High Pressure-High Temperature (HPHT) synthesis of diamond became a commercial reality during the 1950s and remains the major manufacturing method for synthetic diamond products whose main application is in tooling solutions for industry.

In essence, HPHT emulates the way in which natural diamonds are created deep in the earth where the enormous pressure means that carbon crystals with the denser structure of diamond are thermodynamically more stable than those of other carbon allotropes such as graphite.

Recreating such processes in the laboratory, and then the factory, came a step closer to reality when scientists began to understand the relationship between the graphite and diamond forms of carbon and the conditions for transforming one into the other. A phase diagram plotting pressure against temperature revealed the regions where each type of carbon was likely to be the preferred form. The Berman-Simon line defines the boundary between these regions and gave scientists the first clues to the conditions under which graphite might be converted into diamond. At room temperature and normal pressure, whilst graphite is the thermodynamically stable form, diamond is only slightly less stable; but making the transition between these states requires overcoming a huge intermediate energy barrier; this, figuratively speaking, was the hurdle that scientists had to be able to jump in order to synthesise diamond. The existence of such a large barrier for the transition from diamond to graphite is why natural diamonds have lain on the surface of the earth for hundreds of millions of years without any signs of turning into graphite.

It is, then, the combination of high pressures measured, in fifty or more kilobar; tens of thousands of times normal atmospheric pressure, and temperatures of around 1400°C that create the conditions for diamond synthesis. Developing machines that combine heat and pressure to this degree was spurred by the pioneering high-pressure work of Percy Bridgman in the US who developed presses that could generate pressures of 70 kilobars and more. He never synthesised diamond because his presses did not combine pressure with sufficient temperature.

A little bit of magic

It is also the science of catalysis which has also contributed to commercial diamond synthesis because the addition of a catalyst allows presses to run at lower temperatures than would otherwise be needed to produce diamond. Certain metal catalysts such as iron, cobalt and nickel dissolve carbon and this facilitates diamond precipitation out of solution when the other conditions for diamond formation are met.

Modern diamond presses

Two main press designs have been developed to synthetic diamond on a commercial scale: the belt press and the cubic press.

The belt press is popular because a relatively large volume of material can be produced in a single press run. In this design the ceramic canister tube that contains the starting materials for diamond fits into a ring or belt of tungsten cobalt that is further restrained by a large steel band. This ensures that the pressure is confined so that the pressure delivered by two upper and lower anvils acts only over the top and bottom surfaces of the canister. The belt is profiled so that it fits snugly in contact with the anvils.

The tapered anvil design results in the delivery of higher pressures at the point of contact with the canister as pressures of around 80 tonnes per square metre are concentrated over an area one hundred times smaller at the heart of the machine. Thick copper cables deliver electric current to resistively heat the metal canister to the required high temperature.

The second type of press design is the cubic press. A cubic press has six anvils that provide pressure simultaneously onto all faces of a cube-shaped volume. Cubic presses are typically smaller than a belt presses but can achieve the pressure and temperature necessary to create synthetic diamond faster. However, cubic presses cannot be easily scaled up to larger volumes.

The ingredients for diamond making

What goes into the canister and the length of press runs largely depends on the size, morphology and type of diamond you want to get at the end of the process. Generally diamond-makers are like wine-makers and jealously guard their particular recipes. However the principles are well known. If you want to create diamonds less than one millimetre in size – such as those required for used as abrasives in grinding and cutting applications - the aim is to have a high nucleation rate so that you can produce lots of tiny, individual diamonds as quickly as possible. In this case, the canister is loaded with ‘slugs’ comprising blended catalyst metal and graphite powders. The inclusion of diamond seeds also ensures that nucleation occurs.

For an increasing number of industrial and research applications, there is demand for diamonds larger than 2 mm in size. To produce these diamonds you need to start with smaller seed diamonds and grow these to the size you want. In this case, the seed diamonds are loaded into the canister and then covered with a layer of catalyst metal followed by a graphite/catalyst metal mixture. This second layer provides the carbon source so that new diamond will grow onto the seeds. Also, the temperature heating profile has to be altered to create favourable conditions for this growth to occur.

At the end of each press run, the synthetic diamond produced has to be extracted or “recovered” from the mixture of residual graphite and catalyst metal in the canister and this is a chemical process. Finally, the synthetic diamond is graded by size and shape.

Using this knowledge

The knowledge gained through high pressure-high temperature synthesis has been applied to create other novel materials such polycrystalline diamond or PCD, a composite of diamond and cobalt, and cubic boron nitride whose range of applications lies outside those of synthetic diamond.