| Surface states of materials have great impacts on their properties. Surface abrasion and corrosion of steel components due to long-term exposure to working environment could seriously affect the service life. Fabrication of protective ceramic coatings is an effective way to improve the properties of metal matrix and to extend the application of ceramic materials. Metal-based ceramic coatings could make up for the deficiency of metal materials by effectively improving the surface properties. Al2O3ceramic is recognized to be ideal material of wear-resistant coating as its excellent performance in friction and wear testings. The great discrepancy of thermophysical properties between metal matrix and ceramic coating could be relieved by the addition of a transition layer. Fe-Al intermetallic compound is a potentially good material for the transition layer because of its in-between physical properties, but its poor ductility at room temperature and the difficulty of its molding are the main factors that restrict its scope of application. This paper made use of plasma spraying to produce intermetallic compound by in-situ reaction of Fe-Al core-shell composite powders so as to achieve the transition from metal matrix to ceramic coating.Q235steel was used as the matrix, and Fe-Al/Al2O3gradient coatings were fabricated by plasma spraying. Testing methods such as SEM and XRD were employed to analyse the microstructure and phase constitutions of the coatings, and experiments were performed to test the microhardness, bond strength, porosity and thermal shock resistance properties. It was noticed that it was an effective way of prolonging the service life of coatings to achieve componential and structural gradient between metal matrix and ceramic coating through comparative analysis of coatings in variation of constituents and structures.The precursor for the transition layer were prepared through electroless plating in which the Fe/Al core-shell powder was prepared. In this reaction, FeSC>4was reduced by sodium phosphite to deposite on the surface of A1powder to form the core-shell structure. Through the molulation of activation methods of A1powder, pH of the solution, plating time and the complex, the optimized parameters were found. As the results indicated, the removal of the oxide surface could not only increase the catalytic activity of the electroless plating but also decrease its influence on the nucleation of iron. In addition, the introduction of complexes could prevent the precipitation of Fe2+thus promoting the stability of the plating. As a result, the duration could be enlonged and the precipitation effeciency could be increase effectively. Through the two-step plating, both the thickness of the plating and the content of iron in the composite could be increased thus increasing the bonding strength between the plating and the matrix. The optimized parameters for electroless plating were listed as follow. pH9.5～10, temperature75～80℃, durating40min.The Fe-Al/Al2O3gradient coating was prepared through plasma spraying by altering the ralative proportion of Fe/Al composite powder to Al2O3. The DC, TC and GC gradient coatings were prepared and characterized by XRD and TEM. The microhardness, bonding strength and thermal shock resistance of the gradient coatings were tested. As the results indicated, the refining of the gradient coating could release the differences between the coating and the matrix thus promoting the stability of the coating. In addition, the introduction of TiO2into the coating could decrease the abrasion loss through tuning the abrasion resistance and friction coefficient of the coating. |
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