High temperature oxidation of metal, alloy and cermet powders in HVOF spraying process

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Doctoral thesis (monograph)
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Helsinki University of Technology publications in materials science and metallurgy, 160
In thermal spraying, the coatings, alloys and metals oxidise during the spraying process. Oxidation often results in a decrease in the mechanical and corrosion properties of the coating. In this study, oxidation of HVOF coatings is examined. In the beginning of the study, the HVOF spraying process is analysed, including the combustion process, gas temperature, oxygen concentration, gas velocity, and gas momentum. Temperature and velocity of the spray particles are measured. Thermodynamic and kinetic equations of metal oxidation are introduced. In the experimental part of the work, calculations and coating experiments were carried out. Thermodynamic calculations of metal oxidation were done in the HVOF spray gun environment using different fuel/oxygen ratios, fuel quality and metals. The metal oxidation calculations were performed with HSC thermodynamic software. The thermodynamic calculations showed that Ni and Co can be sprayed without any oxidation in the HVOF gun nozzle whereas Cr, Ti, Al and Mg are always oxidised. No significant differences were found in the oxidizing potential of fuels such as H2 and kerosene. In the spraying experiments, oxidation of WC-Co17 and NiCr80/20 coatings was studied. It was shown experimentally that the short process dwell time does not enable coatings to oxidise completely. It was also found that spray powder oxidation occurs in three steps. First, primarily in the HVOF gun nozzle, second, to a small extent in the plume and third, hardly at all on the surface being coated. The thermodynamic calculations, reaction kinetics and experimental work showed that oxidation of spray powder is a complex phenomenon involving selective and volatile oxidation, and formation of solid oxide. Oxidation of the coating is strongly related to the temperature of the particles and combustion gas. Oxidation of the spray powder can take place in solid or molten state. Experimental work showed that the oxidation processes in the HVOF spray process can not be completely prevented, but can be controlled. The degree of oxidation is reduced by lowering the temperature of the flame, by using a higher gas velocity and density, and employing effective substrate cooling.
WC-Co17 coating, NiCr80/20 coating, oxidation, HVOF spraying, thermal spraying, oxidation kinetics, high temperature oxidation, coating oxidation, decarburization of WC-Co17
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