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The Operating Rules And Principles Of PDC(Polycrystalline Diamond Compact)

August 23,2023

1.The Concept And Advantages Of Polycrystalline Diamond Compact

PDC (polycrystalline diamond compact) is a superhard composite material that is sintered under high temperature and high pressure conditions through a specific synthesis process with diamond micropowder as the raw material and cemented carbide as the matrix.
Compared with single crystal diamond, polycrystalline diamond (PCD) and polycrystalline diamond compact have the following advantages:
1.1. A layer of diamond powder is sintered on the cemented carbide, and the diamond crystals are arranged in disorder, isotropic, and have no cleavage plane. Therefore, it is not as small as single crystal diamond, has cleavage planes, anisotropy, great differences in strength, hardness and wear resistance on different crystal planes, and is brittle due to the existence of cleavage planes.
1.2. With high impact strength, cemented carbide as the matrix material of PCD not only has good toughness and certain hardness, but also has weldability, machinability and compatibility with PCD. When the polycrystalline diamond composite sheet is under a large impact, only small grains will be broken, and it will not be broken like a single crystal diamond. Diamond composite sheet has the advantages of high diamond hardness, low friction coefficient and good impact toughness of cemented carbide.
1.3. Through a reasonable process, the size of the polycrystalline diamond composite sheet can be enlarged to meet the requirements of large-sized cutting tools.
1.4. Through laser cutting, it can be made into specific shapes to meet different processing requirements.
1.5. Special properties can be designed by adjusting the synthetic formula to make the material suitable for different purposes.
Polycrystalline diamond composite sheets are widely used in geological drilling, mineral deposit exploration, non-ferrous metals and alloys, hard alloys, graphite, plastics, rubber, ceramics, wood and other materials. Gradually replace single crystal diamond and become an important member of the superhard material family.

2.Mechanism Of Synthesis Of Polycrystalline Diamond Compact

The early PCD products of Debeers in the UK were to sinter diamond micropowder into polycrystalline diamond blocks first, and then weld PCD to cemented carbide or other matrix materials by high-temperature brazing or secondary high-pressure sintering. Later, the company’s Syndite product canceled the secondary sintering process and adopted ultra-high pressure and high temperature primary sintering.

Taking the synthesis mechanism of PDC with Co as a binder as an example, during the ultra-high temperature and high pressure sintering process, Co and W in the cemented carbide substrate diffuse and infiltrate into the diamond layer, making the physical combination of WC-Co and the diamond layer change. Into the chemical combination of WC-Co-D.

During the ultra-high temperature and high pressure sintering process, when the temperature rises to about 800°C, because the cobalt atoms in the WC-Co layer begin to diffuse toward the diamond, the diffused cobalt atoms interact with the carbon atoms on the diamond surface, making the WC-Co Graphitization occurs on the diamond surface near the surface. The graphitization of the diamond surface, on the one hand, is conducive to the relative sliding of the diamond powder, which makes the rearrangement and densification of the diamond particles; on the other hand, it is beneficial to the subsequent diffusion and penetration of the cobalt-carbon eutectic liquid between the diamond particles. The sintering temperature rises to the cobalt-carbon eutectic point (1336°C), forming a cobalt-carbon eutectic liquid. Under ultra-high pressure and capillary force, the cobalt liquid penetrates into the diamond layer. During the infiltration process, on the one hand, the graphitized carbon on the diamond surface is dissolved; on the other hand, due to the good wettability between the cobalt liquid and the diamond, the cobalt liquid interacts strongly with the carbon atoms on the diamond surface, so that the surface of the diamond particles that is still under low pressure is rapidly graphitized. At this time, the diamond particles are rearranged again under the action of the cobalt liquid; after the sintering system enters the stable zone of the diamond, the graphite is infiltrated by the cobalt liquid and forms an interstitial solid solution or a CoxC interstitial phase. The carbon atoms in the cobalt melt diffuse towards the WC-Co interface under the combined action of concentration and temperature gradient, and during the diffusion process, different atoms or atomic groups further interact to form carbon atoms or atomic groups with sp3 structure.

In the diamond-cobalt liquid sintering system, the diamond-graphite transition is a reversible phase transition process, and its phase transition probability and degree are closely related to the phase transition driving force Δp. In the area far away from the WC-Co interface, the pressure is low and the temperature is high, and the driving force of the G–D phase transition is small; near the WC-Co interface area, the pressure is high and the temperature is low, and the driving force of the G–D phase transition is large, which is more conducive to the G-D phase transition. The -D process occurs, and the crystallization process can proceed more stably. Therefore, the cobalt melt far away from the WC-Co interface region will continuously dissolve graphite and continuously diffuse to the WC-Co interface region, making the cobalt melt in this region Supersaturation continuously precipitates carbon atoms with sp3 structure. These sp3 atoms can grow either by adsorption on the pristine diamond surface or by deposition on the recrystallized diamond surface with three-dimensional nucleation. When the carbon concentration in the cobalt melt between the growth planes exceeds the critical concentration of carbon, the solid-liquid interface energy between the diamond-cobalt liquid will be greater than the solid-solid interface energy between the diamond particles, and the cobalt melt between the grains will It is squeezed out of the particle gap, so that the growth surface is in direct contact. In this way, direct diamond-diamond bonding is achieved, forming a polycrystalline diamond structure.

Diamond microcrystals are caused by the dissolution and precipitation of diamond in cobalt melt during the sintering process of diamond at ultra-high temperature and high pressure. This kind of diamond crystallite overlaps between the raw diamond grains in the form of a bonding bridge, forming a direct D-D bond, and the diamond crystallite and cobalt liquid stay in the intergranular area together, forming a so-called D-M-D bond.

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