Research On The Microstructure Regulation And Properties Of Graphene/ZrB₂ Ceramic Composites

Non-ablative ultrahigh temperature ceramic composite is a kind of thermal protection material used in the extreme thermal environment.Zirconium diboride(Zr B2)based ultrahigh temperature ceramic(UHTC)is considered as a promising candidate non-ablative ultrahigh thermal protection material due to its perfect thermophysical properties.However,intrinsic brittleness and poor thermal shock resistance of Zr B2 based ceramic composites limit their wide range of engineering applications.Recently,carbon materials have gotten more and more attention due to their excellent properties,such as high electrical conductivity,high thermal conductivity,chemical inertness,low thermal expansion coefficient,light-weight,et al.Especially,a boom of ‘Nano-carbon' research has been stirred up in the world scientific community accompanied with the discoveries of Fullerene,Carbon nanotubes and Graphene.In the field of UHTC,various form of carbon materiasls have been employed to improve the sintering characteristics,mechanical properties and thermal shock resistance of Zr B2 based ceramic composites.However,these studies are far from being comprehensive and thorough.Aiming to improve the intrinsic br ittleness of Zr B2 based ceramic composites,this work explores various methods to introduce uniformly carbon nanomaterials into the ceramic composites.Many effort has been paid to analyze the effect of carbon nanomaterials on the mechanical properties of Zr B2 based ceramic composites,and the corresponding toughening mechanisms.Graphene oxide/Zr B2 ceramic films are self-assembled using vaccum-assisted filtration method.The influence of microstructure of the thin fims on the tensile properties and electrical conductivity is analyzed.Inspired by biological structures,hierarchical architectures spanning several length scales are fabricated in the Zr B2 based ceramic composite.The mechanical properties of laminated graphene/Zr B2 ceramic composite are evaluated,and the corresponding toughening mechanisms are analyzed in detail.The quality and distribution of graphene nanosheets in the ceramic matrix directly determine their toughening effect.In order to achieve uniform dispersion of graphene nanosheets in Zr B2 based ceramic composite,graphene oxide(GO)with a large number of hydrophilic oxygen containing groups on its surface is insitu thermally reduced to graphene nanosheets during sintering process.The influences of graphene nanosheets content on the microstructure,strength,toughness and hardness of graphene/Zr B2-SiC ceramic composites are analyzed in detail.Since the ceramic composite composed of limited grain,graphene sheets will inevitably bind to each other during sintering process when the volume content of graphene oxide is too high.The increase of thickness of graphene nanosheets will impact the mechanical properties of graphene/Zr B2-SiC ceramic composite.Indention crack paths are employed to analyze the toughening mechanisms of graphene nanosheets.The core-shell structure of nanocarbon coated Zr B2 hybrid particle is prepared by chemical vapor deposition method.The effects of deposition temperature,time and gas flow rate on the morphologies of carbon nanosheets/Zr B2 hybrid particles are explored.FV model and SK model of epitaxial growth are used to analyze the growth process of carbon nanosheets/Zr B2 hybrid particles.The carbon nanosheets/Zr B2 particles and SiC particles are employed to fabricate ceramic composite using spark plasma sintering process.In the process of SPS,carbon nanosheets promote the densification through reacting with the surface oxides of SiC particles to purify the grain boundary.The effects of carbon nanosheets content on the fracture toughness and hardness are discussed in detail.The fracture mechanisms of carbon nanosheets/Zr B2-SiC ceramic composites are analyzed based on the indentation crack growth path.Vacuum assisted filtration method is utilized to fabricate GO/Zr B2 ceramic films using graphene oxide.The influences of film composition,PVA content on the hierarchical architectures are analyzed.Tensile tests show that the maximum tensile strength and modulus of fims appear when the GO content is 30 vol.%.The reduce or increase of GO content in the film is not conducive to build the hierarchical architectures,leading to the reduction of tensile properties.After thermal reduction of GO/Zr B2-SiC ceramic films in the range of 300-1300 ?,the electrical conductivity of these films increases with the rise of reduction temperature,and the increase of GO content.Brittle mineral platelet and ductile protein are utilized to build hierarchical architectures by Shell to possess both high strength and toughness.Inspised by this,one ‘bottom-up' assembly method is utilized to fabricate graphene/Zr B2-SiC ceramic composite with hierarchical architectures spanning several length scales to improve the fracture toughness.Firstly,GO and ceramic particles are self-assembled into GO/Zr B2-SiC films with nano-microscale hierarchical architectures.Then,spark plasma sintering method is utilized to construct micro-macro structural order through densifying two types of alternately stacked GO/Zr B2-SiC films containg different volumes of graphene oxide.Alternately compressive and tensile layers are achieved during the cooling step of the sintering process due to the different thermal expansion coefficients of GO/Zr B2-SiC films containing different volumes of graphene oxide.The influence of microstructure on the strength and fracture toughness of laminated graphene/Zr B2-SiC ceramic composites is analyzed in detail.In addition,multiscale toughening mechanisms of the laminated graphene/Zr B2-SiC ceramic composite are analyzed based on the crack propagation path.