Diamond Tool History
     Diamond, the  most sought-after gemstone in the world, may well also be the world's most versatile engineering material. Diamond is the strongest and hardest known substance, but hardness is not the only superlative property of diamond that makes it important in industry and technology. It also has the highest thermal conductivity of any material at room temperature, has a low-friction surface, and optical transparency. This unique combination of properties cannot be matched by any other material.

     Knowledge of diamonds started in India, where it was first mined. The word most generally used for diamond in Sanskrit is transliterated as vajra, "thunderbolt,"  where the flash of lightning is used as a comparison for the light thrown off by a fine diamond octahedron and a diamond's indomitable hardness. Early descriptions of vajra date to the 4th century BCE which is supported by archaeological evidence. By that date diamond was a valued material. The earliest known reference to diamond is a Sanskrit manuscript, the Arthasastra ("The Lesson of Profit") by Kautiliya, a minister to Chandragupta of the Mauryan dynasty in northern India. The work is dated from 320-296 before the Common Era (BCE). Kautiliya states "(a diamond that is) big, heavy, capable of bearing blows, with symmetrical points, capable of scratching (from the inside) a (glass) vessel (filled with water), revolving like a spindle and brilliantly shining is excellent."

     No diamonds have been found in ancient sites, but holes in ancient beads show diamond's "footprint," cylindrical holes with conspicuous concentric grooves left by a twin-diamond drill. The holes are unlike the marks of any other modern or ancient drilling technique. Beads from sites in Sri Lanka, India, Thailand, Yemen and Egypt show the marks of diamond drills prior to 700 CE and as early as the 4th century BCE in Yemen.

     Chinese interest in diamond was strictly as an engraving or carving tool, primarily for jade, or as a drill for beads and pearls. Chinese writings on diamond refer to "kun wu" and "kin-kang" as jade-cutting knives, the diamond "coming from Rome in iron scribes".

     The presence of diamond in Rome by about 100 CE is established by the writings of Pliny the Elder (23--79 CE), by sapphire engravings, and by talismanic diamond rings. Originally known by the Greek word "adamao", meaning "I tame" or "I subdue", the adjective "adamas" was used in early writings to describe sapphires and corundum, and although there was contact between India and the Mediterranean in ancient times, the timing of the association of  "adamas" with "diamond" has not been established. Pliny the Elder, who died during an eruption of Mount Vesuvius, wrote the encyclopedia "Historia naturalis," a fundamental source of classical information. He states: "The substance that possesses the greatest value, not only among precious stones, but of all human possessions, is adamas; a mineral which for a long time, was known to kings only, and to very few of them... Pliny also discussed diamond fragments: "These particles are held in great request by engravers, who enclose them in iron, and are enabled thereby, with the greatest facility, to cut the very hardest substances known." Roman engraved sapphires, cameos, and intaglios from the first century are undoubtedly the product of diamond engraving points.

     Diamonds were traded out of India by both sea and land routes. Traders employed circuitous trade routes a in an attempt to mask the ultimate source of diamonds, India. For centuries after, rulers of the intervening lands also kept finer diamonds from being carried across their territories, thus diminishing the quantities of diamonds that could reach the Mediterranean region.

     Diamonds disappeared from European history for nearly 1,000 years after the rise of Christianity because the symbolism associated with Roman talismans and Eastern magic made diamond abhorrent to the new religion.  At the same time, Persia and the new Middle Eastern states gained control over much of the trade, and diverted any diamonds from India.

     Diamonds survived conceptually, and in the Middle Ages, medieval treatises called lapidaries presented the qualities of different stones; their power; their efficacy as medicine, poison, or antidote; whether they could reproduce; and sundry other properties. Lapidaries were written until the Age of Enlightenment, in the 18th century.

     One of the earliest diamond-cutting industries is believed to have been in Venice, a trade capital, starting sometime after 1330. Diamond cutting may have arrived in Paris by the late 14th century; for Bruges -- on the diamond trade route -- there is documentation for the technique in 1465.

     The discovery in the 1870s of diamond deposits of unprecedented richness in South Africa changed diamond from a rare gem to one potentially available to anyone who could afford it.

     From the time Smithson Tennant showed that diamond was carbon in 1796, experimenters attempted to synthesize diamond from graphite or lamp black. Although the experiments at high pressure and temperature were in the right direction, for 150 years these attempts were fruitless. The invention of tungsten carbide in the 1930s provided a material that could achieve the pressure containment necessary for growing diamond. Experiments in the 1940s by Harvard professor Percy Bridgman were unsuccessful, but finally in the early 1950s two teams succeeded. The first was led by Baltazar von Platen, at the Allmanna Svenska Elektriska Aktiebolaget (ASEA) Laboratory in Stockholm, Sweden, in 1953. This initial success was neither publicized nor published, and therefore, on February 15, 1955, the General Electric team of Francis Bundy, Tracy Hall, Herbert Strong, and Robert Wentorf claimed credit for the first reproducible transformation of graphite to diamond.

Tracy Hall had this to say when he finally succeeded on December 16, 1954:
"I attempted many hundreds of indirect . . . approaches over a period of about a year but to no avail, and I was becoming discouraged. Then, one wintry morning, I broke open the sample cell after removing it from the Belt. It cleaved near a tantalum disk used to bring in current for resistance heating. My hands began to tremble; my heart beat rapidly; my knees weakened and no longer gave support. My eyes had caught the flashing light from dozens of tiny triangular faces of octahedral crystals that were stuck to the tantalum and I knew that diamonds had finally been made by man. After I had regained my composure, I examined the crystals under a microscope. The largest, about 150 microns across, contained triangular etch and growth pits such as I had observed on natural diamonds. The crystals scratched sapphire and other hard substances, burned in oxygen to give carbon dioxide, and had the density and refractive index of natural diamond. A few days later, an x-ray diffraction pattern unequivocally identified the crystals as diamond."
(Reference: KURT NASSAU, "Gems Made by Man," Gemological Institute of America, Copyright 1980).

