Electrodeposition And Properties Of Ti₃SiC₂and Ni/Ti₃SiC₂

Composite coatings obtained by incorporating ceramic particles into metal matrix can give excellent properties. Ti3SiC2is the most typical material in MAX pahses, which exhibits high stiffness, easy machinability, damage tolerance, thermal shock resistance, good high-temperature oxidation resistance, as well as relatively low hardness and shear strength. It is thus expected that the incorporation of TisSiC2particles into Ni matrix can improve properties of Ni-based composite coatings, and expand their actual and potential applications of Ni-based composites at high-temperature. Moreover, compared with SiC, the coefficient of thermal expansion of Ti3SiC2(9.2×10-6K-1) is close to that of Ni resulting in low residual stress in the composites. Because diffusion/reaction between Ni matrix and Ti3SiC2may occur at high temperature, high-temperature pyrometallurgy or powder metallurgy is not good method to prepare Ni/Ti3SiC2composite coatings. Thus, low temperature process such as electrodeposition is needed. Ti3SiC2possesses good electrical conductivity (4.5×106Ω-1·m-1) so that Ni can directly deposit on the surface of particles and Ti3SiC2particles are easily embedded into the composite coating. Related work is summarized on the following points:1. Ni/Ti3SiC2composite coatings were prepared by electrodeposition from nickel Watts bath containing different amount of Ti3SiC2particles. The coatings were characterized by X-ray diffraction, laser confocal microscopy and scanning electron microscopy. The effect of deposition current and Ti3SiC2concentration in the solution on the composition, grain size, preferred orientation and surface morphology of the electrodeposited coatings were investigated. It was found that (200)-oriented Ni coatings could be deposited at high deposition current (62.5mA), while (220)-oriented Ni coatings could be obtained at low current (25mA). However, the presentence of Ti3SiC2particles disturbed the surface texture of Ni crystallites in the composite coatings. The crystallite shape of Ni was determined by the relative growth rates on {111},{200} and{220} faces according to the simulation using a morphology code in the Cerius24.2computational program.2. Microhardness and friction property of pure Ni coating and Ni/Ti3SiC2composite coatings were measured on Dynamic Ultra-micro Hardness Tester and UMT-2microtribometer through scratch tests, respectively. Ni/Ti3SiC2composite coating has higher microhardness than pure Ni coating, which is attributed to the grain-fining and dispersive strengthening effects of the deposited Ti3SiC2particles. Ti3SiC2particle size, which influences the surface roughness of coatings, also has significant effect on the friction. Ni/Ti3SiC2composite coating has good lubrication.3. Corrosion resistance is an important property of coatings. The electrochemical corrosion behaviors of pure Ni coating and Ni/Ti3SiC2composite coating in3.5%NaCl solution were investigated by means of electrochemical measurements such as open circuit potential, polarization curves and electrochemical impedance spectroscopy combined with scanning electron microscopy. Corrosion mechanisms of pure Ni coating and Ni/Ti3SiC2composite coating in3.5%NaCl solution were preliminarily discussed. For pure Ni coating, corrosion medium filters into the coating through fresh corrosion pore resulting in further damage. Corrosion reaction for Ni/Ti3SiC2composite coating in3.5%NaCl solution is controlled by diffusion process and just occurs in the thin area without great damage.Synthesis, properties and applications of bulk Ti3SiC2have been widely investigated. It is also meaningful to prepare Ti3SiC2thin film or coating. Electrophoretic deposition (EPD) is a feasible method to prepare ceramic film or coating at room temperature. It can avoid disadvantages of high temperature methods, such as diffusion reactions and decomposition of MAX phases. The economical and environmentally benign benefits of using water to prepare electrolyte solution have encouraged researchers to consider aqueous medium for EPD. Work about Ti3SiC2film electrodeposition is summarized on the following points:1. The influence of pH and dispersant polyacrylic acid (PAA) on the Ti3SiC2suspension was studied including measured particle size, Zeta potential and ion conductivity. It is shown that Ti3SiC2particles can be well dispersed in the wide pH range for the suspension with PAA. However, there is no obvious improvement on Zeta potential through adding the dispersant PAA in the suspension. Ion conductivity of the suspension was increased upon PAA adding, which may have no positive effect on the electrophoretic deposition of Ti3SiC2.2. Ti3SiC2films were electrophoretically deposited from the suspensions with PAA in different contents at pH7. SEM surface micromorphology showed that less Ti3SiC2 was electrophoretically deposited on the substrates and particles were inhomogeneously distributed in the films.3. The mechanisms of surface charge and stabilization for Ti3SiC2particles in the aqueous suspension without PAA were investigated. Ti3SiC2films were electrophoretically deposited at3V on indium-tin-oxide (ITO) conductive glass from Ti3SiC2aqueous suspension with1vol.%solid loading at pH9. The surface morphology, cross section microstructure and preferred orientation of the films were investigated by scanning electron microscopy and X-ray diffraction. The as-deposited Ti3SiC2films are dense with (00ll) preferred orientation. Thick and crack-free Ti3SiC2film was obtained by three deposition-drying-deposition cycles.