FeW coatings with 4, 16 and 24 at.% of W were electrodeposited under galvanostatic conditions from a new environmental friendly Fe(III)-based glycolate-citrate bath. This work aims to find correlations between composition including the light elements, internal structure of the electrodeposited Fe-Walloys and functional properties of material. The obtained alloys were characterized by Glow Discharge Optical Emission Spectrometry (GD-OES), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD). Compositional depth profiles of 10 mm thick coatings obtained by GD-OES show that the distribution of metals is uniform along the entire film thickness, while SEM imaging depicted the presence of cracks and O-and W-rich areas inside the Fe-Wcoating with 4 at.% W. In the samples with 16 and 24 at.% of W, oxygen and hydrogen are present mostly at the surface about 1 mm from the top while traces of carbon are distributed within the entire coatings. With increasing W content, the structure of the coatings changes from nanocrystalline to amorphous which was shown by XRD and TEM analysis. Also, the surface of coatings becomes smoother and brighter, that was explained based on the local adsorption of intermediates containing iron and tungsten species. Annealing experiments coupled with XRD analysis show that the thermal stability of FeW alloys increases when the W content increases, i.e. the coating with 24 at.% W retains the amorphous structure up to 600 _C, where a partially recrystallized structure was observed. Upon recrystallization of the amorphous samples the following crystalline phases are formed: a-Fe, Fe2W, Fe3W3C, Fe6W6C, and FeWO4. Hence, the FeW coatings with higher W content (>25 at.%) can be considered as suitable material for high temperature applications. Fig. 4. Compositional depth profiles as obtained by GD-OES of FeW samples of different composition: 4 at.% of W (a), 16 at.% of W (b) and 25 at.% of W(c).
Among W alloys, FeW has seen much attention recently, due to the need of moving toward the design of environmentally friendly materials. Coatings with 4, 16 and 24 at.% of W were electrodeposited from an environmental friendly Fe(III)-based glycolate-citrate bath. The samples were annealed in vacuum at different temperatures up to 800 °C. Different crystalline phases are formed upon annealing: α-Fe, Fe2W, Fe3W3C, Fe6W6C, and FeWO4. Their grain size and distribution within the coating was studied by means of Electron Backscattered Diffraction (EBSD) technique. The effect of annealing on the mechanical properties of the coatings was analyzed performing nanoindentation measurements. The results show a considerable increase of the hardness followed by a rapid decrease at higher temperatures. The highest hardness value, i.e. 16.5 GPa, is measured for the sample with 24 at.% of W after annealing at 600 °C owing to the precipitation of α-Fe crystallites. This study indicates the possibility to substantially increase the hardness of electrodeposited FeW coatings by optimization of the annealing treatment. In addition, the critical influence of the carbide and oxide phases on the mechanical properties of alloys is discussed. Hence, FeW coatings rich in W can be applied as a possible candidate for protective coating applications at elevated temperatures.
The influence of the microstructural transformations upon heat treatments on the wear resistance of Fe-W coatings is studied. The coatings are electrodeposited from a glycolate-citrate plating bath with 24 at.% of W, and the wear resistance is investigated under dry friction conditions using ball-on-disc sliding tests. The samples were annealed in Ar atmosphere at different temperatures up to 800 °C. The microstructural transformations were studied by means of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Electron Backscattered Diffraction (EBSD) technique. Except for the coating annealed at 800 °C, all the tested coatings suffered severe tribo-oxidation which resulted in the formation of deep cracks, i.e., ~15 μm in depth, within the wear track. The precipitation of the secondary phases, i.e., Fe2W and FeWO4, on the surface of the sample annealed at 800 °C increased the resistance to tribo-oxidation leading to wear tracks with an average depth of ~3 μm. Hence, the Fe-W coating annealed at 800 °C was characterized with a higher wear resistance resulting in a wear rate comparable to electrodeposited hard chromium coatings, i.e., 3 and 4 × 10−6 mm3/N m, respectively.
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