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RESEARCH PRODUCT

Optimization and Improvement of a Novel HVOF Process Fueled with Ethanol

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The high velocity oxy-fuel (HVOF) spray has become a process of choice for producing high performance cermet or alloy coatings. Common HVOF thermal spray systems classically use the combustion of gases, such as hydrogen, propane or a liquid fuel such as kerosene. However, there is currently a limited amount of literature on the use of ethanol as a fuel within HVOF processes and the performances of the resulting coatings are not well documented. Ethanol benefits from environmentally friendly features and is less polluting compared to conventional fossil liquid fuels (i.e. kerosene) as its combustion generates less nitrogen oxides and soot particles. In this work, we decide to investigate such an ethanol-fueled HVOF device (called “eGun HVOF”), with the aim to define the merits and limitations of that technology. In addition, we managed some design modifications of the eGun device in view of improving the quality of the coatings.At first, commercial WC-10Co-4Cr powder was sprayed using the eGun HVOF process. Investigations were carried out to determine the influence of different oxygen/fuel ratios on the evolution of the velocity and temperature of in-flight particles in correlation with the properties of the resulting coatings. The variation of the ethanol flow rate appeared to have a greater influence on the velocity and temperature of the particles than that of oxygen. We elaborate detailed correlations between particle parameters and coating properties to deduce spray parameters providing the best performing coatings.Next, we used the eGun to prepare Cr3C2–25wt.%NiCr coatings on a 304 stainless steel substrate. The Taguchi method was employed to adjust the spray parameters providing the best erosion resistance at 90° impact angle. We obtained then the optimal spray parameters (OSP) for minimum erosion wear. And the porosity, fracture toughness, and bonding strength of the coatings prepared with the new eGun HVOF are comparable to those obtained with the conventional HVOF devices (DJH2700, JP5000, K2). Interestingly, the microhardness proved much better. The erosion wear testing of the “optimized coating” was conducted at 30°, 60° and 90° impact angle. It turned out that the erosion rate rises with the increase of the impact angle. In addition, we performed investigations to determine the influence of different stoichiometric conditions, on the coating microstructure and in relation with the resulting coating properties.In the last part of the thesis, we conducted some developments intended to further improve the capabilities of the eGun, which we used this time as an HVOAF system (i.e. with air added as a milder oxidizer). To be able to feed the eGun torch with compressed air, it was decided to design and install a second-stage combustion chamber. NiCoCrAlYTa coatings were prepared using this modified, ethanol-fueled HVOAF device. Investigations were conducted to determine the influence of different flow rates of compressed air on the microstructures and the properties of the resulting NiCoCrAlYTa coatings. We elaborated a detailed correlation between the compressed air flow rate and the coating properties to identify the conditions securing the best performing coatings.Keywords: HVOF; ethanol; erosion resistance; HVOAF; NiCoCrAlYTa; oxidation; sliding wear resistance