Are diamonds forever?

Materials World magazine
,
1 Jul 2014

With 3,500 drilling rigs operating worldwide, the pressure on drilling tools to offer robust performance has never been higher. In the face of this challenge, one material standing out from the crowd is diamond. Dr Yuri Zhuk from Hardide Coatings, UK, explains how new research is helping extend the lifetime of offshore drilling tools.

A key issue for drilling tools in the oil and gas sector is the extreme abrasive wear and erosion they experience, and even the most hard facing materials cannot survive the required equipment lifespan. Cemented carbide has long been the material of choice for these industries, with its combination of hardness and toughness. The hard tungsten carbide grains make the cuts while the ductile cobalt, with its soft metal matrix, dampens the impact. This combination is effective, but not hard enough to last in rock drilling. Its lifespan varies according to geology and drilling rate, and it is worn by rock formation because its hardness is not much higher than some minerals. The net result is that some critical drill string components made using cemented carbide need replacing after every drilling programme.

The oil exploration industry was the first sector to take advantage of diamonds in drilling, and today more than 50% of drilling tools use diamond drill bits. The reason is clear – in an industry where rig and staffing costs can run into millions of pounds per day, there are significant savings to be made if drilling is quicker and uninterrupted.

Diamond, while up to 10 times more wear resistant than cemented carbide, has always had issues associated with cost, graphitisation, oxidation and bonding to tool surfaces, which have hampered its mainstream adoption in industrial hardfacing materials. This is where diamonds – in particular polycrystalline diamonds – come to the fore. Monocrystalline diamonds have anisotropic properties with brittle facets, meaning that during drilling, when the diamond is constantly impacted, it may fracture if the directional impact is along one of the weaker crystalline planes. Polycrystalline diamonds comprise thousands of crystals sintered together, so the crystalline planes are in random formation.

There are two types of polycrystalline diamonds – polycrystalline diamond compact (PDC) and thermally stable polycrystalline (TSP) diamond. To create polycrystalline diamonds, nickel is often used as a catalyst to sinter the diamonds to fuse and recrystallise using high pressure and high temperature. The nickel is then leached from the compound using acids. The main commercially available coating for TSP diamonds is electroless nickel. This coating attains a weak bond to diamond, mainly through mechanical key, and the coating itself is a catalyst for diamond graphitisation. Nickel has a thermal expansion coefficient six times greater than that of diamond, meaning that when it is heated, the coating is strongly pushed away from the diamond surface. The lack of strong adhesion results in the coating ballooning upon heating and the bond between the materials is lost.

Taking the shine off diamonds
Although diamonds are one of the hardest materials on the planet, attempts to use them in hardfacing opened up some weaknesses. The first is their non-stick properties and the fact it is notoriously difficult to form a strong chemical bond with them. Diamond is chemically inert, so does not react with most other materials. It also has poor wettability by molten metals. In other words, a brazing alloy or molten metal infiltrant does not form a strong bond to the diamond surface, and mainly adheres by filling porosity and forming some key with the uneven surface of polycrystalline diamonds. Single-crystal diamonds and grains of diamond grit have to be encased in a metal binder, with the metal surrounding more than half of each grain. The drawback of this approach is that the metal wears and the diamonds fall out. The metal surrounding the diamonds has a thermal expansion coefficient much or six for most traditional alloys and coatings used. This means that, even if the metal binder is effective at room temperature, the metal brazing or coating skin expands up to six times more than the diamond under heat, creating huge stresses between the diamond and the coating.

The second problem is that because they are carbon-based, they succumb to oxidation and graphitisation, making them unstable. If diamonds are heated to around 400°C then graphitisation occurs, with the surrounding ferrous metals acting as a catalyst and accelerating the process. At room temperature it would take millions of years for this degradation to happen, but when high temperature is applied, graphitisation takes the shine off this extreme material in minutes, especially if in contact with iron or similar metals, turning the diamond from the hardest material into one of the softest – graphite. The result of this is that cutting tools using diamonds often have to be cooled by a constant flow of water, which decreases the risk of the diamond being compromised by heat.

