Preparation Of Core-shell Boron Carbide Powders And Study On Properties Of Composites

Boron carbide?B₄C?has many outstanding properties such as low density,low wear coefficient,high hardness,high elastic modulus,high melting point,good chemical stability,and high neutron absorption cross-section.B₄C ceramics are important engineering materials and serve as lightweight armour,wear-resistant blasting nozzles,grinding wheels,and neutron absorber in mechanical,chemical,and military fields.However,B₄C has poor sinterability that is caused by strong covalent bonds over 90%and low self-diffusion coefficient.The applications of B₄C ceramic are limited because its brittleness cannot meet requirements of engineering.In order to reduce sintering temperatures and improve properties of B₄C,this study aims to uniformly package secondary phases on the surface of B₄C particles by molten-salt method instead of traditional ball-milling method.According to phase diagrams,the processes,compositions,properties,and microstructure of prepared powders and compacts were investigated.B₄C,Ti,and Al were used as raw materials in this work.First,we prepared secondary phases on the surface of B₄C particles by in-situ reaction with NaCl/KCl as molten salt medium at high temperatures.The B₄C-Al₃BC and B₄C-TiB₂ composites were successfully fabricated by subsequently pulsed electric current sintering?PECS?at 1700°C.And then we studied the compositions,microstructure,and properties of sintered B₄C composites.The experiment results show that:the appropriate ratios of raw materials,prepared temperatures,and holding time had significant influence on the compositions and distribution of secondary phases.In terms of the XRD and SEM data,we found that the residual Al was always detected in final powders due to the poor wetting between B₄C and Al below 1100°C.And the in-situ formed Al₃BC?<3?m?nucleated on the surface of B₄C particles with granular morphology.However,Ti cannot be detected in final powders,suggesting that Ti is completely consumed and has well wetting with B₄C at 1000°C.The in-situ synthesized secondary phases-TiC&TiB₂ uniformly distributed on the surface of B₄C with core-shell structure model.The properties of composites illustrated:the relative density,elastic modulus,Vickers hardness,and fracture toughness of samples reached as high as 100%?near?,495 GPa,37.0 GPa,and 6.32 MPa·m^1/2 at 1700°C,respectively,when the content of Al₃BC was 18 wt.%.B₄C-29.8 vol%TiB₂ composite sintered at 1700°C by using core-shell structural powders?B₄C@TiC&TiB₂?,the relative density of 98.2%,elastic modulus of 515.6 GPa,Vickers hardness of 32.1 GPa,fracture toughness of 4.38MPa·m^1/2,electrical conductivity of 4.06×10⁵ S/m,and a high thermal conductivity of33 W/mK were achieved.The Al₃BC and TiC&TiB₂ wrapped on the surface of B₄C particles converted into other Al-containing compounds and TiB₂ which distributed on the grain boundaries of B₄C during PECS process,respectively.Due to the differences of thermal expansion coefficient,the micro-crack was produced at interface that was weak.The crack cannot deflect until it propagated to the weak interface.Finally,the fracture mode changed from the single transgranular mode in pure B₄C to a mixture of transgranular and intergranular modes,so the fracture toughness was greatly improved.