Densification Kinetics of Titanium Nitride Nanopowder During Nonisothermal Spark Plasma Sintering

Abstract

The time dependence for densification of titanium nitride nanopowder during nonisothermal spark plasma sintering at an external pressure of 79.2 MPa in a nitrogen atmosphere was experimentally studied under controlled heating at a constant rate of 0.833 K/s. The densification kinetics was analyzed within the continuum theory of bulk viscous flow of a porous body using computational modeling. 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 by an increase in the root-mean-square strain rate following a curve with a maximum. Prior to the onset of densification, when thermodynamic temperature reaches 783 K, a stage involving annealing of the strain-hardened matrix forming the porous titanium nitride is observed. In the temperature range of 950–1040 K, weak densification occurs, governed by plastic flow, with a linear dependence of the strain rate on stress and low apparent activation energy (35.1 kJ/mol). At higher temperatures, dislocation climb becomes the acting mechanism, characterized by a power-law dependence (n = 2) of the root-mean-square strain rate on the root-mean-square stress, with an activation energy of 280.8 kJ/mol. The activation of this mechanism at relatively low temperatures, along with the nanosized structure, is attributed to the influence of the electric field. Titanium nitride samples produced by spark plasma sintering exhibit a nanosized structure with an average grain size of 60 nm, which ensures its enhanced mechanical properties.