Tue, 08 June, 2021
Drilling is one of the major costs for geothermal and mining operations. During drilling, down-the-hole (DTH) hammers break the rock through repetitive impact and cuttings are removed via a high-velocity stream. The main failure mechanism of hammer parts was found to be erosion and wear. To protect drilling equipment, various coating technologies have been applied. The selection of appropriate coatings should increase drilling times, reduce repair times and minimise cost. High velocity oxygen fuel (HVOF) spraying is one of the most popular techniques to deposit thick coatings for drilling tools.
What is HVOF Spraying?
HVOF spraying was developed in the 1980s and is one of the most-applied thermal spray processes. It works by mixing fuel and oxygen, which is fed into a combustion chamber and ignited. The combustion of gases produces a high temperature (2500-3200°C) and pressure in the chamber, which is ejected through a nozzle at supersonic speeds (Figure 1). This velocity may reach values near 2000m/s (for HVOF) in contrast to about 100 m/s for flame spraying or 1000m/s for plasma spraying. This can result in dense and good adherent coatings. Thus, it is largely used for applying powder materials with a relatively low melting point and those that are subjected to thermal degradation at high temperature .
Materials and Applications
HVOF is primarily used to deposit cements, metals and alloy coatings, such as WC-CoCr, WC-Ni, CrC-NiCr, Ni alloys and steels. HVOF also demonstrated capability in depositing dense ceramic coatings such as hydroxyapatite, Al2O3, ZrO2, Cr2O3. Coating thickness is usually in the range 0.1-2mm and with porosity typically lower than 2.0 vol%. These coatings can be used across a range of engineering and manufacturing applications to enhance surface properties of components to enable cost-effective and high-performance approaches. This can include, but is not limited to, protecting surfaces against wear, friction and/corrosion, biocompatible layers, oxidation resistance, electrical insulation, and repair of damaged components.
A wide range of HVOF coatings, including both cermet coatings and alloy coatings, are deposited by TWI Ltd (Figure 2) to explore their possibility in protecting drilling components in Geo-Drill.
You can find out more about thermal spraying here.
The Geo-Drill Project is working to develop “holistic” drilling technologies that have the potential to drastically reduce the cost of drilling to large depths (5km or more) and at high temperatures (250ºC or more). This will increase the efficacy of existing technologies to help grow geothermal energy use while reducing the environmental impact and achieving cost-savings.
The Geo-Drill project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 815319
 MAGNANI, Marina et al . WC-CoCr coatings sprayed by high velocity oxygen-fuel (HVOF) flame on AA7050 aluminum alloy: electrochemical behavior in 3.5% NaCl solution. Mat. Res., São Carlos , v. 10, n. 4, p. 377-385, Dec. 2007.