Study On Structure And Properties Of The Fe/Al₂ O₃ Gradient Coatings Composite

The metal-based ceramic coating is a kind of protective layer or surface film coated on the surface of metals. But the great difference of physical properties between ceramics and metals and the brittleness of ceramic materials in essential attributes can result in low adhesion strength between coatings and substrates, and restricted seriously the extensive application of coating materials. Functional gradient material is a kind of non-homogeneous composite. The gradient change between composition and structure from one side to another side can alleviate the thermal stress because of thermal expansion caused mismatch in the materials. Based upon the preparation method and design concept of functional gradient material, the composite of Fe/Al₂O₃ gradient coatings on steel substrate was prepared. The composition is changing continuously in the direction of coatings thickness. The internal stress can be alleviated and combined properties of coatings can be improved.The sintering technology of Fe/Al₂O₃ composites was studied in this text. The Fe/Al₂O₃ composites were prepared by sintering buried in the graphite. The influence of different mounding pressure, percent of sintering assistant CMS, sintering temperature and holding time on the properties of Fe/Al₂O₃ composites were researched through designing orthogonal experiment. The results show that the properties of Fe/Al₂O₃ composites reach the best when composites are sintered at 1450℃for 90min with the pressure of 125 MPa and CMS percent of 4%.The properties of Fe/Al₂O₃ composites with different constituents and the effect of Ni on the properties of Fe/Al₂O₃ composites were studies. The bending strength and fracture toughness of Fe/Al₂O₃ composites increased with Al₂O₃ percent, and then decreased. The bending strength and fracture toughness of Fe/Al₂O₃composites reach respectively 602.49MPa and 9.33MPa·m1/2 when alumina content is 70wt%. The more alumina content is, the better is corrosion resistance of Fe/Al₂O₃ composites. The toughening way of Fe/Al₂O₃composites mainly depend on shell effect, crack deflection and bending and ductility particles. The cracks were developed from line to face and body. The composites were toughened by increased the number of cracks. A kind of shell formed Around Fe particles in the sintering process. The shell was broken firstly when Fe/Al₂O₃ composites were applied to external stress; Then crack bending took please along the micro- crack between the shell and Fe particle. The composites were toughened by shell broken and crack bend. Introduction of Ni resulted in alumina ceramic grain refined and the compactness of composites declined. So mechanical property of composites decreased, but the corrosion resistance of Fe/Al₂O₃ composites were improved greatly by introduction of Ni. The Fe/Al₂O₃ gradient coatings were prepared by spraying on Q235 steel substrate. The effect of sintering temperature on the adhesive strength of Fe/Al₂O₃graded coatings was researched. The adhesive strength increased and then decreased with sintering temperature increased. The adhesion strength of Fe/Al₂O₃ gradient coatings reaches 25.3MPa when the sintering temperature is 1220℃. The organizational distribution of coatings is uniform, similar to the branches intertwined.The composition of coatings changed continuously, internal stress was alleviated availably. The breakage of gradient coatings is neither peeled regularly by each layer, nor in the gradient coatings or steel substrate respectively. But the breakage of coatings happened in the transitional layer that formed by diffusion and chemical combination nearby the interface of the coatings and steel substrate. The coatings are mainly composed ofα-Al₂O₃, AlFeO₃ and NiFe2O4. The bonding of Fe/Al₂O₃ gradient coatings and steel substrate mainly depends on adsorption, diffusion and chemical combination.