Reasearch On Production And Mechanism Of ZrSiO₄/SiC Coating On C/C Composites

Carbon/carbon （C/C） composites have many merits such as hightemperature mechanical properties, light weight, excellent heat conductivity,good wear resistance as well as thermal stability, which make them applicablesuggestions are very applications and so on. However, C/C composites are proneto oxidation in ambient environment with the temperature in above400℃,which limits their application. Therefore, it is important to enhance oxidationresistance at high temperature. Research shows that an oxidation resistantcoating is a reasonable choice for protecting C/C composites at high temperature.However the traditional technologies, such as chemical vapor deposition,plasma or thermal spraying, sol-gel and pack cementation, for the preparation ofmultilayer coatings are usually carried out at high temperature or involvedpost-high-temperature heat-treatment. These methods will result in thegeneration of micro-cracks in the multilayer coatings due to the mismatch ofthermal expansion coefficient between SiC bonding layer and the outer coating.For the application of coatings, hydrothermal electrophoretic deposition is anovel method which works at low temperature. In the present work, a ZrSiO₄coating was prepared by the hydrothermal electrophoretic deposition process onthe surface of pack cementation SiC bonding layer. The phase compositions andmicrostructures of the coated C/C composites were characterized by XRD, SEMand EDS analyses. The influence of the technological factors on the phasecomposition, microstructure and oxidation resistance of the coating wasinvestigated. And the anti-oxidation properties, oxidation mechanism and failuremechanism of the coating were investigated and discussed. The main contentsand conclusions are as follows:The SiC coating was prepared by a pack cementation process with Si, C,SiC and Al₂O₃powders. The results showed that the bonding coating prepared bypack cementation is mainly contain of Si, α-SiC and β-SiC phases. There aresome micro-cracks on the surface. The antioxidant capacity of SiC coating ofC/C composites significantly improved. The SiC coating can protect C/C composites from oxidation at1773K for56h with a mass loss rate of only1.32%. The failure of the coating is due to the formation of the holes in thecoating surface, which is generated by the escaping of SiO and CO.The ZrSiO₄outer layer was prepared on SiC pre-coated C/C composites bya hydrothermal electrophoretic deposition. The stability of ZrSiO₄suspension isthe best in the isopropyl alcohol. The influences of the ZrSiO₄coatings werestudied, such as the I₂content, deposition voltage, hydrothermal temperature anddeposition time on the phase composition, microstructure and high temperatureoxidation resistance. The optimization technological conditions for preparing theZrSiO₄coating were the I₂0.8g/L, deposition voltage200V, hydrothermaltemperature373K and the deposition time9min. The deposition activationenergy of the ZrSiO₄coating is calculated to be25.45kJ/mol duringhydrothermal electrophoretic process. The ZrSiO₄/SiC coating displays excellentoxidation resistance and can protect C/C composites from1573～1773K for380h. The mechanical properties of the specimens are decline in26.05%. The law ofoxidation activation energy increases at first and then decreases during theoxidation time. The ZrSiO₄/SiC coatings have good thermal shock resistancebetween the room temperature to1773K. The failure of the coating between1773K and room temperature is due to the generation of cross-cracks in the coatingduring the test. At the initial oxidation is controlled by the diffusion rate ofoxygen in the coating defects of ZrSiO₄. The mid-oxidation is controlled by thediffusion rate of oxygen in the phase of ZrSiO₄and SiO₂. At last, the failure ofthe coating is due to the formation of penetrative holes between the SiC bondinglayer and the C/C matrix during the oxidation period.