Preparation,Microstructure And Mechanical Properties Of Reaction Bonded Boron Carbide Composites Fabricated By Gel-casting Process

Boron carbide(B₄C)exhibits attractive properties such as low density,high hardness and high strength,and therefore serves as an excellent candidate material for lightweight structural ceramic components such as protective armor materials.With the increasing demand for armored protective materials in the field of military protection,many researches have oriented at optimizing the properties and reducing the manufacturing costs of boron carbide composites.So far,the low-cost preparation of boron carbide composites by infiltrating molten silicon into moulded B₄C green bodies.Regarding increasing demands for homogeneous structure,the high reliability and quality ceramics,this research aims to fabricate reaction bonded boron carbide composites(RBBCs)via gel-casting process by studying:the influence of gel-casted green bodies on the microstructure and mechanical properties of RBBCs,the process parameters of SiC coating on B₄C particles via CVD,and the reaction mechanisms and controllability during the molten silicon infiltration process.The optimized microstructure and mechanical properties are expected to provide theoretical support and expanded applications for RBBCs.In the present work,a new low-toxicity gel-casting system consisting of resorcinol and formaldehyde is developed for the casting of B₄C-C green body.The chemical processes involved in the reactions between resorcinol and formaldehyde have been studied.The results show that,with increased content of catalyst(sodium carbonate)and resorcinol-formaldehyde(RF)polymer,the polymerization rate and polymerization degree increased.The porosity,pore diameter and carbon content of B₄C-C green bodies can be controlled by controlling the sol composition.A novel kind of 3D interconnected porous carbon bonded B₄C green bodies or hierarchically porous B₄C-C green bodies can thus be fabricated by such gel-casting method.The highest flexural strength of B₄C-C green bodies can reach up to 40 ± 1.5MPa.The molten silicon infiltrated gel-casted B₄C-C green bodies have been studied.The results show that,phase composition of the composite are:BxC?B?2(C,Si,B)3?SiC and residual silicon.The obtained RBBCs show that the uniform microstructures with SiC particles bonded boron carbide scaffold and an interpenetrating residual silicon phase.The gel-casted B₄C-C green bodies help control the size and the distribution of residual silicon and then helps optimize the properties of RBBCs.The Vickers hardness,flexural strength and fracture toughness of composites fabricated by gel-casting method are 19.4± 1.3 GPa,389±17 MPa and 4.37±0.1 MPa·m1/2,respectively,which are statistically higher than those of reaction bonded boron carbide composites fabricated by the conventional compression moulding produced B₄C-C green bodies.The effect of carbon contents on mechanical properties of the gel-casted B₄C-C green bodies for RBBCs via molten silicon infiltration has been studied.The results show that,the bulk density,hardness and flexural strength of the specimens increase with increased carbon contents of the RBBC composites.However,the fracture toughness initially improves then slightly decreases.The bulk density,Vickers hardness,flexural strength and fracture toughness of the RBBCs prepared from perform with carbon content of 16%(mass fraction)are 2.58 g/cm3?24±1.5 GPa?452±27 MPa and 4.32±0.2 MPa·m1/2,respectively.Thanks to SiC-bonded boron carbide skeleton structure and a reduction of the amount and size of residual silicon,the RBBCs show improved mechanical properties.The gel-casted B₄C-C green bodies for reaction bonded boron carbide composites via Si_0.78Ti_0.22 alloyed melt infiltration have been studied.The results show that,phase composition of the composites are:BxC?B₁₂(C,Si,B)₃?SiC?TiB₂ and residual silicon.The size and content of B₄C particles play an important role in Si_0.78Ti_0.22 alloyed melt infiltration process.An increase in B₄C particle size and solid loading can mitigate reaction between B₄C and molten Si_0.78Ti_0.22 alloy.With increasing solid loading of B₄C particles(d50=20?m),the aspect ratio of the plate-like SiC reduces.Compared with conventional moulded B₄C green bodies,the gel-casted B₄C-C green bodies favor Si_0.78Ti_0.22 alloyed melt infiltration.The reaction process of boron carbide composites fabricated by Si_0.78Ti_0.22 alloyed melt infiltration proceeds in this way:the reactions in the Si-Ti-B₄C system start with interface reaction between B₄C-C green body and Si_0.78Ti_0.22 alloy,as a result,B₁₂(C,Si,B)₃?granular SiC and TiC form at the initial stage.Subsequently,molten Si_0.78Ti_0.22 promotes further diffusion of C and B atoms away from B₄C particles,as a result,TiB₂ and plate-like SiC are gradually precipitated out from the molten metal.The dissolved B atoms react with Ti or TiC forming TiB₂.The formation of plate-like SiC in the Si-Ti-B₄C system can be attributed to the reaction between dissolved C atoms and silicon.CVD-SiC coating on B₄C particles and the effect of interfacial SiC coating layer on the reaction between B₄C and molten silicon during the molten silicon infiltration process have been studied.The results show that,when the gas ratio is fixed:H₂:MTS=13.56,Ar:(H₂+Ar)=0.14,deposition temperature is 1100?,the SiC coatings with different thicknesses can be obtained by controlling the deposition time.Phase compositions of RBBCs fabricated from SiC-coated B₄C particles are:B₄C?B₁₂(C,Si,B)₃?SiC?and residual silicon.SiC coatings act as a barrier and prevent the contact between B₄C particle and molten silicon,silicon must transport through the pre-formed SiC coatings and then dope into B₄C lattice to form B₁₂(C,Si,B)₃ phase.