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Polycrystalline Diamond (PCD)

December 26,2022

Polycrystalline diamond (PCD)is not only close to the hardness and wear resistance of single crystal diamond, but also has better anisotropy than single crystal diamond and equivalent impact resistance to cemented carbide. Therefore, it is widely used in oil and gas exploitation, coal geological exploration, mechanical processing and other fields.

The origin of polycrystalline diamond
Since the discovery of natural single crystal diamond, another kind of diamond, “Cabonado” (black diamond), has also been discovered one after another. This kind of diamond is rare in nature. It is found that this is a massive polycrystal consisting of a large number of diamond single crystal particles and trace impurities. The disordered arrangement of diamond particles makes it have the characteristics of no cleavage surface, and the hardness, strength and wear resistance are superior to the traditional single crystal diamond. The inspiration for the manufacture of polycrystalline diamond comes from the cognition of its structure. During 1961-1970, scientists from the former Soviet Union and the United States invested a lot of energy in the synthesis experiment of artificial polycrystalline diamond, and made some achievements. In 1964, GE applied for a US patent for the first time with the idea that “some metal additives can directly combine diamonds”. The first practical polycrystalline diamond chip in history was invented by GE in 1971. PCD was further promoted in 1972-1973, mainly making tools for machining. Subsequently, the polycrystalline diamond bit with higher performance was introduced to the drilling field in 1976.

Synthetic method
1. The explosive method is used to synthesize polycrystalline diamond in a pressure resistant container, which uses the high temperature and kinetic energy generated by the explosion to push the metal sheet to hit the graphite sheet at a high speed and create a transient high-temperature and high-pressure environment, so that the graphite can be transformed into a fine powder state. The polymer synthesized by this method can only be used for the preparation of abrasive grade polycrystalline diamond with low quality requirements because there will be residual graphite in the polymer and regular large size polycrystalline diamond can not be synthesized.
2. Low pressure vapor deposition method The core principle of this method is that when the pressure is lower than 1 standard atmospheric pressure (1.01 × The gaseous raw materials containing carbon elements are introduced into the reaction chamber in the environment of 105Pa). Under this condition, through a series of complex chemical reaction processes, the carbon atoms in the gaseous raw materials are finally deposited on the surface of a certain substrate with diamond phase, forming a film like polycrystalline diamond. This method is called low-pressure chemical vapor deposition. The polycrystalline diamond films prepared by CVD are often used in the electronic and optical fields because they do not need the involvement of transition metals, have high thermal stability, and can be doped with other elements to prepare semiconductor materials. However, the existing problems are long synthesis cycle and low production capacity.
3. Direct conversion method This method is to directly convert high-purity graphite powder into polycrystalline diamond under ultra-high pressure and temperature (2000 ℃, above 13GPa). This method can synthesize high-purity diamond with performance indicators close to natural diamond. However, this method has high requirements for synthetic equipment, harsh conditions, low production capacity and high cost. At this stage, industrial mass production cannot be realized, and it can only stay in the experimental stage.
4. High pressure and high temperature solvent method This method is to transform raw materials such as diamond powder and graphite powder into diamond polycrystalline composite under constant high pressure and high temperature (5-7GPa, 1300~1700 ℃) under the action of transition metal or alloy catalyst. The advantage of this method is that the synthesis cycle is short, the performance index of the used synthesis equipment is not required high, and it plays an indirect role in saving costs, which is very suitable for the promotion of industrial production. However, there is still a certain gap in mechanical and electrical properties between the polycrystalline diamond produced in China and similar foreign products.

Characteristics and application
Polycrystalline diamond belongs to polycrystal and has the commonness of polycrystal. Due to the irregular arrangement of grains in the internal environment, they have the characteristics of internal long-range disorder, but short-range order, making their physical and chemical properties unchanged in all directions. Compared with large single crystal diamond, it is more impact resistant, less limited in application and more widely used due to its non cleavage characteristics.
The synthetic cost of polycrystalline diamond is lower than that of natural and man-made large single crystals. At the same time, it can be made into different shapes according to needs, and has the characteristics of high hardness and high strength. It is an ideal material for industrial applications. Now, diamonds are used in industries, science and technology, national defense and other fields. However, at present, polycrystalline diamond is mainly used in industrial applications.

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