Thursday, 24 January 2013

Diamond is Harder than Steel

The diamond wire-drawing die is one of the most important single tools in modern industry, for it is responsible for the production of the fine wires on which the world's electrical and electronics industries depend. Only natural industrial diamond crystals of the highest quality can withstand the stresses of drawing wire at speeds of up to 8,000 feet per minute (40 meters per second) while maintaining the close tolerances and high-quality finish required in continuous wire production. Very few people think of the diamond as anything but a gem and yet only about 20 percent of total natural diamond production is used in jewelry. The other 80 percent, together with as much again in synthetic production, goes to industry where it is used in an almost infinite number of applications. The widespread use of diamonds in industry is a relatively recent development, however. They had been used for centuries in simple cutting tools and in powder form within the diamond industry itself, but it was the rapid growth of industrial technology during and after World War ll along with intensified research effort that revealed the true value of the diamond to industry. As a result, the world's annual industrial consumption of diamonds has grown from something like 5,000 carats in 1939 to a figure well in excess of 80 million carats today. The research effort was very much the brainchild of Sir Ernest Oppenheimer. 

The growing demand for gem diamonds inevitably involved the mining of a proportionately greater number of non-gem quality stones, and it was clearly in the interests of the diamond producers to encourage the development of industrial uses for these diamonds. It was not long before the research and marketing activities of De Beers began to make an impact on industry, and their efforts were given considerable impetus by two inventions. One was the discovery of how to incorporate crushed diamond into a resinoid bond and then to form it into the shape of grinding wheels. 

The heart of the Science Research Council's new £2 million infrared telescope, which is being installed on Hawaii's Mauna Kea mountain, is a glass ceramic disc 152 inches (3.8 meters) in diameter. In this picture the telescope mirror's 40-inch (1-meter) diameter center hole is being machined with a 450 edge bevel using a wheel containing De Beers natural diamond abrasive.The other was the creation of an abrasive material by impregnating metal powder with crushed diamond, heating the compound to sintering point and pressing it. Diamond powder and grit rapidly became the standard materials used in the engineering industry for grinding, polishing and sawing of hard, abrasive workpieces and some 85 percent of industrial diamond consumption is accounted for in this way.

The principal uses for whole diamonds are in rock drilling, the turning and boring of nonferrous metals, the dressing of abrasive wheels, and wire drawing, tasks which before had been carried out mainly by carbide tools. The adoption of diamond tools in these roles took longer to achieve because of their initial high cost but it soon became recognized that the enormous gains both in precision and in tool life made them far more economical in the long run. 

One example which is typical occurred in the machining of carbon molds used to seal electrical transistors. When carbide tools were employed, they needed sharpening after every 100 operations. The diamond tool went on unaffected for 100,000 runs. The finest wires for the electrical and electronic industries are made by drawing the metal at speeds of up to 100 mph through tiny holes in diamond dies. The durability of these dies means that precision is guaranteed and that they do not have to be constantly replaced. Only a natural diamond crystal of good quality is strong enough to withstand the stresses involved in the operation, and even when the die begins to wear it can be repolished and used for larger diameter drawing. 

Cross-section of a diamond wiredrawing die showing the critical angles to which it must be polished.
As many as seventeen resizings are possible with a single diamond die. Similarly, the introduction of a diamond-impregnated wheel in the manufacture of the stainless steel sleeves of the jet engine thrust reversers on the Lockheed C5A not only halved production time but also achieved a far better finish. The diamond wheels replaced aluminum oxide wheels which had required two operations to complete the job. But it is not only in metal working industries that the diamond is used. it has a great variety of applications in ceramics and in glass and plastics, where once again it does the work of alternative materials in much less time and with far greater precision. 

A good example is in the production of optical lenses. Diamond-impregnated milling and grinding tools are used in the shaping of lenses like the five-ton paraboloid mirror at the Greenwich Observatory in England, down to the smallest camera lens. For centuries artists have used diamond-tipped tools to engrave glassware and today they are widely employed along with diamond grinding wheels to provide delicate effects of line and tone. Applications range from the highest-quality domestic tableware to the windows of Coventry Cathedral.  

In certain types of glass engraving such as stippling the vibrator tool, equipped with a diamond point, can work much faster than the more traditional diamond-tipped hand scriber. Glass designer lane Webster, seen here at work on a presentation piece, employs the vibrator to supplement her regular use of the hand scriber in the engraving of fine detail. Diamond-impregnated blades are used to cut grooves in highways and aircraft runways in order to prevent aquaplaning in wet weather and they do it with a degree of speed and precision far superior to that which could be attained by conventional abrasives. They are also used to cut expansion slots in walls of buildings, again with a high degree of speed and precision: in one day a diamond saw blade can cut eighteen expansion slots compared with the ten days needed to cut just one slot using a hardened metal blade. 

Mining is another important application. Diamonds mounted in the heads of drills can cut through thousands of feet of the hardest rock. Because of the large diameter of many of the holes needed to be bored in rock, it is in this application that the sizes of diamonds used are the greatest. The head of a rock drill may be studded with brown and yellow diamonds of up to 5 carats each. The first recorded use of diamonds in rock drilling was in 1864 when the Mount Cenis Tunnel was driven through the Alps.

