The Densification Kinetics of Porous Zirconium Diboride in Vacuum Pressure Sintering

The time dependence of densification for a zirconium diboride powder during the isothermal sintering in vacuum assisted with applied pressure varying from 12 to 60 MPa in the range 2110– 2300°C and the nonisothermal pressure sintering with temperature being constantly increased by 20 and 40°C/min was studied experimentally. The densification kinetics was analyzed within the framework of continuum theory for bulk viscous flow taking the powder particle shape influenced the rheological properties of porous bodies into account. The densification kinetics was represented by equations describing nonlinear steady-state creep of the matrix forming a porous body. In isothermal sintering conditions, the root-mean-square strain rate is proportional to the fourth exponent of the root-mean-square stress. The creep is controlled by the dislocation climb mechanism with an estimated activation energy that is equal to 626 kJ/mol and is consistent with the activation energy for self-diffusion of atoms in the metal sublattice of borides. The estimated Laplace pressure is virtually comparable with the average applied pressure. The root-mean-square shear stress induced by additive pressure decreases with increase in the relative density and approaches zero at neartheoretical density of the material. The root-mean-square strain rate sharply decreases in the isothermal sintering process and increases reaching the maximum on the nonisothermal sintering curve. Two temperature ranges with different activation energies were found for nonisothermal pressure sintering. The higher temperature range is characterized by energies that are significantly greater than those for isothermal sintering and is indicative of endothermic process. The estimated critical cold brittleness temperature for zirconium diboride to transform to plastic condition is 1345°C.