Poly-crystal Diamond (PCD) not only possesses hardness and wear resistance close to that of monocrystalline diamond, but also exhibits better anisotropy and impact resistance comparable to that of hard alloys. Therefore, it is widely used in fields such as oil and gas extraction, coal geology exploration, and machining.
Since the discovery of natural monocrystalline diamonds, another type of diamond, "carbonado" (black diamond), has also been discovered, although it is very rare in nature.
Research found that it is a bulk polycrystal composed of numerous diamond single-crystal particles and trace impurities. The disordered arrangement of diamond microcrystals endows it with the characteristic of having no cleavage planes, making its hardness, strength, and wear resistance superior to traditional monocrystalline diamonds.
The inspiration for humans to manufacture poly-crystal diamond originated from the understanding of its structure. Scientists from the former Soviet Union and the United States invested considerable effort in synthesizing artificial poly-crystal diamond between 1961 and 1970 and made some achievements. In 1964, GE company was the first to apply for a U.S. patent with "certain metal additives can facilitate direct bonding between diamonds".
The world's first practical poly-crystal diamond product was invented by GE in 1971. PCD was further promoted between 1972 and 1973, primarily being transformed into cutting tools for machining. Subsequently, higher-performance poly-crystal diamond drill products were introduced for the drilling field in 1976.
In a high-pressure-resistant container, the high temperature and kinetic energy generated by an explosion are used to propel metal sheets to impact graphite sheets at high speed, creating an instantaneous high-temperature and high-pressure environment that converts the graphite into micropowder poly-crystal diamond. The polymer synthesized by this method will contain residual graphite and cannot form regular large-sized polycrystals, so it can only be used to prepare abrasive grade poly-crystal diamond of lower quality.
The core principle of this method is to introduce gaseous raw materials containing carbon into a reaction chamber under an environment of less than 1 standard atmospheric pressure (1.01×105Pa). Through a series of complex chemical reactions under these conditions, the carbon atoms in the gaseous raw material eventually deposit in diamond form on the surface of a substrate, forming a film-like poly-crystal diamond. This method, known as low-pressure chemical vapor deposition, produces a poly-crystal diamond film with high thermal stability due to the lack of involvement of excessive metals from the transition metals group. It can also be doped with other elements to prepare semiconductor materials, making it commonly used in the electronics and optics fields.
This method involves directly converting high-purity graphite micropowder into poly-crystal diamond under conditions of ultra-high pressure and ultra-high temperature (2000°C, above 13GPa). This method can synthesize high-purity diamond with performance indicators close to that of natural diamonds.
Polycrystalline diamond is a polycrystal, possessing the common characteristics of polycrystals. Since the arrangement of grains in its internal environment is irregular, it has the characteristic of long-range disorder but short-range order, ensuring consistent physicochemical properties in all directions. Its characteristic of having no cleavage planes makes it more impact-resistant than large single-crystal diamonds, resulting in fewer application limitations and broader application ranges.
The synthesis cost of poly-crystal diamond is lower than that of natural and synthetic large single crystals, and it can be fabricated into different shapes as needed. Combining high hardness and high strength, it is an ideal material for industrial applications. Currently, diamond is applied in the industrial, technological, and defense fields, with poly-crystal diamond being mainly used for industrial applications.
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