Many road and airport runway accidents which occur in bad weather are attributable to aquaplaning, a phenomenon which occurs when a tire mounts a film of water, losing all contact with the road or airport runway surface. Planning machines equipped with diamond cutting heads will grind a pattern of grooves into the surface with skid resistance characteristics equal to, or even better than, those of a newly laid surface.The general public knows the industrial diamond best in its noncutting uses, and without doubt the most familiar example is the record player stylus. Minute though they are, the styli are shaped with the utmost precision. Automatic equipment is used to ensure that a 78 rpm diamond tip is machined to a radius of 0.0035 inches, while that for long-playing record fines down to 0.0007 inches. The highest-quality stereo reproduction requires a 0.0002-inch radius tip. Precision is also essential in research laboratories and diamond feeler styli are used on surface-measuring equipment which is able to check surface finishes to the extent of detecting scratches no wider than two-millionths of an inch. 

An 85 inch (2.2 meters) diameter circular saw blade, whose cutting teeth contain De Beers diamond grit, saws through a block of Welsh slate at high speed.
One great spur to industrial diamond manufacturers has been the challenge provided by steel. Because of a tendency to overheat and fracture, diamonds had never been considered suitable for the grinding of steel; only silicon carbide and aluminum oxide were used. Eager to obtain a share of the market for grinding the most common of metal alloys, De Beers' engineers embarked upon a series of experiments which eventually produced a synthetic diamond grit capable of doing the job. They called it the Ductile Materials Experimental Diamond Abrasive (DXDA) and then improved it by a revolutionary new process of metal-cladding devised by the Swedish company ASEA, partners of De Beers, who were pioneers in the manufacture of synthetics. Metal cladding involves the application of a fine metal coating to the diamond before it is bonded in resin. 

The result is a stronger bond which holds the diamond in place for much longer periods, prevents premature fracturing, and slows down the transfer of heat from the diamond points to the resin bond. DXDA-MC also works successfully on combinations of car-bide and steel. There are also many varieties of micromesh natural and synthetic diamonds with an almost infinite variety of industrial uses which are dictated by the nature of their bond or cladding. Even the smallest diamond crystals have their distinctive shapes and are graded accordingly, and as far as synthetic diamond is concerned, great progress has been made toward actually tailoring the product for the job. 

Diamond crowns 12 inches (30.5 millimeters) in diameter or more, containing up to 900 carats of high-quality drill stones, are used in drilling through hard rock for oil wells. Smaller bits, such as the one illustrated here, are used for core sampling and mineral prospecting.
But if the diamond has a great advantage over other materials in certain industrial uses because of its extreme durability, there are new areas of technology being developed where there is no alternative to using diamond tools. In the field of space technology, for example, no other material is capable of achieving the degree of precision required. There is a story from the early days of the space program about a specification being issued to potential precision tool suppliers. A tool was required by the McDonnell Corporation to machine the reentry configuration of the heat shield of the Gemini spacecraft which would have to operate at white heat without a coolant. 

The suppliers claimed that no such tool could be devised and that even a diamond one would rapidly burn up. In the event, McDonnell decided to make up a tool to their own specifications. They bonded a 3/4 carat cut diamond to a copper stock and mounted it at the corner of a triangular steel block. With the copper acting as a heat sink, the 3,500°F cutting temperature was tolerated long enough for the diamond-tipped tool to complete three to four heat shields before renewal.

Dice are diamond-finished to ensure "fair shakes."  Ultrafine finishes are also vital in the manufacture of air bearings for space vehicle guidance and control devices. These, too, are almost impossible to obtain without the use of diamond compounds because the surfaces to be polished are made of hard-anodized aluminum and are themselves extremely hard. The diamond compounds were found to produce completely scratch-free surface finishes of 0.5 microinch as against the 2- microinch finishes which had been the best attainable by other methods. 

The high thermal conductivity of the diamond, which is superior to that of the best metallic conductors (six times as effective as copper), makes the substance indispensable in many industrial operations in which extreme heat is generated. The diamond is capable of carrying heat away almost instantaneously from areas where high-speed, high-precision work is necessary, as in the example of the machining of the Gemini heat shield already mentioned. It can also act as a heat sink for the high-power transistor microwave radio transmitters used in guided missiles. These are very tiny devices and their efficiency depends entirely upon the ability of the whole system to carry away the heat that they generate.

Diamond dressing tools are widely used in the engineering industry to remove clogged or dulled surfaces on conventional abrasive grinding wheels.A marked difference in the physical properties of individual diamonds has prompted their classification into Type I and Type II. Broadly, Type I diamonds are those that contain slight traces of nitrogen (up to 0.3 percent), while those in Type II contain it only in the most negligible amounts. The former are much more common, but because Type II diamonds are three to five times more efficient as conductors, they are much sought after by industry. 

Of Type ll diamonds, those with exceptional optical and thermal properties are called Type IIa, and those with semiconducting properties, Type Ilb. Both types of diamond are exceptionally pure and very rare in nature. The latter are much rarer and generally blue in color: the famous Hope diamond is of this type. A new use devised for Type Ila diamonds is as windows for infrared detecting sensors. These sensors "see" in the dark by focusing on areas or objects generating heat whether they he the human body, a gun barrel or the exhaust pipe of a vehicle. They then convert this infrared image into a visual one. Since Type Ila diamonds are highly transparent when exposed to infrared radiation, thin slabs of diamond make ideal windows for the devices. They are also used in the same way in the Golay cell, another heat-detecting system used in the laboratory for the measurement of infinitesimal amounts of heat, and in the windows of weather satellites circling the earth.

A copper-coated photogravure printing cylinder weighing nearly a ton is diamond-turned to a mirror finish and a concentricity of 0.001 inches (0.03 millimeters).
There ‘is, of course, no such thing as perfection in any natural material but the purest diamond approaches it nearer than anything produced synthetically. As a result, the engineer and the scientist in a nuclear or space laboratory today regard the diamond with much the same respect as did their ancestors more than two thousand years ago. The diamond is as much a scientist's best friend as it is a girl's

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