For composites, diamond cutting tools can be divided into two types: PCD tools, which are built from PCD segments of a specific size and shape, and CVD diamond coated tools, which are built entirely in a grinding operation and then coated with CVD diamond.
Poly-Crystalline Diamond is a synthetic diamond, unlike single crystal diamond, produced by sintering together many (Poly) diamond particles with a metal binder (usually Cobalt) at high temperatures and pressures. 90%-95% of the particles are diamond, and the rest are cobalt.
Chemical Vapor Deposition – A layer thickness of 6 to 16 microns of a meter is applied to a Tungsten carbide substrate through Chemical Vapor Deposition. It must contain a low cobalt content, such as 6%, and have a specific surface treatment. To create sufficient adhesion between the diamond coating and the substrate, the treatment reduces the cobalt content in the outer layer.
CVD & PCD Comparison:
|CVD Diamond coating
|PCD is a composite diamond composed of 90-95% diamond powder + cobalt binder, which has a lower hardness than CVD. Hardness of around 6000 Vickers.
|Since CVD is 99% pure diamond, it has the highest hardness. Hardness of around 8500 Vickers.
|PCD contains cobalt, so the edge wears faster until a certain edge radius is reached and remains constant for a long time.
|Due to CVD’s pure diamond coating, edge radius is maintained sharper for a longer period of time. Since the material beneath the coating is tungsten carbide, when the coating wears off, edge sharpness deteriorates much faster.
|The cobalt metal binder in the PCD material adds to the strength of the material, as compared to the CVD diamond. Therefore it is likely to have better resistance to chipping in milling operations and in unstable machining conditions.
|The almost pure diamond layer has lower elasticity and strength, therefore would be more likely to fracture and delaminate than the PCD.
|The geometry of PCD tools in the form of wafer segments is limited by the shape of the segment. PCDs with full nibs, however, do not have design limitations.
|The geometry design of CVD tools is not limited due to the fact that they are shaped first in a grinding operation.
Diamond is an ideal material for manufacturing cutting tools due to its extreme hardness, wear resistance, and thermal conductivity. All non ferrous materials can be machined with PCD tools, including chipboards, HDF, laminated boards, materials used by the automotive industry to make aluminium components, and lightweight materials, such as carbon fiber reinforced plastics (CFRP), metal matrix composites (MMC) and stacks for aircraft construction.
Unlike milling, drilling has constant contact with the material, so the cutting edge is less likely to chip or fracture. As a result, CVD diamond drills minimize delamination at the hole exit by combining hardness and sharpness. In highly engineered parts, such as aerospace, drilling composites is a major concern. CVD drills outperform PCD drills when delamination is the failure criterion. If hole diameter is the failure criteria, PCD drills will “survive” longer than CVD drills. This is because CVD diamond is a coating. Edge wear accelerates rapidly when it delaminates from tungsten carbide. PCDs, on the other hand, are solid diamonds. Wear develops almost continuously. Stack materials like CFRP/Al, where the exit hole is in aluminum, may also benefit from a PCD drill. The best choice is PCD, since it will last longer and can also be reconditioned a few times, since delamination is not an issue. Unlike wafer drills whose geometry is determined by PCD segments, CVD diamond drills would also provide design flexibility. A Fullnib PCD drill, however, has no geometry limitation and design flexibility is the same as a CVD diamond drill as long as PCD nib height (which is limited) does not create additional limitations.
PCD or CVD in Countersinking operations:
The countersink body is made from steel due to the thread connection, despite countersinking being a manual operation. Since CVD diamond cannot be coated on steel, most composite countersinks are PCD. The steel body of the countersink is brazed with PCD segments. Due to the difficulty of grinding the thread on the tungsten carbide body, coating full carbide countersinks is rarely seen.
There are pros and cons to both PCD and CVD cutting tools. Neither of them can fully replace the other. Other factors such as tool cost, reconditioning, tool diameter limitations, and more were not discussed in this article. We will discuss some of these factors in the following articles. We offer PCD and CVD diamond Drills, End Mills, Routers and countersinks for all your composites applications at General Cutting Tools.