| Amorphous alloys are a new type of metallic materials with many potentialapplications due to the combianation of a set of unqique properties, including highstrength and hardness, good elasticity, excellent wear-and corrosion-resistance. However,amorphous alloys suffer from the limited product size and poor room-temperatureplasticity due to the disordering of atomic structure. Fabrication of amorphous coating isthought to be an allernate route to overcome the shortcomings of bulk amorphous alloys.The aim of this dissertation is therefore to develop high performance amorphous coatingsbased on the Fe-based amorphous system by thermal spray technique. The preparation,microstructure and properties, including corrosion resistance, friction/wear properties andwetting behavior, of Fe-based amorphous coatings have been systematically investigatedby means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) anddifferential thermal analysis (DTA), optical microscopy (OM), scanning electronmicroscopy (SEM), transmission electron microscopy (TEM), nanoindentation,micro-hardness measurement, friction/wear testing system and electrochemicalworkstation.Fe48Cr15Mo14C15B6Y2(nominal compostion) amorhous coatings was successfullyfabricated on mild steel substrate by the high velocity oxy-fuel thermal spraying (HVOF)process. The effect of the size of feedstock powders on the microstructure and corrosionresistance of the coatings was investigated. The results showed that the coatings are highlydensed with the porosity of less than1%, and the coatings are nearly fully amorphousexcept a few nano-oxdies embedding in the intersplat regions. All coatings exhibit goodcorrosion resistance in3.5%NaCl solution with wide spontaneous passive rejion andrather low passive current density. However, the particle size of the feedstock powders hadsignificant influence on the microstrture and the corrosion proterties of the resultantcoatings. The coatings sprayed with the finest powders show the most compact structure;while the coating with the coarser powders exhibits a better corrosion resistance. It wasalso found that the corrosion resistance of the coatings is closely related to the wetting behavior which is affected by the oxygen content and the roughness of coatings. Thecoatings with hydrophobicity exhibit a better corrosion resistance.To understand the corrosion mechasim of the amorphous coating, the pitting initiationof the coating in a6M NaCl solution was studied in detail via high resolution transmissionelectron microscopy (TEM) coupled with nanobeam energy-dispersive X-ray spectroscopy(EDX). It was found that pitting was always initiated in a narrow region about100nmwide near the intersplat regions, but not exactly at the regions as have been expected.Nanobeam EDX indicated that a Cr-depleted zone exists near the intersplat due to theoxidation effect. More interestingly, pitting was found to occur only on one side althoughCr depletion is equal on the two sides of the intersplat. This can be well explained in termsof the galvanic effect between the Cr-depleted zone and the Cr-rich intersplat regions.The wear behavior of the amorphous coatings was studied under dry slidingconditions in a ball-on-plate mode using alumina ball as the counterpart. It was found thatthe friction coefficient and the wear rate of the coating are around0.3-0.4, and (3~19)×10-5mm3N-1m-1, respectively. Compared with traditional steels and other wear-resistantcoatings, such as hard Cr and Al2O3coatings, the Fe-based amorphous coating showshigher wear resistance. The wear rate is independent of the applied load, but increaseslinearly with the increase of sliding speed. The wear mechanism of the Fe-basedamorphous coating was found to be dominated by oxidative wear due to relatively highflash temperature and large oxygen affinity of alloy elements. Further analysis revealedthat the oxidation process is governed by inward diffusion of oxygen. In addition, thedelamination wear also occurred at the intersplat regions during wear process.By carefully designing the spraying parameters and the powders sizes, a variety ofFe-based amorphous metallic coatings with different surface topographies and roughnesscould be fabricated via thermal spraying. The results showed that the wetiing behavior ofthe coating could be altered by surface rougness, the coating behaves hydrophilic when theroughness is less than5μm，while becomes hydrophobic when the roughness is larger than9μm. The largest static water contact angle can be reach as high as130°~140°in the study.Further modification with low surface energy treatment, i.e, supptering a nanostructuredAu layer+splating a dodecanethiol film, results in the realization of superhydrophobicity of the coating, which shows the contact angle greater than150°and the sliding angle lessthan16°(for a8μL water droplet). The superhydrophobic coating exhibits excellentself-cleaning effect. Finally, the roughness effect on the wetting behavior of theamorphous coatings was discussed in terms of Wenzel and Cassie-Baxter theory. |
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