Papers by Keyword: Coating Microstructure

Abstract: Permeability is important to ensure the protective properties of coatings based on styrene-acrylic dispersions. This indicator characterizes the complex of insulating properties of coatings, their ability to prevent the penetration of liquids, vapors and gases from the environment to the surface to be protected. It was studied the effect of aluminosilicate microspheres, which are characterized by the hydrophilic nature of the surface and highly dispersed silicate filler aerosil with a hydrophobised surface on the water absorption of styrene-acrylic coatings. Decreased of water absorption of styrene-acrylic coatings filled with aluminosilicate microspheres with the introduction of hydrophobised aerosil is linked to the fact that the fine aerosil with a high specific surface area provides the formation of a more densely packed structure. Thus, partially filling the interspherical space, which is formed by particles of microspheres with a diameter of 10-100 μm and reduces the surface defect of the styrene-acrylic coating. Localization on the surface of defective structures of particles of hydrophobised aerosil leads to a decrease in wetting of defective structures with water. Resulting deteriorating wetting the surface of the styrene-acrylic coating. Micrographs were taken to assess the nature of the distribution of aerosil on the surface of the styrene-acrylic coating. The analysis of the obtained micrographs confirms that the introduction of microspheres form large agglomerates, between which there are vacancies, which will negatively affect the technological and operational properties of the developed coatings. At the same time, the introduction of aerosil allows to obtain a more orderly structure, which allows to obtain a coating with lower internal stresses, increased aggregate stability and, as a consequence, with improved technological and operational properties.

Abstract: Thermal spraying is one of the most promising methods for obtaining thermal barrier coatings for aerospace applications. Providing the necessary set of coating properties and their stable reproducibility are an actual task of the modern production. The purpose of this work was to determine the influence of the initial powders fractional composition on the structure and properties of the protective coating. The microstructure and the elemental composition features of the heat-resistant and ceramic layers of the thermal barrier coating are investigated. It is shown that the microstructure, porosity and microhardness of the coating ceramic layer depend on the ZrO₂+8%Y₂O₃ initial powder fractional composition. The porosity of the coating and the average pore size increase with increasing particle size of the powder. The maximum value of the ceramic layer microhardness is observed when using a powder fraction of 40-80 μm. The studies have found that microstructure and the necessary combination of coating physical and mechanical properties are achieved during the deposition of zirconia powder fractions 40-80 microns.

Abstract: Al-Si coating in different hot dip process conditions were made by a hot-dip galvanizing simulator, and distribution regularities of chemical elements in coating were studied by means of GDS, SEM, EDS and XRD. The results show that the hot dip temperature has no obvious impact on elements distribution in coating, but has some impact on Si content in surface coating. The hot dip time has no obvious impact on surface coating element content, but has distinct impact on deep coating element content. With the hot dip time increasing, Al content decreases, Fe content increases, and Si content decreases. Al-Si coating is composed of 3 layers, surface layer contains fine and close Al2O3 film, which has good anti-oxidation property on high temperature and hot stamping property, middle layer contains high melting point phase ,such as rich Fe phase , FeAl3 ,which has excellent anti-oxidation property on high temperature. The elements in diffusion layer can even be transited to basic steel, so coating has good adhesion property.

Abstract: Simulation experiment was done to investigate the effects of rare earth on hot-dipped Zn-55%Al alloy coating. The results show that the rare earth has little effect on the zinc dross and its burning loss is about 10%. The microstructure of coating is similar to that of solidification bath, and which is made up of phases of rich aluminum, rich zinc, rich silicon and rare earth, and intermetallic layer of Al-Zn-Fe-Si. The rare earth phase is needle or rod, and mainly distributed inside rich zinc phase and on the interface between the coating and steel substrate. The rare earth has no obvious influence on coating grain and spangle size. The appropriate addition of rare earth would be helpful to improve the coating bending formability and corrosion resistance.

Abstract: In this article, the effects of galvanizing process parameters, especially cooling rate, on the microstructures of coating was investigated. The microstructures of Al-Zn-Si coating were analyzed by the following methods: surface microstructure, cross-section microstructure, thickness and composition of alloy layer by SEM and its accessories, distribution of each element along depth by glow-discharge emission spectrometer, micro-area elemental distribution image analysis of each element distribution on surface and cross-section by electron probe, crystallization orientation by X-ray diffraction instrument. The results showed that coating was made up of aluminum-rich dendrite, interdendritic zinc-rich phase and alloy layer, that cooling rate affected depth of alloy layer in Al-Zn-Si coating directly, that zinc concentrated in the surface layer of coating and silicon in the alloy layer, while the coating has larger depth it also concentrated in the surface layer, that aluminum-rich dendrite has preferred orientation in the solidification process.

Abstract: Effects of steel coatings on weldability in resistance spot welding of galvannealed steel sheets were investigated. The steels with different coatings, where Fe content changed from 7.0 wt% to 11.4 wt%, were welded by alternating current (AC) welding machine. The results showed that the weldability in RSW of the GA steel with 11.4 wt% Fe was the best in three researched steels. Higher Fe content in the coating resulted in increased contact resistance and hence reduced welding current needed and increased the range of weld current in RSW.

Abstract: In the paper, nanostructured, multimodal and conventional WC-12Co cermet coatings were sprayed by HVOF and the properties and structures of the coatings such as microhardness, microstructure, phase composition were compared. Finally sand solid and slurry erosion wear tests were carried out and their wear failure mechanisms were explored by XRD and SEM analysis. Research results show that microstructures of nanostructured and multimodal WC-12Co coatings prepared by HVOF are dense with little porosity, and their microhardness values are obviously higher than conventional WC-12Co coating. As well, it was found that nanostructured and multimodal WC-12Co coatings exhibited better sand solid and slurry erosion wear resistance in comparison with conventional coating and nanostructured WC-12Co coatings possessed the best sand solid erosion resistance properties at large impact angles and slurry erosion wear resistance. Testing results also show that although decarburization of WC occurred during spraying multimodal and nanostructured WC-12Co powders, the decarburization of WC for the nanostructured powder was more severe.

Abstract: A blended NiCr-Cr₂O₃-Ag-BaF₂/CaF₂ feedstock was sprayed using an atmospheric plasma spraying process. Due to the differences in physical and thermophysical properties of each constituent, coating microstructures and resulting coating properties were largely dependent on the interactions between each constituent phase and plasma jet in view of the physics of thermal spraying. Thus, hydrogen gas flow rate was changed to affect the plasma jet characteristics such as gas enthalpy and gas thermal conductivity in this study. According to it, evolutions of chemical composition and microstructures of the as-sprayed coatings were observed. As the hydrogen gas flow rate was increased, Cr₂O₃ weight fraction was increased with the decrease of Ag and porosity. Vickers microhardness and bond strength of the coatings showed consistent behaviors to the phase composition and microstructures. Friction coefficient and weight loss during a pin-on-disc dry sliding wear test were measured from room temperature to 500°C at the interval of 100°C. Finally, the effects of phase fraction and coating properties on the friction and wear were investigated.