iTSSe TSS ADVANCED MATERIALS & PROCESSES | APRIL 2024 33 iTSSe TSS FEATURE High-velocity oxygen fuel (HVOF) is a thermal spray coating technology that was developed in the late 1970s and early 1980s. It was initially created as an advanced coating solution for aerospace applications, specifically to improve the durability and performance of aircraft components. Traditional applications primarily focus on wear and corrosion protection by applying tungsten carbide, chromium carbide, and metallic coatings. Kerosene and hydrogen are two common HVOF fuels but other fuels such as propylene, propane, ethylene, natural gas, and ethanol are also used based on availability and cost. A list of standard HVOF torches and common parameters is provided in Table 1. NEW DEVELOPMENTS IN HVOF THERMAL SPRAY Recent advances in high velocity oxygen fuel torches in the form of miniaturization, joining, automation, and digitization are increasing the application opportunities for this thermal spray coating technology. Alan Burgess,* Spraywerx Technologies Inc., North Vancouver, Canada Andreas Wank, GTV Verschleiss-Schutz GmbH, Luckenbach, Germany The traditional HVOF process is characterized by large spray distances, typically 200-400 mm. Parameters are varied by adjusting the oxygen-fuel ratio, λ. λ is defined as: where (O/F) is the operating oxygen/fuel ratio and (O/F)st is the stoichiometric oxygen/fuel ratio. For λ < 1 combustion is fuel-rich and for λ > 1 it is oxidizing fuel-lean. Adjusting the λ value by varying the oxygen flow, flame, and inflight powders therefore also affects coating microstructures. Advances in HVOF are achieved by adjusting the flow and combustion state of the HVOF flame by alternative means and miniaturizing the process to allow for short spray distances. This in turn allows for internal surface HVOF spraying. These advances are exemplified with the Spraywerx ID-Nova and the GTV K2 technologies, respectively. ID-NOVA HVOF FOR INTERNAL SURFACES There is an increasing demand for HVOF coatings for internal surfaces and hard to reach areas that are not accessible with traditional HVOF torches due to their size and long spray distances. The torch needs to access the area to be coated through miniaturization and it must be able to produce high quality coatings, which *Member of ASM International 4 TABLE 1 — HVOF TORCH PARAMETERS AND POWER LEVELS Fuel type Fuel flow rate, l/h Oxygen flow rate, nlpm Power, kW λ Liquid fuel K2 Kerosene 26.0 900 253 1.00 JP5000 Kerosene 20.8 883 202 1.23 Gaseous fuel TopGunGas Propane 50 215 72 0.86 TopGunGas Hydrogen 432 182 78 0.84 TopGunGas Acetylene 75 150 70 0.80 Jet Kote Propane 50 300 72 1.20 Jet Kote Hydrogen 432 182 78 0.84 DJH 8 Hydrogen 633 285 114 0.90 DJH 9 Propylene 77 330 117 0.96 ID-Nova Hydrogen 500 250 90 1.00 Mini-Nova Hydrogen 350 175 63 1.00
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