Preparation And Properties Investigation Of SiO₂ Sealing Coating Prepared By Sol-Gel Process

In this paper, SiO2 sol sealing agent was developed by hydrolyzing of organic alkoxide, the dipping-costing method was employed to prepare the SiO2 sealing coating onto thermal spraying coating and microarc oxidation (MAO) coating of magnesium alloy. The effect of sealing coating on the soundness and the surface smoothness of the porous ceramic coating was analyzed, and the corrosion resistance and oxidation resistance at elevated temperature of the thermal spraying coating and MAO coating of magnesium alloy treated by sealing were investigated. Some conclusions can be obtained.The transparent and homogeneous SiO2 sol was prepared from tetraethoxysilane (TEOS) by sol-gel method with ethyl alcohol as solvent and hydrochloric acid as catalyst. When SiO2 sol has exposed on the air for 200 minutes, its viscosity increases remarkably, it begins to form SiO2 gel. The result of the Fourier transformed infrared spectrometry (FTIR) analysis shows that SiO2 sol contains dilation flutter absorption peak of Si-O-Si in image of infrared which didn't change with the temperature, so we can obtain that solution exists SiO2 particle and the dried gel has a great stability in high temperature. X-ray diffraction patterns indicated that the microstructure of SiO2 dried gel is similar to the molten silica, shows amorphous of short-range order which is from 3nm to 8nm. The micro-particle size of dried gel treated at different temperature almost has no change.A sound SiO2 sealing coating can be obtained for atmospheric plasma spraying (APS) coating sealed by SiO2 sol, the soundness and surface smoothness of APS coating is evidently improved. The corrosion loss for the sealed APS coating reduces from 1.561mg/cm2 to 0.662mg/cm2. It shows that the sulfur acid corrosion resistance of coating is improved by sealing. At the same time, the oxidation resistance at high temperature is improved and the substrate of magnesium alloy was effectively protected by sealing. The sealing coating has excellent adhesion onto APS coating when the specimen is dipped in 750℃aluminum molten. The MAO coating was sealed by SiO2 sol. It can be seen that the SiO2 sol immerses into and fills up the hole of MAO coating through the observation and composition analysis of the cross-section of coating. The corrosion result shows that the corrosion rate of MAO coating sealed by SiO2 sol reduces from 106mg/m2h (MAO unsealed) to 32mg/m2h. On the other hand, the anti-oxidizing performance of sealed MAO coating at 410℃is greatly improved. Therefore, the coating sealed by SiO2 sol offers the effective protection for magnesium alloy substrate.