The Regulation Of Microstructure And Thermal Shock Resistance Of Ultra-high Temperature Zircrium Diboride Based Ceramic Composites

The central problem of low thermal shock properties for ZrB₂ based ceramic composites restricts its widespread applications at the extreme thermal and large gradient environments associated with hypersonic flight, atmospheric re-entry and rocket propulsion. Previous Methods of improved fracture toughness and enhanced thermal conductivity had been developed to increase mechanical stability at extreme environments, but they still not satisfied the requirements for human-made spacecraft, such as invalid phase transformation toughening, damaged whiskers and fibers, poor particle dispersion. Hence, it must seek an alternative method to enhance the thermal shock property of ZrB₂ based ceramic composites at extreme environments.The target of this work is enhance the thermal shock property via two bioinspired methods, which are composed of biomechanics of architectural design（composing of soft organic and hard inorganic materials） and surface modification（composing of hierarchical structure）. Initiating from the response of ceramic microstructure under the thermodynamic effect, this work revealed their operating mechanisms at thermodynamic environment and raised some new methods that containing microstructure design principles and preparation methods, and ultimately manipulated their damage and occasionality. Two research approaches were proposed. Firstly, the fracture toughness of ceramic matrix had been enhanced by introducing reinforced graphene（soft materials） with high mechical and high thermal properties, which imitated from nature nacre. Additionally, this approach constructed a three-dimensional heat flow path among grain boundarys（GBs）, and thus increased its thermal diffusion. The thermal shock properties had been enhanced by higher toughness and quicker released thermal energy. Secondly, a biomimetic thermal shock（YSZ） coating was fabricated on ZrB₂ based ceramic composites via a sol-gel technology of co-assembly process and heteroepitaxial growth. Their coupling effect of increased thermal diffusion（2.884 times to matrix surface） and released thermal stress on matrix surface enhanced the thermal shock properties of ceramic materials.A modified Hummers method had been proposed to prepare large scale-up graphene oxide. Electrostatic assembly had been proved to be an effective way to obtain the dispersive GO wrapped ZrB₂-Si C ceramic particles. Subsequently, ZrB₂-Si C/graphene composites were obtained by calcinating those GO platelets wrapped ZrB₂-Si C particles at high temperature. The reconstructed graphene promoted its high temperature reduction by the modification of structual defects. there were some operating mechanisms that enhance the ZrB₂-Si C/graphene composites, such as graphene crack bridging, graphene pull-out, graphene fracture, crack branching and deflection. The muli-layer structures of grapheme kept perfectly among GBs. Their weak van der Walls（vd W） interaction efficiently released the dislocation-induced strain energies, and deformation changed from creep to sliding. So the deformation mode changed from dislocation-mediated process（creep of interface diffusion） to a GB-mediated（sliding） process. Owing to constructing a three-dimensional continuous network structure of graphene in matrix materials, Both of thermal diffusivity and thermal conductivity in ceramic matrix were increased rapidly with graphene content.Here we fabricated bioinspired coating of enhanced thermal shock properties via facile sol-gel technology and heteroepitaxial growth. A formed air envelope between biomemic coating and water enhanced the thermal diffusion of matrix materials in the thermal shock process. Under the coupling effect of high thermal diffusion of coating surface and low thermal conductivity of YSZ coating, the dynamic effect of temperature and stress distribution in the substrate changed rapidly. Finite element analysis showed that the maxium stress decrease form bioinspired YSZ coating surface（767 MPa） to matrix surface（336 MPa） in X direction. Furthermore, both of them were larger than that of in X direction（ 0 MPa）. These results demonstrated that crack propagation along the normal direction of substrate surface. Ultimmately, the thermal shock properties of ZrB₂ based ceramic composites was reinforced via reducing the crack driving force under the effect of microstructurual control.Both biominimic methods of microstructual control（structure and morphology） enhanced the thermal shock properties of ZrB₂ based ceramic composites, which verified the avalibility of biomimetic structure appeared in nature, such as nacre and lotus leaf. According to above analysis, bioinspired microstructure design has widely potential applications of ceramic materials in mechanics, thermology, electricity and son on.