COMMENTARY — The diamond-cobalt bond

Diamonds are still the hardest substance available for cutting, but only in a few cases (for example, wheel dressing) are large stones used on their own. Most diamond tooling is made in a manner similar to carbide tooling. Materials are blended, pressed and sintered.

As in the carbide industry, cobalt is valued as a bonding agent; although unlike the hard metal industry, cobalt in diamond tooling does not have a virtually exclusive contract. Other materials, such as bronzes, are used, as many of the factors present in carbide sintering are not relevant to diamonds. Diamond tooling is used in metal-cutting but not in conventional turning, etc. In normal cutting, problems are caused by welding and breakdown of the diamond material with ferrous alloys (remember, diamonds are pure carbon). Its role is more specialized in honing and polishing.

The main function of diamond tooling is cutting non-metallics: stone, marble, tarmac, roads, concrete, glass and crystal. Specialized tools can be made by shaping before sintering. That nice, rounded edge on your car window is simply ground on with a diamond-bonded wheel.

Originally, all diamonds used in tooling were natural, from Africa and Brazil. In 1959, synthetic diamonds became commercially available. The tetragonal, crystal structure of diamonds is not stable at room temperature, but graphite is. In theory, therefore, given time, all that expensive jewelry in the Tower of London will revert to lead pencil. Luckily, however, the time is infinity, unless one raises the temperature. This is why diamonds are not used in high-productivity ferrous machining. The tip temperatures are too high and the diamond material would revert to graphite, which is not known for its hardness.

Diamond material is formed by reproducing the conditions under which natural diamonds must have been produced high temperature and pressure. Even then, graphite, which is the starting material, does not convert easily. To accelerate the change, it is mixed with a catalyst or solvent metal cobalt. For the slow conversion to occur, a container of layers of graphite and cobalt is squeezed between carbide anvils at around 2,000C at pressures of 40,000 atmospheres (600,000 lb. per sq. inch), and held.

From a recent issue of Cobalt News, published by The Cobalt Development Institute in the United Kingdom.