Nonisothermal Pressure Sintering Kinetics for a Powder Mixture of Boron and Silicon Carbides and Structure and Fracture Behavior of the Sintered Composite

Abstract

The kinetics of nonisothermal pressure sintering of a power mixture of boron carbide with 20 wt.% silicon carbide under controlled heating is studied. The isothermal sintering kinetics of the mixture at temperature of 2240 K under applied pressures of 36.1, 49.6, 63.2, and 72.2 MPa was analyzed to determine the Laplace pressure. It is found that the kinetics is controlled by steady-state creep mechanism in the matrix forming the porous body, with the viscous flow rate being proportional to the stress square. The relatively low value of the estimated Laplace pressure (5.6 MPa) explains the difficulties in producing the boron carbide composites with pressureless sintering. The current temperatures and height of the samples during pressure sintering were used to determine the heating rate and the relative density derivatives with respect to temperature, which allowed the pressure sintering kinetics to be described within the theory of volume viscous flow of the porous body in a die. The estimated activation energy of the intermediate and late stages of pressure sintering of the composite for different heating rates ranges from 610 to 710 kJ/mol. These values indicate that the sintering kinetics is controlled by dislocation climb mechanism. The structure and fracture behavior of the sintered samples show that they depend on the heating rate. The higher the heating rate during B4C–20% SiC sintering, the greater the heterogeneity in the distribution of structural components and the larger the fraction of transcrystalline fracture of sintered samples.