Study On The Designing Toughening And Amorphization-resistant Of Boron Carbide Ceramics And Its Structure And Properties

Boron carbide（B₄C）is commonly used in armor protection and engineering applications due to its excellent properties of low density and high hardness.However,B₄C is very brittle,and the strong covalent bonds between its atoms limit the relaxation of the lattice under strain and lead to brittle fracture.B₄C lacks the local plastic deformation mechanism and energy dissipation mechanism,such as dislocation slip,deformation twins and microcracks,as shown in the fracture process of SiC and TiB₂,so as to adapt to irreversible deformation.The high covalence ofB₄C and the low symmetry of its crystal structure mean that it is difficult to achieve local dislocation motion.Engineering applications require certain compliance of materials,and the lack of plasticity hinders the wide application ofB₄C in many engineering applications.In addition,the nano-amorphous reaction ofB₄C under high impact/high shear stress results in a significant loss of strength and toughness and limits its ballistic applications.High shear stress triggers icosahedral collapse,leading to the formation of nanoscale amorphous band networks.The loss of shear strength ofB₄C at pressures beyond its elastic limit（18-20 GPa）is attributed to the nucleation and propagation of cracks within the amorphous zone.This thesis proposes a design strategy for fabricating the boron carbide with enhanced toughness and resistance to amorphization.The multi-scale toughening effect was realized by combining atomic doping（micro-nano scale plastic deformation toughening）and the addition of reinforcing phase（macroscopic toughening）,thus the intrinsic brittleness and amorphous sensitivity of boron carbide were solved.It provides theoretical direction and solid experimental basis for the design and preparation of high-performanceB₄C based armor ceramics.The specific work mainly includes the following four parts:1.Boron carbide with different stoichiometric ratios was prepared by hot pressing sintering process at high pressure and low temperature.The lattice parameters and stoichiometric values were obtained by chemical composition analysis,XRD analysis and Rietveld refinement,and revealed the excessive B atoms caused lattice expansion.The lattice vibration characteristics were obtained by Raman spectroscopy.The microstructure characterization reveals that the densification mechanism of boron carbide with the addition of B gradually changes from atomic diffusion mechanism driven by thermal energy to plastic deformation mechanism dominated by the proliferation of dislocation and substructures.The introduction of chemical composition changes,by dissolving excessive B into boron carbide further affected the microstructure and consequently the mechanical response.The Vickers hardness,elastic modulus,and sound velocity all decreased with increases in B content,while the fracture toughness increased.The flexural strength of the samples was optimised at the B/C stoichiometric ratio of 6.1.2.The effects of quasi-static loading on the crystallization of boron carbide(B_4.2C)and B-rich boron carbide(B_6.1C andB_8.6C)and the anti-crystallization ability of boron carbide with higher stoichiometric coefficient were studied.Compare the Raman spectra of the pristine region with the residual indented area,the peak intensity ratio of the amorphous peak(1340 cm^-1)to the IBM peak(1088 cm^-1)decreases greatly with the increase of boron content.it is concluded that boron-rich boron carbides can prevent the amorphization more effectively.The Raman pseudo-color map of amorphization intensity beneath quasistatic multiple loads Vickers indentations on boron carbide with different boron content suggest that the amorphization area of B-rich boron carbide is uniformly distributed,and the amorphization intensity is not directly proportional to the increase of the load,and also much less than pure boron carbide.B_8.6C exhibits the best resistance to amorphous under low load conditions（2.9-4.9 N）.The anti-amorphous ability of B-rich boron carbide is still much stronger than that of pure boron carbide,although it will weaken under high load,andB_6.1C shows the most stable anti-amorphous ability.3.Microalloying is used to reduce stress-induced amorphous formation by changing the chemical composition of icosahedral and triatomic linear chains of boron carbide.The addition of Ti-Al alone did not led to Al doping inB₄C,and it did not reduce the degree of amorphization ofB₄C generated under high shear stresses.In contrast,the co-addition of Ti-Al and B contributed to both B and Al solid solutions into the boron carbide lattice and changed the chemical structure of the 12-atom icosahedron and three-atom linear chains ofB₄C,thus effectively mitigating the amorphization ofB₄C.The microstructure of theB₄C+B+5 wt.%TiAl sample showed that a nano–core–shell structure with aggregated dislocations in the shells which was generated at TiB₂grains due to the segregation and dissolution-precipitation between TiB₂and B-rich boron carbide,and contributing to the strengthening of grain boundary by inducing compression strain therein.The short-range ordered substitution of B and the disordered doping of Al inB₄C led to lattice distortion and expansion,resulting in the formation of high-density defects including stacking faults and nano-twins,which improved the mechanical properties of the composite.The composite with a low apparent density（2.52 g/cm³）provided an attractive combination of homogenous morphology and excellent mechanical properties,including a Vickers hardness of 35.53±0.83 GPa and a fracture toughness of 4.37±0.35 MPa·m^1/2.4.The effects of the addition amount of the second phase and sintering modes（low temperature liquid phase and high temperature transient liquid phase）on the structure and properties ofB_6.5C（B₄C+B）composites were studied by introducing different contents of Ti-Al into the matrix.Change sintering temperature and material composition to control material phase and interface composition.TheB_6.5AlC-TiB₂composite exhibits the best mechanical properties at the low density（2.51 g/cm³）at the high temperature transient liquid phase sintering（1900℃）and the addition of 3 wt.%Ti-Al.Compared with pureB₄C,its Vickers hardness,fracture toughness and flexural strength are increased by 24%,85%and 183%respectively.At micro-nano scale,B and Al dissolved into the lattice ofB₄C simultaneously,which changed the chemical structure of the 12-atom icosahedron and three-atom linear chain ofB₄C,thus effectively slowing down the amorphous reaction ofB₄C under high shear stress.By analyzing the microstructure of BAT3 composite,it was found that the segregation of solute atoms promoted the migration of grain boundary（Ti B2）,while the subgrain propagation was promoted by atomic doping of boron carbide crystals,resulting in the formation of a large number of nanotwins and stacking faults.The migration of grain boundaries and the plastic deformation induced by stacking faults and the resulting fine structure introduce compressive stress in the material which is beneficial to the mechanical properties and contributes to the plasticity,strengthening and hardening of the material.At the macro scale,the mismatch of thermal expansion coefficient betweenB₄C matrix and TiB₂reinforcer induces crack deflection,crack branching,crack bridging and microcrack toughening mechanism.The multi-scale toughening effect was achieved by combining the micro toughening of atom doping with the macro toughening of particle reinforcement.