Preparation And Properties Of Functional Composite Coatings Embedded ZrO₂ Ceramic Particles

Composite coating is a new kind of composite materials with special function, which has excellent characteristics of the metal matrix as well as special features scattered particles. In this paper, Ni-P-ZrO₂ microcomposite coatings and Ni-ZrO₂ nanocomposite coatings were prepared on low carbon steel (Q235) substrates by electrolessdeposition and electroplating. The effects of ZrO₂ concentration in bath and stirring rate on properties of composite coatings were studied. And the optimal ZrO₂ concentration in solution was fixed. The surface and cross-sectional micrographs of the composite coatings were observed with optical microscope and scanning electron microscopy (SEM). And the chemical composition and phase transformation of composite coatings were analyzed with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). It was shown that a relatively uniform concentration of ZrO₂ particles along the cross section of electroless Ni-P-ZrO₂ composite coatings, and the continuous and uniform composite coatings with about 55μm of thickness showed good adhere to substrate. The structure of Ni-P-ZrO₂ composite coatings as deposited was amorphous. When the ZrO₂ concentration in solution ascend to 9g/L, particles content in coating and deposition rate reached the maximum, 14.3wt.% and 21.2μm/h, respectively. The presence of ZrO₂ particles can enhance significantly microhardness,corrosion resistance and wear resistance performances of Ni-P alloy. After heat-treated, the matrix of composite coatings crystallized into nickel crystal and nickel phosphide(Ni₃P), which result in an increase in the coating hardness, with a maximum hardness and excellent tribological behavior being achieved after annealed at 400°C.The effects of different kinds of dispersants on nano-ZrO₂ composite electroplating bath as well as effects of stirring means in plating process on dispersion of nano-ZrO₂ in composite coatings were studied. It was shown that the ZrO₂ suspension which was added to the dispersant 752W with complex structure had better dispersion and stability. The mechanical stirring+intermittent ultrasonic had obvious stirring scatteredrole,strengthen and promoting role and refining the nickel grains. The oxidation resistance of the coatings was discussed by their weight gains in a muffle furnace. It was shown that the products of nanocomposite coatings consist of NiO with trait of small plane after high temperature oxidation over 800°C. These NiO which linked to each other to form a dense oxide film played a very good protection to the substrate. The reason why the Ni-ZrO₂ nano-composite coatings can improve the high temperature oxidation resistance maybe that the rate-controlling Ni²⁺ diffusion along short-circuit paths is blocked by embedded nano-ZrO₂ particles that originally existed in the Ni-ZrO₂ nanocomposite coatings, which exerted a reactive-element effect on the growth of NiO scales on the composite coatings. What is more, the disperation of ZrO₂ particles in the Ni-P matrix reduces the effective area of Ni alloy contacting with ambient oxygen, thus significantly decreasing weight grains of the composite coating. In addition, oxide layer grain size of nanocomposite coatings was smaller as compared with pure nickel coating, which can greatly enhance the deformation rate. During the circulating oxidation process, oxide layer of nanocomposite coatings can be released by the diffusion creep thermal stress. The growth of NiO layer was formed through the O₂^- inner diffusion (oxidation initially may be caused by some O₂^-) from the surface to the internal nanocomposite coatings.