Construction and Toughening Mechanism of ZrB2-SiC-Graphene Biomimetic Composite Microstructure

Author:Cheng Ye Hong

Supervisor:han wen bo

Database:Doctor

Degree Year:2019

Download:70

Pages:134

Size:12151K

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The temperature of the local hot components of the aircraft is as high as 2000°C or higher during high-speed flight.More stringent requirements are placed on the oxidation resistance and temperature tolerance limit,high reliability and thermal shock resistance of thermal protection materials.Traditional refractory metals,graphite,C/C,C/SiC and SiC/SiC thermal protection materials have been difficult to meet the needs of future thermal protection systems.The refractory boride or carbide of transition metal with a melting point above 3000°C is one of the most promising non-ablative ultra-high temperature thermal protection materials above2000°C,among which ZrB2-based ultra-high temperature ceramic(UHTC)have been investigated extensively and deeply due to their excellent anti-oxidation ablation performance.However,the intrinsic brittleness and poor thermal shock resistance of ZrB2-based UHTC materials have always been the key to limit their engineering application.Domestic and foreign researchers have tried to employ a variety of additions to improve the fracture toughness and thermal shock resistance of ZrB2 ceramics,however,the toughening effect is limited,which couldn’t meet the needs of extreme thermal environments.To further improve the fracture toughness and thermal shock resistance of ZrB2-based ceramic materials,a lot of work has been done on biomimetic layered or“brick-and-mortar”structured ceramics at home and abroad,while,most of them used polymer,metal or two-dimensional graphite sheets and BN with high-volume fraction(>70 vol.%)as boundary layers instead of protein layers in natural biomaterials,which severely destroyed the high temperature performance and oxidation ablation resistance of ceramic materials.In addition,the laminated ceramics have significant out-of-plane anisotropy,and the fiber monolithic structure only has good mechanical properties along the fiber axial direction,and the mechanical properties attenuated dramatically when tested off from axial orientation.In view of the above problems,this paper uses the bionic microstructure to toughen UHTCs,while using the weak interface of high ceramic components to ensure the excellent oxidation and ablation performance of ZrB2 UHTCs,and to alleviate the remarkable anisotropy of biomimetic ceramics through microstructure design.Finally,multi-component,cross-scale microstructure design achieves multi-scale toughening and alleviation of the anisotropy of biomimetic ceramics,and improves the thermal shock resistance while maintaining its excellent oxidation and ablation performance.ZrB2-SiC-Graphene ceramics with varying microstructure and mechanical properties were obtained by adjusting the content of graphene.ZrB2-20vol.%SiC with high elastic modulus and strength was selected as the strong matrix,and ZrB2-20vol.%SiC-30vol.%Graphene with high fracture energy and thermal shock resistance was selected as the weak interface.ZrB2-SiC-Graphene@ZrB2-SiC(ZSG@ZS)“brick-and-mortar”ceramics was obtained by innovative granulation-coating method and exhibits less anisotropy and superior thermal shock resistance.ZSG@ZS“brick-and-mortar”ceramics can resist crack initiation caused by local damage before the crack tip by the bridging of graphene at the micro-nano scale.On the macroscopic scale,the mismatch of elastic modulus between multiple components inhibits crack propagation and forces any initial crack to change direction continuously,which reducing stress at crack tip,inhibiting crack growth and improving fracture toughness through crack deflection and bifurcation.Finally,the multi-scale composite toughening effect is achieved by the micro-toughening behavior of graphene extraction and bridging and the macro-toughening behavior of crack deflection and bifurcation.Non-axially aligned ZrB2-SiC/ZrB2-SiC-Graphene(ZS/ZSG)short fiber monolithic ceramics with in-plane isotropic property were prepared by hot pressing of disordered ZS/ZSG short fibers to overcome the in-plane significant anisotropy of conventional axial aligned ZrB2-based fiber monolithic ceramics.The effect of relative density and fiber diameter on the microstructure and properties of ZS/ZSG short fiber monolithic ceramics was investigated.The relative density and mechanical properties of ZS/ZSG short fiber monolithic ceramics increased gradually with the increase of sintering temperature.As fiber diameter decreasing,the flexural strength of ZS/ZSG short fiber monolithic ceramics first increases and then decreases,and the fracture toughness gradually increases.ZS/ZSG short fiber monolithic ceramics were ablated by oxyacetylene flames with a maximum temperature of over 2150°C for 615s,and the shape remains intact without macroscopic cracks,showing non-ablative properties.ZS/ZSG continuous fiber monolithic ceramics with various microstructures were obtained by adjusting the fiber arrangement angles.The axially aligned ZS/ZSG-0ofiber monolithic ceramics have excellent mechanical properties along the fiber axis,their fracture toughness and fracture work are as high as 10.42±0.98MPa·m1/2 and 737.8J/m2,respectively,but have significant in-plane anisotropy.The fracture toughness of ZS/ZSG-15o,ZS/ZSG-45oand ZS/ZSG-90ofiber monolithic ceramics are lower than that of ZS/ZSG-0o,but exhibits relatively uniform in-plane properties.ZS/ZSG continuous fiber monolithic ceramics resist damage by internal and external toughening of multi-scale structures from nanoscale to microscale.The internal toughening mechanism mainly works on the nanoscale,graphene can resist crack initiation caused by local damage before the crack tip.The external toughening mechanism plays a role on the micron scale and is mainly behind the crack tip.When the crack begins to expand,the external toughening mechanism,such as crack deflection and bifurcation,and pulling out of graphene,reduces the stress at the crack tip and inhibits the crack growth,so that ZS/ZSG continuous fiber monolithic ceramics show a significantly rising R curve behavior.