Pressureless Sintering And Dynamic Impact Damage Mechanism Of Laminated SiC-based Ceramics

The excellent properties of ceramics such as low density,high hardness,high strength,high melting point and high compression make the impact resistant materials enter the era of rigid and flexible composite materials from the era of single metal material with steel grams of rigidity,especially in the field of armor protection.However,the brittleness of ceramics limits its ability to resist multiple impacts,so it is urgent to improve the toughness of ceramics.The emergence of laminated ceramics provides an effective technical way to improve the impact resistance of ceramic materials.Laminated ceramics have high structural and performance designability,which can improve the damage tolerance and toughness,and then improve the impact resistance of ceramic composite armor.However,the current experimental methods of dynamic mechanics can not fully reveal the dynamic response and damage failure process of ceramic materials,which makes the study of dynamic failure not in-depth,and can not establish a suitable damage model to guide the structural design and optimization of materials,affecting the wide application of ceramic materials in the field of protection.At present,the experimental data of dynamic compression of laminated ceramics are seriously lacking,and the dynamic failure mechanism and damage model of laminated ceramics are still in the initial stage.The main research object of impact resistance of laminated ceramics at home and abroad is materials prepared by pressure sintering.The efficiency,cost,simple shape and size of pressure sintering greatly limit the application of laminated ceramics.In this paper,laminated SiC-based impact-resistant ceramics were prepared by pressureless sintering.The feasibility of the preparation of laminated SiC-based impact resistant ceramics was verified by liquid phase sintering,solid phase sintering and reactive sintering.The process parameters were optimized and the relationship between interface design,structure design and properties of laminated ceramics was studied.The mechanical behavior and damage process of laminated ceramics under dynamic compression were studied by means of SHPB and high-speed camera.The relationship between interface type and strength,structural design,wave impedance design and dynamic response of laminated ceramics was compared,and the dynamic failure mechanism and damage model of laminated ceramics were established.The main research contents are as follows:(1)The effects of the content of sintering aids and sintering temperature on the properties of liquid phase sintered SiC ceramics were studied.The mechanical properties of laminated L-SiC/BN ceramics were controlled by the interface strength and structure design.The optimum values of sintering additives and sintering temperature are 20 wt.%and 1900 ^oC,respectively.With the decrease of interfacial strength,the mode of crack propagation changed from brittle fracture to non-brittle fracture,the toughness of the material increased and the strength decreased,and the increase of the thickness ratio decreased the mechanical properties of the material(2)The effects of type and content of sintering aids,initial grain size and sintering temperature on the properties of solid phase sintered SiC ceramics were studied,and the mechanical properties of the materials were controlled by interface type and layer structure design.The optimum process parameters were pyrolysis carbon of 1 wt.%,boron of 2 wt.%,initial SiC particle size of 0.5 um,sintering temperature of 2050 ^oC.Interface design included C,BN,B₄C and TiB₂.The optimum interface type and structure design was S-3SiC/C with the fracture toughness of 10 MPa·m^1/2 and bending strength of 350 MPa.Mechanical properties decreased with the increase of thickness ratio.(3)The interfacial reaction law of SiC ceramics prepared by pressureless reactive sintering was studied.Laminated ceramics with different interfaces were prepared by controlling interfacial reaction.When the thickness of TiC layer was 150?m,the prepared layer structure was R-SiC/TiSi₂,the thickness of TiC layer was 450?m,and the prepared layer structure was R-SiC/Ti₃SiC₂.The interface of B₄C prepared by pressureless reactive sintering was controllable and the near-net-size forming of the material was completed.The optimal value of the optimized structure of R-SiC/B₄C ceramics was fracture toughness of 6.2 MPa·m^1/2 and bending strength of 210 MPa.(4)The effect of three interfacial layers of SiC ceramincs prepared by pressureless sintering on the dynamic compression properties was studied.The interlayer interface included the weak interface of SiC-YAG sintered in liquid phase,the strong interface of SiC-SiC sintered in solid phase and the brittle interface of SiC-Siprepared by reactive sintering.The experimental results showed that the three bulk SiC ceramics had obvious strain rate strengthening effect.The strain strength of the strong interface was the highest,the weak interface was the second and the brittle interface is the worst.The dynamic damage mode of ceramics was axial splitting fracture,according to the stress-strain/modulus curve,the failure process was divided into three stages:initial loading stage,strain hardening-damage accumulation stage and material failure stage.(5)The effects of wave impedance design and layered design on the dynamic failure mode and damage evolution mechanism of laminated SiC-based ceramics were studied.The periodic structure and interface wave impedance design of laminated ceramics increased the dissipation and attenuation of stress waves.The dynamic failure process was divided into four stages and had one more strengthening stage than block ceramics.There were two strengthening peaks in the stress-strain curve.The introduction of interlayer interface had changed the damage failure mode of materials,and the weak interface exhibited strain rate insensitive phenomenon.Through the initiation and propagation of interlayer cracks,the axial crack deflection was induced.The strong interface had strain rate hardening effect.The crack began at the end of the reflective tensile stress and deflected under the residual stress at the interface.(6)The effect of interfacial strength on the dynamic damage failure process of materials was studied.With the decrease of interfacial bonding strength,the dynamic strain was increased and the strain strength was decreased.The dynamic damage mode changed from axial penetrating crack propagation to interlaminar and stepped crack propagation,which restrained the penetrating axial crack and improved the structural integrity and the dissipation of kinetic energy.(7)The effect of interface type and layer structure design on dynamic compressive properties of laminated ceramics was studied.Interface type affected dynamic compressive properties of materials by changing wave impedance design,interface strength and types of residual stress.With the increase of impedance mismatch between layers,the dissipation of stress wave was increased.Weak interfaces depended on interlayer damage to guide axial crack deflection.Strong interfaces depended on interlayer residual stress to deflect axial crack,which reduced the moving speed of transmission stress peak,attenuates stress wave and improved the impact resistance of materials.The layer structure design could improve the strain strength of the weak interface,but reduced the deflection of the stress wave,so that the ability of the laminated ceramic to maintain structural integrity was reduced.The layer structure design increased the residual compressive stress at the strong interface and decreased the impact resistance of materials.(8)By comparing the dynamic compressive properties of laminated SiC-based ceramics prepared by pressureless sintering,the impact resistance database of laminated ceramics was established.According to strain strength-strain results,laminated materials were divided into three types:low strain strength and high strain(>10%),medium strain strength and low strain(<3%),high strain strength and medium strain(>6%)laminated structure.The dynamic damage mode of laminated SiC-based ceramics was controllable and adjustable.A database of dynamic compressive properties of laminated SiC-based ceramics was established,which provided experimental and theoretical support for the application of laminated SiC-based ceramics.