DENSIFICATION KINETICS OF THE TiB₂–20 wt.% MoSi₂ COMPOSITE DURING NONISOTHERMAL SPARK PLASMA SINTERING

Abstract

The densification of a powder mixture comprising titanium diboride and 20 wt.% molybdenum disilicide was experimentally studied as a function of time during nonisothermal spark plasma sintering. The sintering process was assisted with an external pressure of 50.93 MPa in vacuum under controlled heating at a constant temperature increase rate of 1.67 and 2.72 K per second. It was established that sintering occurred when the thermodynamic temperature reached 1155 K, which should be taken as the critical brittle–ductile transition temperature for molybdenum disilicide, a less refractory material. The densification kinetics was analyzed using the continuum theory of bulk viscous flow of a porous body, considering the effect of powder particle shape on the rheological properties of the sintered body. In general, the sintering process is characterized by a decrease in the root-mean-square stress within the porous body matrix to the limiting zero value as it approaches the nonporous state and an increase in the root-mean-square strain rate along the curve with a maximum. Computational modeling of the densification kinetics for the powder composite, involving determination of the dependence between the activation energy of viscous flow of the composite matrix and temperature and root-mean-square stress allowed the initial, low-temperature, and medium-temperature stages of spark plasma sintering to be identified. At the initial stage up to 1220 K, the activation energy increased nonlinearly and sharply, which can be caused by active spark flashes with the formation of plasma within the loose random packing of the powder particles as a similar stage is not observed in conventional pressure assisted sintering. At the next low-temperature temperature stage, the activation energy increased as the root-mean square stress decreased. In the temperature range from 1300 to 1389 K, the activation energy for the viscous linear flow of the composite matrix was 223 kJ/mol. In the medium-temperature range from 1414 to 1485 K, the activation energy increased to 255 kJ/mol.