufacturers can also tune the properties of PCD by controlling the shape and size distribution of the diamond grains and the quality of the binder. Monitoring these characteristics is one area where high-quality manufacturers might distinguish themselves from low-cost competitors. For example, more rounded diamond grains or more faceted diamond grains will interact with one another and with the binder differently. In some cases grains are chosen to provide a more intimate bonding between neighboring grains during sintering, which improves hardness; in others, a more intimate bond with the binder is desirable, improving toughness. During sintering, the cobalt binder also acts as a catalyst to promote recrystallization, which may or may not be desirable depending on the needs of the application. On the other hand, more sharply faceted grains will not compact as closely and will reduce the bond between neighboring grains. The average size distribution also plays a role because occasional larger grains — if plucked out during the final shaping process — will leave a larger “hole” that can negatively affect the quality of the cutting edge and surface finish.

Diamond grains and binder content aside, PCD is always manufactured by sintering at a high temperature and an extremely high pressure. In some cases, PCD is formed on top of a tungsten carbide wafer or disc, after which larger wafers are “diced” into shapes using a laser or wire EDM (electrical discharge machine).

Eventually these shapes are brazed onto the body of a cutting tool and sharpened. In other cases, PCD is formed inside a “slit” in a tungsten carbide cylinder. These cylinders can be brazed to drill bodies and carefully shaped into drills where the PCD forms only the cutting edges.

Given the fact that PCD is so hard, shaping PCD into an accurate cutting tool has always proved challenging to manufacturers, and many view this step as something that makes their tools superior to others. Three methods are used.

Many PCD cutting tools are physically ground using diamond grinding wheels. While this method can create cutting edges where individual diamond grains actually become sharpened, it requires physical contact that can break delicate tools and can occasionally “pluck” diamond grains out of the binder. Also, the diamond grinding wheels wear nearly as fast as the PCD.

Other PCD tools are shaped using electrical discharge machining (EDMing). Being non-contact, this process can accurately and safely shape delicate tools; but individual diamond grains are not electrically conductive, which means they simply fall out as the conductive binder around them is removed. Ultimately this means that cutting edges don’t contain sharpened diamond grains.

Lastly, PCD tools can be shaped using laser ablation, which is becoming more common. Although ideally suitable for shaping PCD, some manufacturers argue that the heat from laser ablation will cause nearby diamonds to decompose into graphite, dramatically reducing the usefulness of the cutting edge. While this is a legitimate concern, there is strong evidence that modern femtosecond lasers eliminate this problem by pulsing the laser so quickly that heat doesn’t have time to travel into the nearby diamond (one femtosecond is to one second as one second is to 30 million years). Using femtosecond lasers, ablation is recognized as the most sophisticated way to shape PCD cutting edges, and can be used to make micro-tooling where the entire tool body is solid PCD and cutting edges actually contain sharpened diamond grains; but currently the equipment is very expensive.