Effect Of Transition Metal Compounds On Sintering Behavior,microstructure And Properties Of Boron Carbide Multiphase Ceramics

Boron carbide（B₄C）ceramic is the fifth hardest known substance after diamond,cubic boron nitride,fullerene compounds and diamond monolithic fibrils.Owing to its good strength,wear resistance,and the abundant natural resources,it has been gradually used as one of structural ceramics in the military industry,aerospace and other field.However,based on the characteristics of chemical bonding of B₄C ceramics,high sintering molding temperature,large energy consumption,and difficulty of density all leading to high research costs.In addition,B₄C ceramics have low fracture toughness,high brittleness,and lack standards for failure prediction,which means that application reliability needs to be improved.In conclusion,reducing the sintering cost and promoting higher toughening are of great significance to the academic research and engineering applications.Therefore,it is of high research value to systematically study the effect of the sintering method of B₄C ceramics among the microstructure and properties.The purpose of this article is to solve some basic research problems hidden behind the application of B₄C multphase ceramic.In this paper,control mechanism based on liquid phase sintering,eutectic sintering and original reaction.Variety of transition metal compounds are introduced into the B₄C powder,and high-performance boron carbide-transition metal boride ceramic materials with uniformly dispersed second phase are prepared through different sintering method.The effects of the same second phase through different reaction paths and different kinds of second equal processes through the same reaction path on the microstructure and mechanical properties of boron carbide ceramics were studied by XRD and SEM.To explore the bonding strength between the second boride phase and the matrix produced by different ways under the same conditions and its influence on the microstructure and properties,adding the compounds of Ti and Zr.In Ti and its compound system,the sample added with Ti O₂will generate gas during the reaction,so the grains are finely and uniformly distributed.The samples added Ti,TiC and TBC（TiB₂+TiC）,because the nucleation is easy to grow in the process of generating TiB₂in situ,the second phase grains are larger and have agglomeration.In Zr and its compound system,the Gibbs free energy of the sample added Zr O₂and Zr C is higher than the reaction of WC introduced from ball milling and the matrix.There are many W₂B₅phases in the product,and the density of the Zr-B-W solid solution phase is low.The Zr-added samples have lower Gibbs free energy,and the ZBC-added samples do not need to undergo nucleation,so the products of these two groups of samples are mostly dense Zr-B-W solid solution phases.Expanding the exploration of Nb and Ta elements based on Ti and Zr,and systematically studying the effects of their oxide introduction on the sintering behavior of boron carbide complex-phase ceramics and their tissue mechanical properties,B₄C-Me B₂（Me=Ti,Zr,Nb₂O₅and Ta₂O₅）complex-phase ceramics were prepared by pressureless sintering at 2250°C using a mixture of B₄C and Me O_x（Ti O₂,Zr O₂,Nb₂O₅and Ta₂O₅）powders,Nb and Ta)complex-phase ceramics were prepared by pressureless sintering.The result shows the addition of transition metal oxides can promote the densification of sintering,and the second phase grains are fine and dispersed,but it is easy to leave pores in the matrix.The samples prepared by adding Ti O₂have the highest elastic modulus and flexural strength,411 GPa and336 MPa;When Zr O₂was added,the sample prepared had the highest hardness,which was21.1 GPa;The samples prepared by adding Ta₂O₅have the highest fracture toughness,which is 6.08 MPa·m^1/2.In order to systematically study the effects of different metal element carbides on B₄C complex-phase ceramics,B₄C-Me B₂（Me=Ti,Zr,Nb,Ta and W）complex-phase ceramics were prepared by in situ pressureless sintering at 2250°C using a mixture of B₄C and Me C（TiC,Zr C,Nb C,Ta C and WC）powders.The result shows the effects of transition metal carbides,namely,TiC,Zr C,Nb C,Ta C,and WC,on the phase composition,microstructure,and mechanical properties of the ceramics were investigated.The results showed that Me C could facilitate the sintering densification of B₄C by distributing second-phase particles uniformly throughout the B₄C.Additionally,the phenomenon of W solid solution into the matrix is observed.The main phases observed were B₄C and（Me,W）B_x（Me=Ti,Zr,Nb,or Ta）due to the doping of a small amount of WC during the ball milling process.As a result,the mechanical properties of B₄C-Me B_xshowed significant improvements when compared with those of single-phase B₄C ceramics.B₄C-Nb B₂ceramics were found to exhibit the best mechanical properties,whose elastic modulus,hardness,flexural strength,and fracture toughness are 393.0 GPa,28.7 GPa,368.0 MPa,6.94 MPa·m^1/2,respectively.Based on the research of the effect of transition metal compounds,the method of reducing the pressureless sintering temperature was further investigated at three composition points of sub-eutectic,eutectic,and super-eutectic,respectively.The B₄C-Me B₂（Me=Ti,Zr,Nb）complex phase ceramics were prepared at 2100°C,2200°C.The results showed that the densities of B₄C-Zr B₂prepared with the peri-eutectic component were more than 93%at2100°C.The process of sintering densification occurred in the temperature interval2100-2200°C,and the hardness values were significantly increased.From the results of the eutectic sintering study,the Zr-containing system showed a rapid densification process at2100°C-2200°C.Induction of this process at lower temperatures is expected to achieve densification of B₄C ceramics at lower temperature.In order to realize the preparation of high toughness boron carbide multiphase ceramics at low temperature.By using B₂O₃-Zr O₂-SiC sintering additives added to the B₄C matrix,the B₄C composite material with better comprehensive performance was successfully prepared by optimizing the hot-pressing sintering process and the ratio of the composite sintering additives.The research results show that the product of this system can use molten boron oxide to promote particle rearrangement before 1200℃,and this process results in promoting sintering densification.Between 1600-1800℃,Zr B₂particles will transform from spherical to lath shape,and the distribution will be more uniform.Two-step hot-press sintering of the material at 1800°C can significantly fine the microstructure of the material,thereby increasing the fracture toughness by 46.63%.The matrix whose vicker’s hardness,elasticity module,flexural strength and fracture toughness are 19.84 GPa,299.73 GPa,404.49 MPa and 7.63MPa·m^1/2,respectively,are successfully sintered under 1800℃。...