Polycrystalline diamond
Polycrystalline diamond (PCD, PCD) is a synthetically produced, extremely hard, intergrown mass of diamond particles with random orientation in a metal matrix. It is by co- sintering of selected diamond particles at high pressure and high temperatures produced. The sintering process is strictly controlled within the stable range of diamond, resulting in an extremely hard and wear-resistant structure. PCD is used as a cutting material in cutting tools for wood, plastic and non-ferrous metal processing. The high affinity of iron for the carbon of the diamond only allows economical machining of steel in rare cases. The carbon from the diamond diffuses into the steel as the temperature rises, whichgreatly limitsthe service life of the tool. Strong cooling (with CO 2 flakes or cold gas) can extend the service life considerably. As an alternative, polycrystalline cubic boron nitride (CBN) is used in steel processing.
The abbreviation for polycrystalline diamond according to ISO 513 is "DP" (with binder) or "DD" (without binder).
Application examples
PCD tools are used to machine:
- Chipboard , fibreboard , plywood and hard natural woods
- Composite materials with a metal matrix
- Aluminum alloys
- Copper , brass , bronze , magnesium alloys
- Fiberglass , carbon fiber
- Plastic , rubber
- Not yet sintered (“green”) and sintered ceramics and hard metals
- Mineral and thermoplastic materials in building renovation
The tools are mainly used in computer-controlled machining centers. PCD cutting edges have a very high durability , but because of their own hardness by spark erosion or grinding with diamond grinding wheels ( ceramic - or non-ferrous metal bond under cooling lubricant ) to sharpen .
Cutting speed up to 6000 m / min for non-ferrous metals
Manufacturing
The production of PCD as a ready-to-use cutting material takes place in two steps:
- HPHT process (high pressure, high temperature synthesis)
- High pressure liquid phase sintering
The high pressure-high temperature synthesis: The diamond synthesis uses the property of graphite ( carbon ) to dissolve itself under certain temperature and pressure conditions in metals of the VIII. Subgroup of the periodic table ( iron , cobalt , nickel ) as well as in chromium and manganese . In this process, a metal-graphite mixture is exposed to temperature and pressure cycles with peaks of 1800 ° C. and 6 GPa in a reaction chamber. Once the metal has reached its melting temperature, it covers the graphite particles with a thin film and the graphite dissolves in it up to the saturation limit. As the temperatures and pressures continue to rise, the solubility of the carbon is reduced and, due to the high pressures, it partially separates out of the melt in a diamond structure. To prevent the diamond from graphitizing again, the temperature is lowered while maintaining the pressure. This cycle is run through several times and the existing diamond crystals act as crystallization nuclei in the further course of the process. This process sometimes produces larger monocrystalline diamonds, but mainly diamond scatters with grain sizes of 2–400 µm .
High-pressure liquid-phase sintering: In this sintering process, pressure and temperature conditions similar to those in diamond synthesis occur. In the process, the diamond layer is applied directly to a cobalt-containing hard metal base. Defined diamond grains with a diameter between 2 µm and 100 µm are used as the starting material for the synthesis of cutting materials. These monocrystalline diamonds are combined with the addition of metallic solvent catalysts and other sintering aids to form a polycrystalline matrix. The diamond grains are plastically deformed and compressed due to the high pressure. Diamond surfaces, on the other hand, which are only exposed to low pressures, graphitize due to the high temperature. The liquid cobalt from the hard metal base penetrates the remaining cavities between the diamond grains and loosens the graphite present there. After exceeding the saturation limit when going through a temperature cycle, the graphite separates from the melt again as a diamond crystal and thus creates a connection in the diamond matrix. The sintering process is only complete when the graphite has been completely converted. The product resulting from the sintering process is a layered composite material made of a polycrystalline diamond matrix on a hard metal base separated by a boundary layer enriched with cobalt.
literature
- Jan Chr. Siebert: Polycrystalline diamond as a cutting material. Hanser Fachbuchverlag, 1991, ISBN 3-446-16435-9 .