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How to distinguish the difference between diamond PCD tools and CVD tools when selecting tools for processing composite materials?
1.Through this article, we try to explain the problems faced by most users of composite materials and cutting tools, and solve the problems of not knowing how to choose the appropriate processing tools during processing.Composite material cutting tools are mainly divided into two categories of products: PCD diamond tools and CVD diamond coated tools，The cutting edges of PCD tools are constructed from PCD segments of specific sizes and shapes; the edges of CVD tools are fully built in a grinding operation and subsequently coated with CVD diamond.
1.PCD (polycrystalline diamond cutting tool)
PCD (polycrystalline diamond) is different from single crystal diamond in that it is synthesized by aggregating many diamond particles, usually 2 to 30 microns in size, and sintering them together with a metal binder (usually cobalt) at high temperature and pressure . The ratio is 90-95% diamond particles and the remainder is cobalt.
2.CVD (chemical vapor deposition) cutting tools
(Chemical Vapor Deposition) is the process of coating nanodiamond particles on a tungsten carbide substrate with a thickness of 6 to 16 microns.Tungsten carbide substrates must contain a low cobalt content such as 6% and must undergo specific surface treatments. This treatment reduces the cobalt content of the outer layer and exposes the edges of the diamond to create sufficient adhesion between the diamond coating and the substrate.
In the table below, a comparison of the two types is summarized
Comparison table of PCD and CVD tools | ||
PCD cutting tools | CVD coated tools | |
Hardness | PCD is a composite diamond, 90-95% diamond powder + cobalt binder, so the hardness is lower than CVD. About 6000 Vickers hardness. | CVD is 99% pure diamond, so the hardness is the highest, about 8500 Vickers hardness. |
Wear resistance | Since PCD contains cobalt, the edges wear out faster, but this happens before a certain edge radius is reached and remains constant over a longer period of time. | Since CVD is a pure diamond coating, its wear resistance is high, so the edge radius can be maintained for a longer time. However, as the coating wears away, the edge's sharpness decreases faster because the underlying material is tungsten carbide. |
Toughness | The cobalt metal binder in PCD materials increases the strength of the material compared to CVD diamond. Therefore, it has the potential to have better resistance to chipping during milling operations and unstable machining conditions. | The almost pure diamond layer is less elastic and strong and therefore more prone to breakage and delamination than PCD. |
3.Application of PCD or CVD in milling
The milling operation involves interrupted cuts into the material, with each sharp tooth coming into contact with the material and exiting in one tool rotation. The "hammering" effect damages the cutting edge of the tool every time the cutting edge enters the material. Judging from the table above, PCD tools are more likely to withstand the "hammering" effect during sharpening. In CVD diamond-coated tools, the CVD coating will eventually delaminate leaving only the tungsten carbide matrix. In PCD, even if some fracture occurs, the diamond is solid and therefore maintains the same properties. However, there is a concern regarding the geometric limitations of PCD. For example, CVD diamond tools have advantages when particularly long alveol lengths are required, or when specific tool geometries are required.
4.Applications in PCD or CVD drilling
During drilling operations, the cutting edge is in constant contact with the material, unlike sharpening, so the cutting edge is less likely to chip or break. With this in mind, combined with hardness and sharpness, CVD diamond drill bits perform better at minimizing delamination at the hole exit. This is a major issue for drilling composites in high-precision components such as aerospace.When delamination is the criterion for failure, there is no question that CVD drills perform better than PCD drills. However, when hole diameter is the failure criterion, then PCD drill bits will "survive" longer than CVD drill bits.The reason is that CVD diamond is a coating. When it finally breaks away from the tungsten carbide, edge wear accelerates rapidly. PCD, on the other hand, is a solid diamond as mentioned above.The rate of wear development is almost constant. Another example where PCD may be advantageous is the drilling of laminate materials like CFRP/A, where the exit holes are in aluminum. In this case, delamination is not an issue and therefore, PCD cutter will be preferred as it can last longer and can be re-finished several times. Another advantage of CVD diamond drills is design flexibility, whereas with standard wafer drills the PCD segment determines the geometry.However, on superhard drills, there are no geometric restrictions and as long as the height of the PCD tip (which is limited) creates no other constraints, the design flexibility is the same as with CVD diamond drills.
