Research On The Effect Of Aluminum-Contained Additives On The Sintering Of Boron Carbide

Boron carbide is a strategic material with high melting point,high hardness,low density,good thermal stability,strong resistance to chemical erosion and neutron absorption ability,which has been widely used in energy,military and bulletproof fields.However,the high covalent bond content makes the densification of boron carbide very difficult,which restricts its excellent performance and practical application.The promotion of the densification and the comprehensive performance of boron carbide has become the focus of current researches.In this paper,the same-system additives AlN,Al and Al2O3 powders were selected as the sintering aids.By comparing the performance of the boron carbide composite ceramics fabricated with different additives,the optimal preparation process of composite ceramics products was obtained.Firstly,the green compacts was prepared at the optimal molding process conditions,which are binder content 0.2 wt%,water content 8 wt%,cold isostatic pressing pressure 215 MPa and pressure-holding time 3 min,respectively.The optimal pressureless sintering process for the preparation of AlN/B4C ceramics with AlN as the additive is AlN addition 5 wt%,sintering temperature 2100℃ and the holding time 60 min.The relative density,flexural strength and fracture toughness of boron carbide composite ceramics are 95.84%,383.30 MPa and 5.45 MPa·m1/2,respectively.The results show that the high-hardness BN and layered composite-structure AlB2 generate by the reaction of B4C and AlN,which can significantly improve the density and mechanical properties of boron carbide ceramics.Al/B4C ceramics was prepared with Al powder as the additive by pressureless liquid-phase sintering method.The optimal process conditions are the addition amount of A1 3 wt%,the sintering temperature 2100℃ and the holding time 60 min.Under these conditions,the optimal properties of boron carbide ceramic composites are 95.32%relative density,326.68 MPa flexural strength and 3.83 MPa·m1/2 fracture toughness.It was found that the main densification mechanism in the process of pressureless sintering under vacuum was the viscous flow of Al.The driving force was provided by the internal stress between particles and the metal boride generated from the reactions between B4C and Al had the effect of dispersion strengthening,which was the reason for the superior performance of the fabricated ceramic composites.In order to improve the performance of boron carbide ceramics,Al2O3 was added to the matrix as the additive to prepare Al2O3/B4C ceramics.The effects of Al2O3 additive and sintering temperature on properties of boron carbide ceramic composites were investigated.Through analysis and comparison of experimental results,it was concluded that the optimal technological conditions for preparing the ceramics were sintering temperature 2050℃,Al2O3 content 5 wt%and holding time 60 min.Under these conditions,the relative density of ceramic products was 87.82%,flexural strength was 231.35 MPa and fracture toughness was 3.10 MPa·m1/2.Because boron carbide raw materials contain impurities such as free carbon,free boron and boron trioxide,the presence of these impurities will reduce the purity of boron carbide and adversely affect its performance.It is found that B2O3 can react with Al2O3 to produce Al18B4O33 phase,which will reduce the performance of the obtained ceramic composites.The action mechanism of additives on boron carbide was analyzed,and the results showed that the addition of aluminum additives could change the structure of rond surface of boron carbide and cause lattice distortion and polarization.