Study On Preparation And Properties Of ZrB₂/B₄C-BNNTs Laminated Composites

The based layer B₄C-BNNTs and the hard layer ZrB₂ were prepared by aqueous tape casting,BNNTs was added in original B₄C powers to be the reinforced phase while amorphous boron powders as the base layer sintering aid.ZrB₂/B₄C-BNNTs layered composite materials were fabricated through the stack,hot pressing,binder removal and hot pressing sintering process.The influence of dispersant content,binder content,R value,solid content and milling time on the rheological properties of hard layer ZrB₂ slurry was studied.The thermal residual stress with the different layer thickness ratios and different number of layers of the based layer B₄C-BNNTs and the hard layer ZrB₂ was calculated by using the ANSYS.The effects of different thickness ratios of based layer B₄C-BNNTs and hard layer ZrB₂ and different sintering temperatures on the microstructure and mechanical properties of the as-synthesized sample were studied.Rheological property of the slurry was measured by rotary viscosimeter.The results showed that the ZrB₂ non-aqueous tape slurry with 50% solid content can be obtained with 1.2wt% castor oil,5wt% PVB and 6wt% DOP,secondary ball milling for 12 h.Butyl alcohol was added into slurry used as defoamer and then handled the slurry by vacuum bubble removing.The green tapes with smooth surface,no pores and no crack can be obtained.It can be concluded by ANSYS that the more layers,the more compressive stress in B₄C layer and less tensile stress in ZrB₂ when the number of layers less 13.But when the number of layers reached 13,compressive stress and tensile stress would barely change.With the increasing of layer thickness ratio of B₄C and ZrB₂,the compressive stress in B₄C would be reduced and the tensile stress in ZrB₂ would be increased.The phases and micro structures of ZrB₂/B₄C-BNNTs layered composite were characterized by XRD and SEM,the relative density of the samples was measured by drainage method,bending strength,fracture toughness and hardness were measured by universal testing machine and hardness tester.The BNNTs in base layers can increase the fracture toughness of samples with the effects of pulling out,crack deflection and bridging.Residual stresses are generated in the interface,due to dismatch in thermal expansion coefficient between the based layer and the hard layer.The energy of the crack tip were reduced and the crack were deflected,through the residual stress played a barrier,absorption,scattering,induction effect on crack.Results showed that,The composites with the thickness ratio of1.8 was sintered at 2000 ?,hot pressure of 30 MPa for 1h and the relative density,bending strength,fracture toughness and Vickers hardness of the as-synthesized samplewas 95.6%,468.8MPa,36.5GPa and 6.58MPa·m1/2,respectively.And the relative density,bending strength,fracture toughness and Vickers hardness of the sample sintered of2040 ? was 97.9%,516.81 MPa,38.4GPa and 7.69MPa·m1/2.With the sintering temperature increasing,the relative density,bending strength,Vickers hardness and fracture toughness were increased by 2.4%,10.2%,5.2% and 16.8%,respectively.