The DENSIFICATION KINETICS of POROUS ZIRCONIUM DIBORIDE . IN VACUUM PRESSURE SINTERING

Abstract

The time dependence for densification of zirconium diboride powder subjected to vacuum sintering at applied pressure varying from 12 to 60 MPa in isothermal holding conditions in the temperature range 2110-2300 °С and to nonisothermal heating at controlled temperature constantly increased by 20 and 40 °C per 1 min experimentally studied. The densification kinetics was analyzed within the continuum theory of bulk viscous flow for a porous body considering the influence of powder particle shape on the rheological properties of a porous body. The analysis showed that the densification kinetics was represented by the equation describing steady-state nonlinear creep of a 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 dislocations climb mechanism with estimated activation energy of 626 kJ/mole, being consistent with similar values for the self-diffusion in the metal sublattice of borides. The estimated Laplace pressure is virtually comparable with the average value of selected applied pressures. The root-mean-square shear stress induced by the action of additive pressure decreases with increase in the relative density and approaches zero at near-theoretical density of the material. The root–mean–square strain rate sharply decreases to small values in the isothermal sintering process and increases until the curve shows a maximum for nonisothermal sintering. 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 critical cold brittleness temperature of zirconium diboride at which it transforms to plastic state was estimated and is 1345 °C.