| Among the fabrication techniques of ceramic coatings, sol-gel (SG) method has been the subject of extensive research studies over many decades worldwide as it provides a high-purity, low temperature synthesis and, especially a precise composition control. Meanwhile, the sol-gel method can overcome lots of the disadvantages of conventional techniques because it is simple, cost-effective and allows coating of complex geometry. The sol-gel method, however, is limited by two main disadvantages: 1) The precursors used in sol making are always aluminium alkoxides or aluminium salts. The aluminium alkoxides are expensive. The aluminium salts-derived sols contain a large quantity of impurity ions. Removing these impurity ions also increases the cost of the whole sol-gel processing. 2) SG technology often fails if the film is thicker than around 1um owing to the damaging shrinkage strains during drying process. Therefore, it is critical to prevent the cracking in the coatings and to improve the adhesivity between the coating and substrate.In our work, the problems as mentioned above are approached by usingcommercial boehmite powder as precursor and applying composite sol-gel (CSG) technology. The well-dispersed slurry used for coating (>10um) is made by adding micro-TiB2 particles into stabilized, active boehmite sols. The adhesivity between coating and metallic substrate is enhanced through sealing treatment with phosphoric acid, and the mechanism of chemical bonding involved is studied. Besides, the mechanisms of sol-drived TiB2 coating on porous carbon anode are preliminarily examined.The boehmite sol is made of commercial boehmite powder, using HNO3 and H3P04 as peptizing acids. The influences of solid concentration, pH value, temperature and other parameters on the viscosity, stability and soling time of the boehmite sols, are investigated. The experimental results show that a stable boehmite sol can be successfully made only by adding HNO3 until pH<4. On the contrary, only suspensions could be obtained under every pH values when H3P04 is used as peptizing agent. The viscosity and stability of the HN03-peptized boehmite sol increase with the decrease of the pH value and the increase of solid concentration and temperature.The peptization mechanisms of the boehmite powders are experimentally studied. In the case of HN03, since the electronegativity of 0 is higher than that of N, the electron density is not uniformly distributed over the nitrate ions. As a result, the - N(V ions are firstly adsorbed on the colloidal particles surface with positive charges, and form the adsorption layer and the diffusion layer. The repulsive force make the colloidal particles separated from one another. In comparison, H3PO4 is unlikely to break the A1-O-A1 bonds among the boehmite particles.TEM and FTIR technology are used to characterize the structures of the boehmite colloidal particles. TEM images show that the boehmite colloidal particles are of around lOnm and form a moniliform structure. Fourier transform infrared (FTIR) analysis reveals that some of N03- ions has reacted with A100H, forming A1-N03 bond, while the rest exists in the forms of dissociated ions.One important way to prevent the cracking in coating is through CSG technology. Namely, a nano-micro (0-3) slurry is obtained by dispersingmicro TiB2 particles into nano-scaled boehmite particulate sol. The displacement of intercalated water in sols by the added ceramic particles prevents the cracks in the coating. The viscosity and stability of the slurry were experimentally measured. The interactions between TiB2 particles and the surrounding colloid are supposed to be chemical adsorption, chemical reaction and penetration.The stainless steel substrates were coated by spraying using conventional pressurized air spray gun, then sealed with phosphoric acid solution, and finally heat treated. The adhesive strength between coating and substrate is obviously improved by in-situ chemical reactions. SEM results of sealed coatings show that the phosphoric acid play... |
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