Cutting edge

A cheap technology can help meet the demand for synthetic diamonds

Published: Wednesday 30 April 1997

Cutting glass: one of diamonds (Credit: Amit Shanker /CSE)two Indian scientists, Rustum Roy of Pennsylvania State University and Pravin Mistry of qqc Process, Dearborn, Michigan, both in the us, are spearheading a cheaper and more flexible process to boost the worldwide demand for synthetic diamonds. A process that creates pure diamonds and binds it to a surface with the ease of paint on a brush looks set to change the manufacture of objects from engines and transmission parts to dyes, sporting goods and cutting tools.

Indian scientists state that most people think of diamonds as naturally occurring stones that must be mined but the human-made diamond has been used in manufacturing, mainly as inserts on metal-cutting tools, since it was invented in the '50s.Annual world sales of synthetic diamond amount to us $900 million, although the value of the cutting tools and grinders which use it is us $3.1 billion.

The new rapid-coating technology called qqc could expand the worldwide market for synthetic diamonds significantly. The qqc approach creates the diamond in an ordinary atmosphere, not the high-temperature vacuum used in standard diamond manufacture. Four laser beams are directed through a cloud of carbon dioxide at a tungsten carbide surface. The lasers break the carbon dioxide into oxygen and carbon. Diamond is formed from the bonding of this carbon with carbon atoms, which the laser energy has mobilised from the rotating tungsten carbide surface.

The object to be coated can be moved around by a robotic arm under the laser, precisely controlling deposition of the diamond. Adjustment of the lasers can control crystal size and structure. Most synthetic diamonds are made by chemical vapour deposition (cvd). But, in spite of years of effort, the cvd process can still coat only a few coin-sized shapes and this requires a vacuum chamber which must be heated to 800 c. And, when they are being used, cvd-tipped tools require liquids to carry away heat and chips of the metal being cut. Coolants are one-quarter the cost of all machines; and manufacturers have found disposal of them a costly headache.

The thickest layer of diamond made so far by the qqc process has been 1,000 microns, compared with the 7.25 micron layers usually created by cvd. Most striking is how fast the diamond forms, at a rate of about one micron per second, while it bonds metallurgically to the surface below. This compares to a few microns per hour for cvd.

News of the qqc process has spread rapidly. The interest is due to the implications of making the hardest known material quickly and in almost any shape. And the coating's metallurgical bonding to the substrate means tools made with it can cut through metal dry, without coolants.

The electronics industry has had its eye on synthetic diamond as a possible substitute for silicon. Tools coated both in diamond and tetrahedrally bonded non-crystalline carbon are being used in production, automation, power, train and chassis components such as gears, shock-absorbers and brake rotors to provide corrosion-proof properties, improved wear and tear, in some cases supplanting chromium and cadmium plating. Golf star, a California-based sports equipment company, plans to use qqc for golf club heads. Astro-Sweep of Woodbridge in New Jersey, a supplier of street-cleaning brooms to municipalities, has used qqc to tip its steel brushes in diamond.

The us Navy sees applications in hard-coated and corrosion-resistant pumps, machine components, bearings and gears. The laser process has created pure diamond on aluminium, plastic, ceramics and high-speed steel used in the home-tool industry. The inventors envision coating virtually any substrate.

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