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What's PCD Polycrystalline Diamond

2020-10-20 08:56:33
What's PCD Polycrystalline Diamond? How to cut and grind PCD?
 
PCD tools is made of super hard material “poly-crystal diamond” and substrate (HSS or tungsten carbide) by special process. 
 
Features of PCD tools
High hardness
High compressive strength
Good thermal conductivity
High wear resistance
 
Characteristics of PCD tools
The hardness is up to 8000HV, which is 8 - 12 times higher than tungsten carbide.
The thermal conductivity is 700W/mK, that 9 -15 times higher than tungsten carbide, and higher than copper and PCBN
The friction coefficient is only 0.1 – 0.3, so PCD tools can significantly reduce the cutting force. 
The thermal expansion coefficient is only 0.9 x 10^-6 – 1.18 x 10^-6, which is only equivalent to 1/5 of the carbide. Therefore, the PCD tool can reach high machining accuracy.
The affinity between PCD tools and non-ferrous and non-metallic materials is very small, and the chips are not easy to bond to the cutting edge during the processing.
 
How to make PCD?
PCD blanks for the PCD tools are made from natural or artificial diamond powder and binder (including cobalt, nickel and other metals) sintered at a certain proportion under high temperature (1,000 – 2,000℃) and high pressure ( 50,000 – 100,000 atmospheres). In the sintering process, due to the binder, the diamond crystals form bonding bridges with TiC, SiC, Fe, Co and Ni as the main components, and the diamond crystals are embedded in the skeleton of the bonding bridge in the form of covalent bond. 
 
How to cut PCD?
Because of the high hardness and wear resistance of PCD tools, we need to use special processing techniques to cut PCD. At present, we usually use EDM wire cutting,  laser cutting, ultrasonic processing. 
 
Comparison of cutting technique for PCD
EDM wire cutting
1, Highly concentrated pulse discharge energy
2, Powerful explosive force of discharge
3, High efficiency
Ultrasonic cutting 1, Low efficiency
2, High consumption for diamond powder
3, Heavy dust pollution
Laser cutting
1, Contactless processing
2, High efficiency
3, Small deformation after cutting
 
In the above cutting technique, EDM wire cutting shows the better effect. The existence of bonding bridge in PCD makes EDM work. With the cutting fluid, pulse voltage is used to make the cutting fluid near the electrode metal form discharge channel, and discharge spark is generated in the part. The polycrystalline diamond can melt and fall off at high temperature in an instant, thus forming the PCD into triangular, rectangular, or square blank. The efficiency and surface quality of EDM wire cutting is influenced by cutting speed, grit size of PCD, thickness of PCD, and quality of electrodes. And the key is choosing the right cutting speed. The experiment shows that increasing the cutting speed will reduce the surface quality; but if the cutting speed is too low, the phenomenon of “Arch Wire” will be produced, and the cutting efficiency will be reduced. Also, increasing the thickness of PCD will reduce the cutting speed. 
 


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