Polycrystalline Diamond (PCD) is a new type of superhard material that has been researched and used internationally since the 1970s. It is sintered at a high temperature of 1400 ° C and a high pressure of 6 GPa.

Polycrystalline diamond is both an engineering material and a new type of functional material; it is both a high-tech product and a highly efficient product. With the development of modern industry and science and technology, polycrystalline diamond has been widely used in modern industry, national defense and high-tech fields with its excellent strength, heat, chemistry, sound, light and electricity.

Polycrystalline diamond tools have become an indispensable tool in modern machining. This is mainly reflected in the following aspects:

(1) High-speed cutting, high-stability processing

(2) Ultra-precision mirror processing

(3) Dry cutting and cleaning processing

(1) High hardness and wear resistance
Polycrystalline diamond has a hardness of up to 10,000 HV. It is the hardest material in the world's man-made materials. It is much harder than hard alloys and engineering ceramics. Because of its extremely high hardness and isotropic properties, polycrystalline diamonds are extremely polar. Good wear resistance.
(2) Low friction coefficient
The coefficient of friction between polycrystalline diamond and some non-ferrous metals is lower than that of other materials, which is about 1/2 of that of cemented carbide. The low coefficient of friction not only reduces the deformation and cutting force, but also causes no built-up edge during cutting, thus reducing the surface roughness of the machined surface.
(3) High thermal conductivity
Polycrystalline diamond has a high thermal conductivity, which is better than silver and copper. It is much higher than that of ordinary hard alloys. Therefore, the cutting heat is easily dissipated during the cutting process, so the cutting temperature is low.
(4) High processing accuracy
Because the polycrystalline diamond tool has a low coefficient of thermal expansion and a high modulus of elasticity, the tool is not easily deformed during the cutting process. Under the action of the cutting force, the tool can maintain its original parameters, keep sharp for a long time, and have high cutting precision. Therefore, when machining with a PCD tool, the cutting force can be reduced and the cutting temperature can be lowered, the tool durability and the cutting rate can be improved, and a good machined surface can be obtained.

The main difficulties: the high hardness and high wear resistance of PCD materials make molding and surface finishing very difficult, which seriously hinders its popularization and application.
Processing methods: EDM, laser processing(TKD laser cutting Machine), chemical processing, ultrasonic processing.
Ideal processing method: grinding

Grinding
In view of the high hardness and wear resistance of polycrystalline diamond, the grinding process mainly includes diamond grinding wheel grinding, electric discharge grinding and electrolytic grinding. The simplest and most effective grinding method is diamond grinding wheel grinding. In the actual machining process, different grinding methods can also be combined according to the requirements.
(1) Diamond grinding wheel grinding

When the diamond grinding wheel grinds the polycrystalline diamond, the contact pressure and the grinding force between the grinding wheel and the workpiece are large, and the grinding processing system is required to have sufficient rigidity and high precision. In recent years, many countries have been developing and developing special grinding machines and grinding wheels for grinding polycrystalline diamond (including polycrystalline cubic boron nitride). Such as high rigidity diamond tool precision grinding machine developed by TKD Co., Ltd. and TKD series high efficiency special diamond grinding wheel

(2) Mirror grinding of polycrystalline diamond
Precision Mirror Grinding Technology is a new ultra-precision machining technology which is widely used right now. The basic principle is to use the non-linear electrolytic dressing action and the dynamic balance of the electrolytic inhibitory effect of the surface oxide insulating layer of the metal bond superhard abrasive wheel during the grinding process, and the grinding wheel is continuously trimmed and trimmed to obtain the grinding wheel abrasive grain. Constant amount of protrusion, so that the grinding wheel is continuously processed in the best grinding state, it is suitable for precision mirror grinding of hard and brittle materials.

The diamond grinding wheel with metal bond and ELID precision mirror grinding technology can obtain good effect. It is a new way of precision processing of polycrystalline diamond, which has great promotion value and application prospect.

(3) EDM grinding

The characteristics of the pulse generator during EDM are very important, especially for processing PCD materials. PCD has a certain conductivity, good thermal conductivity and high melting point, so the traditional EDM processing of polycrystalline diamond is very difficult.


The erosion mechanism of EDM grinding PCD includes: gasification of diamond, oxidation of diamond, conversion of diamond to graphite and amorphous carbon, throwing force by electric spark discharge, thermal stress to microcrack on diamond surface, and diamond Breakage of crystal grains, etc. Among them, the graphitization of diamond plays a key role in the process of electric spark polishing of diamond film, not only to conduct electricity, but also to maintain the existence of discharge channel, so that the top of the diamond film protrudes from the top of the peak, and the graphitization-oxidation process of diamond continuously Perform to remove the diamond.


