The Design And Toughing Mechanism Research Of Heterogeneous Modulus Laminated Boron Carbide Composites

Boron carbide is a suitable material for many high performance applications dut to itsattractive combination of properties such as high hardness, low density, high neutronabsorption cross-section, excellent thermoelectric properties, etc. It has used as shieldingmaterial, armor materials, ceramic bearings, cutting materials etc. Boron carbide is the lowestdensity hard material in ceramics. It is a suitable material as body and vehicle armor. In spiteof its excellent properties, application of B₄C is rather limited,in real life,due to difficultiesin its toughness.In this paper, the toughness designs of boron carbide are combined with natural bionicmaterials and heterogeneous modulus ceramics, based on the macroscopic structure of boroncarbide composite material and the nature of the biological materials of generic structure, anew preparation method of high toughness boron carbide was proposed. Through the designof structure, material systems, densification and molding technology, the heterogeneousmodulus laminated B₄C/BN ceramic matrix composites were prepared by tape casting-laminated-hot pressing. The preparation technology of B₄C/BN prefabricate was discussed.The influence of the preparation technology of tape casting-laminated-hot pressing, thethickness ratio of hard and soft phases and different composition of soft phase on the structureand properties of boron carbide composites. The relationship and optimal components ofprocedure, structure and properties of the heterogeneous modulus laminated B₄C/BN ceramicmatrix composites were obtained. Based on mechanical properties, microstructure, crackinitiation and propagation path, cracking way and the load-displacement curve of thematerials, the mechanisms of toughness about the heterogeneous modulus laminated B₄C/BNceramic matrix composites were proposed. The mechanism of crack propagation of thecontinuum theory as a foundation, through the model numerical calculation and finite elementsimulation, the function of residual stress to fracture resistance and the resistance of crackpropagation in the heterogeneous modulus laminated B₄C/BN ceramic matrix compositeswere studied.The preparation technology of heterogeneous modulus B₄C/BN prefabricated includedboron carbide tape casting, BN composite forming and binder system removed etc.. In B₄Ctape casting process, the slurries was shear thinning fluid. The slurry composition and process parameters were optimized. Smooth surface green tapes with no defects were prepared in thethree-step ball milling, in the room temperature with no air flow condition. The dense BNcoating combined with matrix closely can play the role of the interface layer.The homogeneous modulus B₄C ceramic materials were prepared by tapecasting-laminated-hot pressing. The material was homogeneous and dense, with bendingstrength to585MPa, the fracture toughness of4.8MPa m^1/2. It is both higher than theproperties of the boron carbide by traditional hot pressing sintering.In different layer-thickness of laminated B₄C/BN composites, the best properties of thecomposites with thickness ratio of5.7, with bending strength of468MPa, the fracturetoughness of5.5MPa m^1/2, the fracture work for2398J/m2. The BN soft layer with B₄C andAl2O3can improve the properties of the laminated B₄C/BN composites. The composites with10wt.%B₄C in soft layer is optimal performance. bending strength for420MPa, the fracturetoughness of6.5MPa m^1/2, the fracture work for3248J/m2, close to three times ofhomogeneous modulus B₄C ceramic material. The composites with10wt.%Al2O3in softlayer is optimal performance. bending strength for471MPa, the fracture toughness of7.3MPa m^1/2, the fracture work for3071J/m2The toughening mechanisms of heterogeneous modulus laminated B₄C/BN compositesare different scales in different toughening mechanism of multilevel cooperative tougheningrole. Primary toughening mechanism is cracks deflecting in the soft phase and absorbing a lotof fracture energy. Level2toughening mechanism is the pullout and breakage of layers. Level3toughening mechanism is bifurcation of crack tips.The cracking process of the heterogeneous modulus laminated B₄C/BN composites wassimulated by finite element simulation.