In this article, we’ll try clarifying and shedding more light on a dilemma confronted by many end users of cutting tools to machine composite materials, when they are required to decide which type of tool to use for their application. There are mainly two types of diamond cutting tools for composites: PCD tools, where the cutting edge is built from a PCD segment of a specific size and shape and CVD diamond coated tools, where the tool’s edge is completely built in a grinding operation and later coated with CVD diamond.
PCD coating
PCD (Poly – Crystalline – Diamond) is, unlike single crystalline diamond, produced synthetically by sintering together many (Poly) diamond particles, usually in the size of 2 to 30 microns of a meter, with a metal binder (usually Cobalt) at high temperature and high pressure. The rate is 90-95% diamond particles, and the rest is Cobalt.
CVD Diamond
Chemical Vapor Deposition is a process of coating Nano diamond particles on a Tungsten carbide substrate applying a layer thickness of 6 to 16 microns of a meter. The tungsten carbide substrate has to contain low cobalt content, such as 6% an d has to go through specific surface treatment. The treatment reduces cobalt content in the outer layer, exposing the diamond edges, to create sufficient adhesion between the diamond coating and substrate.
In the table below, summarized is a comparison between the two types:
|
PCD |
CVD Diamond coating |
|
|
Hardness |
PCD is a composite diamond, 90-95% diamond powder + cobalt binder, therefore lower hardness than CVD. Around 6000 Vickers hardness. |
CVD is 99% pure diamond; therefore, hardness is the highest. Around 8500 Vickers hardness. |
|
Wear resistance |
Since PCD contains cobalt, the edge is more likely to wear faster, but that occurs until a certain edge radius is reached and remains constant for a longer period. |
Since CVD is pure diamond coating, the wear resistance is higher, so edge radius is kept sharper for a longer period. However, when coating wears off, since the material beneath is tungsten carbide, edge sharpness deterioration is much faster. |
|
Toughness |
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. |
|
Design |
PCD tools in the form of wafer segment are limited in geometry to the segment shape. However, when a full nib PCD is used, there are no design limitations. |
Since CVD tools are first shaped in a grinding operation, there are no geometry design limitations. |
Trying to make this comparison practical, let’s discuss different applications in regard to these two diamond tools:
Milling operations involve interrupted cutting of the material, where each end mill tooth is engaged with the material and exiting it in one tool revolution. Each time the cutting edge enters the material, the “hammering” effect damages the cutting edge of the tool. Looking at the table above, PCD tools are more likely to withstand the “hammering” effect in milling. In CVD diamond coated tools, the CVD coating eventually delaminates and only the tungsten carbide substrate is left. In the PCD, even if some fracture occurs, the diamond is solid and therefore keeps the same characteristics. There is a concern, though, about geometry limitations with the PCD. For instance, when a particularly long flute length is required, or when specific tool geometry is required, e.g. porcupine router, the CVD diamond tool has an advantage.
In drilling operations, the cutting edge has constant contact with the material, unlike milling, therefore the cutting edge is less likely to chip or fracture. Taking that into account, with a combination of hardness and sharpness, CVD diamond drills perform better regarding minimizing the delamination at the hole exit. This is a major concern in the drilling of composites in highly engineered parts, such as Aerospace. When delamination is the failure criteria, there is no doubt that CVD drills outperform PCD drills. However, when hole diameter is the failure criteria, then the PCD drill will “survive” longer than CVD drills. The reason for this is since the CVD diamond is a coating. When it eventually delaminates from the tungsten carbide, edge wear accelerates rapidly. The PCD, on the other hand, as said, is a solid diamond. Wear development rate is almost constant. Another example where PCD may have an advantage is the drilling of stack material like CFRP/Al, where the exit hole is in the Aluminum. Delamination is a non-issue in this case, therefore PCD would be first choice, since it will hold longer and can also be reconditioned a few times. Another advantage of the CVD diamond drill would be design flexibility, while comparing to standard wafer drill, where PCD segment dictates geometry. On a Fullnib PCD drill, however, there is no geometry limitation and design flexibility are the same as in the CVD diamond drill as long as PCD nib height (which is limited) does not create other limitations.
While countersinking is considered a manual operation, the countersink body is made from steel, mainly due to the thread connection. Steel cannot be coated with CVD diamond; therefore, most countersinks in the market for composites are PCD. PCD segments are brazed to the steel body of the countersink. Full carbide countersinks which can be coated are rarely seen, because grinding the thread on the tungsten carbide body is a complex task.