For composite machining, diamond cutting tools are generally classified into two main types: PCD tools and CVD diamond-coated tools.
PCD tools are manufactured using PCD segments of specific sizes and geometries, while CVD diamond-coated tools are fully ground to shape first and then coated with a CVD diamond layer.
CVD Diamond-Coated Tools
Chemical Vapor Deposition (CVD) is a process in which a diamond coating with a typical thickness of 6–16 microns is deposited onto a tungsten carbide substrate. The substrate must have a low cobalt content, usually around 6%, and undergo a specialized surface treatment prior to coating.
This surface treatment reduces the cobalt concentration at the outer layer of the substrate, which is essential for achieving strong adhesion between the diamond coating and the carbide base. The resulting tool offers excellent wear resistance and is well suited for machining highly abrasive composite materials.
PCD Tools
Polycrystalline Diamond (PCD) is a synthetic diamond material, distinct from single-crystal diamond. It is produced by sintering multiple diamond particles together with a metallic binder—typically cobalt—under extremely high temperatures and pressures.
PCD typically consists of 90–95% diamond particles, with the remaining percentage being cobalt. This structure provides exceptional hardness, wear resistance, and edge stability, making PCD tools highly effective for cutting non-ferrous and composite materials.
CVD & PCD Comparison:
| PCD | CVD Diamond coating |
| Hardness | 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. |
| Wear resistance | 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. |
| Durability | 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. |
| Construction | 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. |
Application
Diamond is an ideal material for cutting tools due to its extreme hardness, wear resistance, and high thermal conductivity. All non-ferrous materials can be machined with PCD tools, including chipboards, HDF, laminated boards, materials used in the automotive industry for aluminum components, and lightweight materials such as carbon fiber reinforced plastics (CFRP), metal matrix composites (MMC), and stacked materials used in aircraft construction.
Unlike milling, drilling involves constant contact with the material, which makes the cutting edge less likely to chip or fracture. As a result, CVD diamond drills minimize delamination at the hole exit by combining hardness with sharpness. In highly engineered parts, such as those used in aerospace applications, drilling composites is a major concern. When delamination is the failure criterion, CVD diamond drills outperform PCD drills.
If hole diameter is the failure criterion, PCD drills will “survive” longer than CVD drills. This is because CVD diamond is a coating: once the diamond layer delaminates from the tungsten carbide substrate, edge wear accelerates rapidly. PCD tools, by contrast, are solid diamond materials, and wear develops more gradually and continuously.
For stack materials such as CFRP/Al, where the hole exit is in aluminum, PCD drills may also be advantageous. In this case, PCD is the preferred choice, as it offers longer tool life and can be reconditioned several times, since delamination is not an issue.
Unlike wafer drills, whose geometry is determined by PCD segments, CVD diamond drills offer greater design flexibility. A Fullnib PCD drill, however, has no geometry limitation, and its design flexibility is equivalent to that of a CVD diamond drill, provided that the limited PCD nib height does not introduce additional constraints.
PCD or CVD in Countersinking Operations
Despite countersinking being a manual operation, the countersink body is typically made from steel due to the threaded connection requirement. Since CVD diamond cannot be coated on steel, most countersinking tools for composite materials are manufactured using PCD.
In these tools, the steel countersink body is brazed with PCD segments. Because it is difficult to grind threads on a tungsten carbide body, fully carbide countersinks with CVD diamond coating are rarely used in practice.
Both PCD and CVD cutting tools have their respective advantages and limitations, and neither can completely replace the other. The optimal choice depends on the specific tool design, material combination, and application requirements.
