2010 Full-Line Drill Catalog - Desanto

From natural diamond to synthetic PCDWonder of natureAs the hardest natural mineral the diamondis ideally suited for machiningother materials. It also possesses extremehardness as well as very sharpcutting edges if machined accordingly.For example, a diamond will scratchany other mineral but cannot be scratchedby any other stone itself.StructureEasily recognisable – the cubic structureTreasure from the deepNatural diamond consists of purecarbon and graphite. It was createdmillions of years ago in the earth’scrust through complex geological processes.At a depth of 130 to 700 kilometresbelow the earth’s surface therewere large quantities of pure carbon.Under certain optimal conditions thecarbon atoms combined and in timecrystallised in the form of diamonds.Responsible for this synthesis werethe temperatures of in excess of 1000ºC and enormous pressures of approx.40,000 bar existing at this depth.Valuable raw materialRaw diamonds cleansed from accompanyingminerals and dirtSource volcano coneThrough convection currents in theearth’s crust – also the engine for theplate tectonics, the movement of thecontinental plates – over millions of yearsthe diamonds steadily rose furtherand further upwards with the magmaand were eventually spat into the atmospherewith the eruption of a volcano.This created relatively small volcanocones with so-called “pipes”, thecurrent source of natural diamonds.Equivalent characteristicsNatural diamonds are less suited forindustrial application due to limitedavailability and heterogeneous structure.For this reason, PCD is appliedin this instance because it possessessimilar characteristics with regard tohardness, wear-resistance and sharpness.PCD – a super-hard tool materialBecause diamonds have always beenvery valuable, mankind have attemptedto artificially recreate diamonds – bymeans of magic and alchemy of coursewithout success. However, natural sciencehas increasingly unearthed thesecret of the diamond and opened thedoor for a synthetic production.20 tons per yearThe first synthetic diamond was createdat General Electric, USA in 1955.More or less simultaneously, ASEA inSweden developed a similar process.With this special process, at a temperaturebetween 1500º C and 1800º Cand a pressure of between 53,000 and100,000 bar, synthetic diamonds up to1 mm diameter are produced from graphite,marketed under the name PCD(polycrystalline diamond). Currently,the annual production of PCD is approximately20 tons, primarily applied forcutting tools, grinding powder and forthe production of cutting wheels.The advantage of PCD is primarily itsconsistent high quality – a fundamentalpre-requisite for the further machiningof tools with the same characteristics.CBN – a tough alternativeCubic boron nitride (CBN) is a highperformancetool material from a polycrystallinemass, that similar to PCDis produced in a high temperaturepressureprocess.CBN, its hardness only surpassed by adiamond, is suitable for the machiningof materials that cannot be machinedwith PCD or monocrystalline diamond.The main application range is ferrousmaterials possessing a hardness fromapprox. 45 HRC as well as grey castiron, Cr-chilled cast iron and wear alloyson a cobalt, nickel or iron basis.In contrast to PCD and diamond, CBNdoes not react with the carbide constituentsexisting in these materials. Alsoof no consequence are the occurringmachining temperatures, because CBNonly reacts with oxygen from a temperatureof approx. 1200º C and subsequentlypossesses an unequalled thermalhardness.Only in so-called super-alloys in theaircraft or reactor industry with a distinctiveaustenitic phase and simultaneouslyhigh toughness do CBN toolsgenerally reach their application limits.Typical representatives are high Nialloyedmaterials such as Inconel 718or Nimonic. For these materials machiningtrials are necessary to clarify theapplication possibility.PCD and CBN Tools243