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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.
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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 to the diamond is lost.
Above: Hardide-coated
thermally stable
polycrystalline diamond
on a reactor.
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
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greater than the substrate, typically by a factor of five
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,
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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 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
Above, from top: The
Hardide coating reactor
being loaded.
Hardide-coated thermally
stable polycrystalline
diamond.
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, [email protected]
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