Nonisothermal Pressure Sintering Kinetics of the B4C–SiC Powder Mixture, Structure and Fracture Behavior of Sintered Composite

Abstract

The kinetics of nonisothermal pressure sintering of boron carbide powder mixed with 20 wt.% silicon carbide in the controlled heating mode 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 square of stress. The relatively low value of the evaluated Laplace pressure (5.6 MPa) explains the difficulties in producing boron carbide composites with pressureless sintering. The current values of temperature and height of the samples during pressure sintering were used to determine the heating rate and the temperature derivatives of relative density, which enabled to describe the pressure sintering kinetics in the terms of the theory of bulk viscous flow of the porous bodies in a die. The evaluated activation energy of the intermediate and late stages of pressure sintering of the composite for different heating rates ranges from 670 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 are shown to depend on the heating rate. The higher the heating rate during B4C–20% SiC sintering, the more heterogeneous is the distribution of powder components and the larger the portion of transcrystalline fracture of sintered samples.