Downhole tools often operate in extremely abrasive, erosive, corrosive and chemically aggressive environments. These conditions reduce tool life and cause significant downtime. This has substantial cost and time implications and can seriously impact well or project viability. Materials used in downhole drilling and completion tools have to combine high hardness with impact and corrosion resistance. Because hard materials are brittle, these properties conflict, and until now one or the other had to be sacrificed. This has meant that the use of traditional materials such as Inconel and stainless steel have limited the life and performance of tools and compromised economics and viability.


Hardide-T is a nano-structured, pore-free coating with a unique combination of enhanced wear and erosion resistance, toughness and impact resistance. The new, patented coating can be applied to steel, including stainless steel and some tool steels, Inconel, copper, stellite and other alloys. Thickness can be controlled to a range between 5 and 100 microns.

Figure 1. High Resolution Electron Microscopy (micrograph produced by Oxford University Department of Materials) shows nano-particle tungsten carbide precipitate (darker area in the center) in Hardide-T coating. (All images courtesy of Hardide)


The coating consists of a metallic tungsten matrix with dispersed nano-particles of tungsten carbide typically between 1 and 10 nanometers in size. An electron microscopic image is shown on Figure 1. Dispersed tungsten carbide nano-particles give the material enhanced hardness that can be controlled and tailored to give a typical range of hardness of between 1,100 and 1,600 (and, with other types of Hardide coating, up to 3,500) Hv (Hardness Vickers). Abrasion resistance is up to 12 times better than hard chrome.


Nano-structured materials are known to possess unique toughness, crack and impact resistant features. Hardide-T has withstood 3,000 microstrain deformation without any damage; this deformation will crack or chip any other thick, hard coating. Coating a component manufactured out of a tough alloy gives an unprecedented combination of surface wear resistance and the ability to survive impacts and shock loads. Figure 2 shows a crater left after a shock impact on a coated sample. The coating survived the impact and deformation without cracking or chipping.

Figure 2. Micro-paragraph of a crater (diameter approximately 1 mm) made by inpact into 50 microns thick hardide coating.


In a drilling tool application for a major oilfield service company, the coating has increased the life of critical components three-fold. The components were operating in an extremely abrasive environment and typically failed after 60 hours because of excessive wear. After being coated, the life of the parts was proven, in the lab and the field, to extend to more than 200 hours. This has enabled uninterrupted drilling for far greater periods of time and reductions in downtime and tooling costs. Traditional hard materials for this application were too brittle and difficult to machine because of the complex part geometry. Other coating technologies were not able to reach important hidden surface areas.


The coating is applied by Chemical Vapor Deposition (CVD) in a vacuum chamber furnace at approximately 932°F (500°C). Once the parts reach temperature, a controlled mix of gases is pumped into the furnace. After a series of chemical reactions between the gases, tungsten carbide is crystallized on the surface of the components as a smooth, binder-free layer with abrasion, erosion and chemically resistant properties. The coating crystallization — atom-by-atom from gas phase — allows coating of internal surfaces such as the inside of cylinder sleeves and complex shapes. This opens up new possibilities for tool design. Figure 3 shows a cross section of the coating on a thread, where the coating uniformly follows the substrate. There are no uncoated shadows or build-up of thicker coating on the sharp edges, which are typical problems for most other coating techniques.


Figure 3. A micro-paragraph of a cross-section of 50-microns thick Hardide-T coating on thread. The uniform coating follows the substrate; even slight imperfections are accurately followed.

Other key properties include resistance to acids (including H2S) and the absence of porosity. The highly mobile reaction products fill pores and defects in the coating as it grows. The porosity, measured as the difference between theoretical and actual material density, is less than 0.04%, while the coating completely covers the substrate without any through pores starting from less than 1 micron thickness. Unlike sprayed tungsten carbide, the coating does not use cobalt, which can be affected by acids; this is especially important for processing sour oil.


The coating was tested for resistance to aggressive media in accordance with the NACE Sulphide Stress Cracking test in a solution of 5% NaCl, 0.5% Acetic acid, saturated with H2S. Samples were tested in deformed conditions with coating elongation up to 3,000 microstrain. During the 30-day test the uncoated control sample cracked across the full 20 mm width and suffered from extensive micro-cracking and pitting. The same substrate coated with Hardide-T showed no micro- or macro-cracking or degradation. This confirmed the non-porous structure of the coating; under 3,000 micro-strain deformation, any existing micro-cracks or defects would open up channels for an aggressive fluid to attack the substrate.


Zero porosity is important for applications with valves where any porosity can result in gas diffusion through the coating layer and lead to a potentially explosive mixture. For this reason, spray coatings are often sealed with polymeric materials to close surface porosity, but this limits their operating temperature. This sealing would still leave unsealed pores deeper in the coating layer, which may open in use later. The coating is produced pore-free across the whole thickness of the coating layer. The coating is used commercially on valves in the oil and gas, food manufacturing and chemical industries and in cryogenics at liquid helium temperatures.


The coating also possesses excellent anti-galling properties. Because tungsten and tungsten carbide are both high melting-point materials (tungsten’s 6,128°F or 3,387°C melting temperature is the highest among metals) they will not micro-weld to another tungsten-based surface or other metals, eliminating the cause of galling. The coating will replicate the “as supplied” customer component surface finish and in the majority of cases no post-coat machining is required. The “as coated” surface finish provides a nodular but smooth surface texture. The coating also reduces the wear of other uncoated materials working in contact with the coating. For example, elastomeric seals last longer when working against the coating compared to the same seals working against stainless steel because of the uniform structure of the coating, its chemical inertness and low friction.


The coating is particularly suitable as wear-protection in applications requiring shock resistance. Testing is at an advanced stage for coated bushes for a drilling tool in such an application. Because the bushes, which bear a rotating pin, have to resist abrasion in drilling mud, they are currently made of cemented carbide. However, in the event of a shock load they shatter and cause tool failure. In lab tests carried out by a major oilfield services company, Hardide-coated Inconel bushes have proven “indestructible” when their abrasion and shock resistance is “hammer tested.” In this case, the ability of Hardide-T to coat internal surfaces is crucial as it is the inside of the bush that suffers from abrasion and wear, and this is where other coating technologies cannot be applied.


The coating is now in commercial application. It is processed and applied at bespoke coatings facilities in Oxfordshire, UK, and Houston, Texas.

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