     The device used by GE to synthesize diamond was termed a "belt device" because tungsten carbide rams were driven into a cavity contained by a doubly-tapered carbide cylinder, contained in turn by a steel jacket - termed a belt. Between the rams is a cylinder of graphite - a furnace - containing the material to be raised to high temperature and pressure. Around the furnace assembly and between the anvils and belt is a compressible material to contain the pressure and accept the deformation; it has traditionally been a natural clay called "pipestone clay" for its alternative use in tobacco pipes. A hydraulic press, capable of perhaps 50 tons, drives the rams into the belt cavity, amplifying the force at the interior to high pressure. An electrical current is passed between the rams and through the conductive graphite, which heats in response; the clay acts as a thermal insulator as well as a container for pressure.

     Modern manufacture of synthetic diamonds utilizes these same methods discovered by Mr. Hall. A mixture of graphite and a catalyst (typically nickel) is subjected to a pressure of approximately 1,000,000 pounds per square inch and a temperature of 1,800 °C for a period of approximately 1 hour. During this time diamond crystals nucleate at many sites in the mixture. The mixture is then cooled., then the pressure reduced to atmosphere. The diamond crystals are then separated from the remaining graphite and nickel using an acid wash.

     The separated crystals are sorted by shape, size, and impurities. This process is called grading. A typical production cycle will yield approximately 300 carat. of synthetic industrial diamond of various grades. The larger diamonds are used for sawing concrete, granite, and marble. Smaller diamonds are used in grinding wheels.

     Even though it is more expensive than competing abrasive materials, diamond has proven to be more cost effective in numerous industrial processes because it cuts faster and lasts longer than any rival material. Diamond that does not meet gem-quality standards for color, clarity, size, or shape is used principally as an abrasive, and is termed "industrial diamond." Eighty percent of the diamonds mined annually are used in industry; 4 times that production is grown synthetically for industry - that's a total of over 500 million carats or 100 metric tons. Synthetic industrial is superior to its natural diamond counterpart because it can be produced in unlimited quantities, and, in many cases, its properties can be tailored for specific applications. Consequently, manufactured diamond accounts for more than 90% of the industrial diamond used in the United States.
     Another use for the diamond crystals is to put a layer of diamond on a carbide substrate by again subjecting this to the high temperature high pressure process. This yields a product called polycrystalline diamond compacts (PCD) which are used for oil well drills and cutters for drilling and milling machines.

     Many new products, like compact electronic devices, windows for optical devices in demanding environments, and "no-wear" bearings, such as in the space shuttle, utilize diamond. For these applications, a synthetic form leads the way. This is CVD, so-named for the growth technique chemical vapor deposition. At present the major commercial application for CVD diamond is in thermal management, where diamond heat-spreaders conduct byproduct heat away from a device. The material can be grown with a thermal conductivity close to that of the best natural and high-pressure synthetic diamonds used until now as heat spreaders. Thousands of suitable heat spreaders can be cut from a single wafer of CVD diamond, making for efficient use. A CVD diamond coating on an object can be polished to yield an extremely smooth diamond surface, ideal for high precision and low friction, such as is needed for precision bearings. CVD diamond wafers with high optical transparency are excellent for viewing a wide portion of the electromagnetic spectrum in environments with extreme temperature, corrosiveness, or radiation.

     Because of their transparency, thermal conductivity, or surface properties, diamonds are used in many research instruments as windows. An application of exceptional value in mineral and material science is a small device that generates extremely great pressures in the space between two diamonds - the diamond anvil cell. These devices are used in experiments on the nature of planetary interiors and dense matter, from mimicking Earth's core to producing solid hydrogen. The mechanics of creating high pressure are simple, involving just an application of force onto a small area, but extreme pressure will not be achieved without a material of supreme hardness, incompressibility, and strength - such as diamond. Most materials, steel for example, will deform or break before reaching pressures that exist deep within Earth. Tungsten carbide is better, but diamond is best. By polishing the ends off two fine round brilliant diamonds to a width of a millimeter or so, and carefully and accurately squeezing them together, pressures comparable to the center of Earth - 4,500,000 atmospheres - can be achieved. At these pressures hydrogen transforms into a metal - a state that might exist deep within Jupiter. Research on planetary interiors and dense matter has been advanced greatly by the use of diamond anvil cells, using lasers, optics, and x-rays to probe these small samples to reveal their mysteries. Cruise missiles and smart bombs utilize infrared detectors to seek their targets. Diamond windows can provide a wide band of transparency and resistance to abrasion and heat thereby improving the targeting of such devices.

     Managing heat, particularly in electronics, with large layers of CVD diamond is a rapidly expanding field. One of the most imaginative of these is the three-dimensional multi-chip module, which holds out the promise of an extremely powerful supercomputer. To gain speed, electronics need to be as compact as possible, concentrating waste heat as well. By stacking sandwiches of electronics and CVD diamond, a supercomputer could be made small and cool enough to function. The use of diamonds as radiation detectors, light emitters in electronic displays, and coatings to make surfaces indomitable or unwettable are being researched now. Beyond their imprint as a tool, diamonds will be showing up in more and more products in the future, probably in  home electronics, appliances, and automobiles.

Reference: American Museum of Natural History; US Geological Survey.

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