Densification Kinetics Of Boron Carbide （B₄C） Ceramics During Spark Plasma Sintering

Boron carbide（B₄C）is one of the special ceramics with excellent properties,which is widely used in aviation,aerospace,military fields.One of its most important applications is as an air bearing material for navigation gyroscopes.Due to its low self-diffusion coefficient and strong covalent bond,it is difficult to make dense B₄C ceramics,and the densification behavior and mechanism of boron carbide sintering at high temperature stage are still unclear.In this paper,we studied the sintering behavior of boron carbide powder with different particle sizes during spark plasma sintering（SPS）,guided by the plastic deformation theory dominated by creep,the densification behavior and dynamic mechanism of boron carbide powders were studied.The influence of grain growth mechanism on densification was quantitatively analyzed.FeMnCoCrNi and FeMnCoCr alloys with high performance and high entropy were used as new ceramic sintering aids.The densification kinetics models of low melting point sintering aids were established.The main contents and innovations of this study are as follows:（1）The densification process and control mechanism of pure B₄C powder during SPS were revealed.The results show that the higher the sintering temperature and the finer the powder,the higher the final relative density of the samples.The stress exponent values and densification activation energy of boron carbide powder can be calculated according to the steady-state creep densification model without significant grain growth.In the early and middle stage of densification,when the stress exponent is1,the densification mechanism of boron carbide is atomic diffusion dominated by boron atom diffusion,and the activation energy of densification is about 117.9-170.9 k J/mol.When the stress exponent is 2,the densification mechanism is the grain boundary slip/grain boundary diffusion mechanism dominated by carbon atom diffusion,and the activation energy of densification is 247.3-337.3 k J/mol.At the later stage of densification,the stress exponent is higher than 3,and the densification is mainly controlled by dislocation climbing mechanism.（2）The grain growth kinetics and control mechanism of boron carbide powders under different sintering parameters were studied.Based on the determination of grain growth“window temperature”T_g,by further increasing the sintering temperature,extending the holding time,guided by the classical grain growth model,the grain growth kinetics during high temperature process of boron carbide was systematically studied.The results show that for ultrafine boron carbide powder（average particle size<1μm）,grain boundary diffusion is the main control mechanism at the initial stage of grain growth,and volume or liquid phase diffusion is the main control mechanism at the later stage of grain growth.Grain boundary diffusion is the main mechanism for grain growth of micron boron carbide powders.（3）The mechanism of non-thermal effects such as electromigration effect and electroplastic effect in the late densification of boron carbide powder were discussed.Results show that the driving force of mass migration caused by the electric migration effect is in the same direction as the driving force of temperature and stress field of boron carbide at the initial densification stage,and the electric migration effect accelerates the densification of boron carbide powder at the initial densification stage.The instantaneous current density in the powder bed decreases gradually,and the higher the current density is,the higher the densification rate of boron carbide powder is at the beginning of the soaking stage.Electric field can promote the densification of materials by reducing sintering activation.At the later stage of densification,the higher the pulse rate is,the lower the instantaneous current density is,and the higher the dislocation density is.The dislocation motion accelerates under the electroplastic effect,therefore the densification rate accelerates.（4）FeMnCoCrNi high entropy alloy（HEA）was successfully used as the sintering aid for SPS B₄C ceramics,and the densification mechanism of boron carbide powder with FeMnCoCrNi-HEA was investigated.FeMnCoCrNi-HEA can not only form liquid phase filling pores between crystals to promote the densification,but also enhance the mechanical properties of B₄C materials.The fracture strength of B₄C-based ceramic is35%higher than that of pure boron carbide.In the low temperature range,the material transport mainly depends on the surface diffusion of boron carbide particles.At the low effective pressure stage,the densification mechanism is controlled by grain boundary slip/interface reaction or lattice diffusion,and the activation energy Q_dof densification at this stage is519.5±82.6 k J/mol.At high effective compressive stress,the densification mechanism is transformed into a dislocation climbing mechanism.（5）A new steady-state creep model with low melting point component was established and the densification mechanism of B₄C powder under the influence of different contents of sintering additives was revealed.When the sintering agent melts,the pressure exerted by the SPS on the graphite mold causes the liquid phase to flow in the open pores of the B₄C matrix.These liquid phases change the actual contact area between particles,which in turn influences the actual effective stress derived.The effective stress exponents of B₄C powder obtained by using the modified model are0.85 at low effective stress-low temperature,and the main control mechanism of densification is grain boundary diffusion or atomic diffusion.At the stage of high stress,the effective stress exponents are higher than 3,and the densification process is mainly controlled by dislocation climbing mechanism.With the increase of the addition amount of high entropy alloy,the activation energy of sintering decreases from 234.4 k J/mol to 75 k J/mol.116 Figures,16 Tables and 244 References...