EDM and EDM grinding are the most efficient and cost-effective methods of processing polycrystalline diamonds, but they do not efficiently process large areas of polycrystalline diamond.

Grinding

There are three main methods for grinding polycrystalline diamond: one is to use a resin bond fine-grained diamond wheel for precision grinding; the other is to use a hardened high-hardness steel plate (or agate plate) without any abrasive. Direct grinding; another is the use of high-speed rotating cast iron discs, supplemented by diamond micro-powder grinding. Grinding can be used as a fine polishing process for polycrystalline diamond, which is an important part of the production of polycrystalline diamond tools.

(1) Grinding with diamond grinding wheel

Grinding of diamond grinding wheel Polycrystalline diamond is similar to grinding of grinding wheel, except that the type of grinding wheel is different. Grinding wheel with resin concentration of 50%-70% and particle size of W40-W7 is generally used for grinding. When the diamond grinding wheel is used to grind polycrystalline diamond, the grinding of the PCD material by the diamond abrasive grains in the grinding wheel is essentially a process of interaction between two objects with similar hardness and properties, which is essentially different from the traditional grinding process. The PCD grinding mechanism and grinding process have their own characteristics.


(2) High-speed steel disc grinding of polycrystalline diamond
High-speed steel disc grinding polycrystalline diamond, the basic principle is that the PCD workpiece is rubbed under a certain pressure and high-speed rotating high-speed steel disc, generating frictional heat, increasing the temperature of the sliding interface, sliding interface (grinding surface) The high temperature generated on the surface is beneficial to the oxidation, graphitization, diffusion, bonding and thermal stress cracking of the diamond grains in the sliding interface of the PCD, thereby achieving the purpose of grinding.


From the chemical properties of diamond, some metal elements can react with them to cause their disintegration. If the material of the grinding disc itself contains the above-mentioned metal element component, it will promote the diffusion, adhesion, thermal erosion and the like of the diamond at the sliding sliding interface, which helps to improve the efficiency of the grinding process. However, the temperature of the grinding sliding interface should not be too high, otherwise it will affect the quality of the grinding surface.

(3) Abrasive grinding of polycrystalline diamond
Abrasive grinding of polycrystalline diamond is one of the traditional processing methods. During the abrasive grinding process, the cleavage of the crystal grains is one of the main removal methods in the process of grinding PCD; the sharp abrasive grains are at a high speed. With proper pressure across the PCD, a large number of damaging scratches are produced on the surface of the diamond grains. These scratches can be divided into plastic scratches and brittle scratches, which is also an important mechanical removal method for PCD materials. In addition to this, there are many thermochemical removal methods such as graphitization removal, thermal erosion removal, diffusion removal, and oxidation removal. However, the thermal chemical removal method does not occupy a major position in the removal of the polished PCD material. Abrasive grinding has high precision, but the efficiency is extremely low. However, when assisted in ultrasonic grinding, the efficiency is greatly improved.

Other processing methods
(1) Wire EDM

The electric discharge wire cutting process realizes the cutting processing of the workpiece material by the pulse discharge of the local region between the electrode wire and the workpiece, and it is one of the commonly used methods for PCD cutting processing. Most of the wire-cut polycrystalline diamonds are used to make machining tools, geological drills and wire-drawing abrasives. These workpieces are complex and variable in shape, and it is economically feasible to form them with EDM machines.

(2) Laser Cutting

Laser cutting is a non-contact cutting. It has excellent characteristics in PCD material cutting. It can cut high-purity PCD materials (such as polycrystalline diamond film), cutting deformation and thermal deformation are small, and laser cutting speed is fast and efficient. It is a more effective processing method. Especially in microporous processing and non-conductive superhard material processing, it shows its superiority. However, there are problems such as difficulty in controlling dimensional accuracy and repeatability, and micro-graphitization on the machined surface, which requires further finishing. Currently only used for roughing or semi-finishing. The laser parameters that determine the function of the laser processing material are pulse width, maximum pulse power, and average pulse power.

TKD brand laser cutting machine is currently the most widely used laser cutting machine in China, and is widely exported to India, Russia, Italy, Taiwan and other countries and regions. It is the world's most cost-effective laser cutting machine.

(3) Processing by chemical methods

(4) Ultrasonic processing

Ultrasonic processing of polycrystalline diamond is processed by using diamond micropowder (1-50um) as free abrasive. The factors affecting ultrasonic processing of PCD are vibration frequency, amplitude, diamond powder size, PCD material, unit pressure and processing area. At present, polycrystalline diamond ultrasonic processing includes ultrasonic grinding and ultrasonic grinding, etc., and is mostly used for processing PCD bores and PCD drawing dies and sand blasting dies. The core component of the ultrasonic grinding method is an ultrasonic grinding device that performs ultrasonic vibration on one side and high-speed rotation on one side.

To be continued-->