More than a decade has been spent working on these materials to resolve some of the issues. The outcome has been the development of a tungsten carbide adhesive and protective coating to enable the use of diamonds in a new generation of hardfacing materials. Coating the diamonds prevents oxidation because the coating has zero porosity, restricting metals such as nickel from coming into contact with the diamonds. The coating acts as a diffusion barrier for both metals and oxygen, which opens possibilities for new, stronger, longer-lasting and better performing diamond tools. It is applied by chemical vapour deposition (CVD) – crystallised from the gas phase atom-by-atom, producing a conformal coating that can coat internal and external surfaces and complex shapes. The coating is made from a metallic tungsten matrix with dispersed 1–10nm particles of tungsten carbide. When required, dispersed tungsten carbide nanoparticles give the material enhanced hardness that can be controlled and tailored to give a typical range of hardness of 1,100–1,600Hv and, with some types of coating, up to 3,500Hv. The resulting coating has very strong chemical bond to diamond, exceeding the bulk TSP material strength, and good wetting by brazing alloys enables diamond attachment. The coating is pore-free and provides an impermeable barrier, protecting diamonds from both oxidation and graphitisation. While the coating still faces issues around thermal expansion, as no material can match the low rate of diamond, it has the lowest thermal expansion rate among all metals and enhanced toughness and ductility helps absorb thermal stresses.

Testing for success Before a new coating can be successfully launched into the marketplace, it must first be rigorously tested. In this case, the initial destructive testing checked the coating adhesion, where coated TSP diamonds were broken by strong impact and the fracture edge inspected under a microscope. The poorly adhered coating would separate from the substrate on the edge forming a free-standing coating, as the fracture would develop along the weakest link bond between the coating and the substrate. Initially the coating failed, as described above, but with further development and process optimisation, a strong adhesion bond was achieved, exceeding the strength of the substrate so that the coating behaved as an integral part of the sintered diamond compact.

Additional testing involved grinding the coated diamonds, testing coating wetting by brazing alloys, testing the thermal stability of the coated diamonds and checking other key coating parameters. The laboratory testing is only a partial reflection of the real-life field tests, when all the factors act together: impact, abrasion, temperature and oxidation. Field testing of the coated TSP diamonds is in progress, with promising signs, as several thousands of hardidecoated diamonds are being used to form a hardfacing skin for oil drilling tools.

Operational challenges
The oil and gas sector is at the frontier of using and adapting hardfacing materials, because the cost of tool failure is so high. PDC diamonds have been used in drilling in this sector for the last 25 years and there has been rapid growth in their use in other tools.

Price has been a factor too, with industrial diamond perceived as an elite tool of the trade. However, this has changed in the last decade, with the proliferation of synthetic diamonds, coming from countries such as China, reducing the price of diamonds by around 50%.

Oil and gas drilling activity is proliferating into deeper regions, facing the challenges of directional and horizontal drilling, and more demanding conditions with higher temperatures – localised temperatures at the drill bit can rise to 500°C. Increased pressures and chemically-aggressive H2S, CO2 and mineral acids also contribute to the greater need for advanced hardfacing. Being able to drill for longer, thanks to use of new advanced materials, can save huge downtime costs, especially for offshore and deep sea operations. A single failure of a drilling tool underground can cost more than £1 million in downtime, offshore platform hire, labour and other costs.

The enhanced attachment factor gives a secondary advantage of enabling more flexibility in engineering design, because the coating allows the diamond to be brazed in a tile formation on the tool. The coated diamond tiles hold better due to the adhesion properties and good wettability by brazing alloys. By forming a denser skin, there are fewer gaps between the diamonds and subsequently a better hold on impact during drilling. This is more robust and enables tool design for more ergonomic and effective engineering, which offers additional project advantages. This could improve the economics of more marginal and declining oil and gas fields, such as those in the North Sea.

The next steps will see the coating tested on other types of diamond such as PDC and diamond grit. The PDC elements consist of polycrystalline diamond discs attached to tungsten carbide/cobalt base by cobalt infiltration. The presence of cobalt in the diamond disk interstitial volumes severely restricts its operating temperature. Above 400°C, it causes graphitisation and mechanical damage to the PDC. Hardide-coated PDC might help achieve improved thermal stability.

While diamond is no stranger to the oil and gas sector, such new developments with the material have and will continue to enable exploration in deeper waters and more extreme environments.

This project was partly funded by the UK TSB under the SMART scheme. For further information, email Dr Yuri Zhuk, info@hardide.com