Abstract
In this dissertation, the optimum processes used for fabricating Fe-based amorphous coatings by HVAF and HVOF have been determined. The structure, corrosion resistance,friction and wear behavior, bonding strength and thermal shock resistance of the two coatings were comparatively investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), oxygen determinator, thermomechanical analyzer (TMA),microhardness instrument, mechanical testing system, electrochemical workstation and friction and wear tester.
Compact and dense Fe-based amorphous coatings with the same porosity and thickness were successfully prepared by HVAF and HVOF. XRD results show that they are partially amorphous, but the crystalline peaks in the coatings are notably weaker than those in the powders. Although HVAF and HVOF coatings have the same porosity, their micro-components and microstrucure are distinctly different, i.e., the oxygen content in HVAF coating is significantly lower than that in HVOF coating; quite a few dark contours exist at the interfaces between the partially melted particles in HVOF coating,which were only occasionally detected in HVAF coating; EDS indicates that the dark contours mainly contain Fe-oxides. Additionly, HVAF coating has a higher hardness than HVOF coating.
Electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis indicate that the HVAF coating has better corrosion resistance than the HVOF coating, which can be attributed to the lower oxygen content in HVAF coating.These metal oxides are mainly located between the partially melted particles, which may hinder the formation of dense passive film on the one hand, and even become the diffusion channels for electrolyte to cause inner corrosion on the other hand.Electrochemical polarization tests indicate that HVAF coating has good corrosion resistance in simulated groundwater, which means that it can be used for protecting the spent nuclear fuel container.
Dry sliding wear tests show that the HVAF coating has a lower lower friction coefficient than HVOF coating because the former can easily flake and cause fatigue wear due to its higher hardness and weak ability to resistant crack propagation while the latter has a higher oxygen content which can decrease the friction coefficient.Nevertheless, the wear weight loss of the two coatings is similar and much lower than that of the substrate, indicative of a good protective ability. Consequently, HVAF and HVOF coatings can be used as nonskid and anti-friction coatings, respectively. With the increase of the load, the friction coefficient and wear weight loss of HVOF coating increased slightly but maintain a low level. Additionally, HVOF coating has a good anti-friction property against metal and ceramic.
Tensile tests show that the bonding strength of HVAF coating is very high, and fracture morphology indicates that it is mechanical bond between the coating and substrate. Thermal shock resistance tests show that HVAF coating has good thermal shock resistance in seawater and air conditions, which can be attributed to the matched thermal expansion coefficient between HVAF coating and substrate.As a new coating with good properties and relatively low cost, HVAF Fe-based amorphous coating can be made for industrial applications, especially in the areas of marine and the storage of spent nuclear fuel.
Keywords: HVAF HVOF Fe-based amorphous coating Polarization curve Electrochemical impedance spectroscopy Friction and wear resistance Bonding strength Thermal shock resistance
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