Fundamental principles of spark plasma sintering of metals: part II – about the existence or non-existence of the ‘spark plasma effect’

Abstract

The mechanisms of densification in spark plasma sintering (SPS) were investigated both analytically and numerically for a model system of two spherical metallic powder particles. From the microscopic temperature distribution, the possibility of a micro-local overheating of the particleparticle contacts was analysed for different particle sizes, contact geometries, materials, and electrical loads. It is shown that, for particles below the size of one millimetre, local overheating is below one Kelvin. Subsequently, the material transport by thermomigration, electromigration, and diffusion driven by surface curvature and external pressure was derived from microscopic field distributions obtained from analytical calculations and finite-element simulations. The results show that, while the mechanical pressure accelerates material transport by orders of magnitude, the electrical current and the temperature gradients do not. It is also shown that pulsing the current has no significant influence on the densification rate.

Thermo-electro-mechanical modeling of spark plasma sintering processes accounting for grain boundary diffusion and surface diffusion

“…Analytical models [9,11,13,[18][19][20][21][22] play a key role in understanding of the sintering process, but drastic simplifications have to be assumed (e.g. oversimplified geometry, isothermal sintering and neglected grain growth) that limit their ability to quantitatively predict the densification of real powder compacts.…”

A critical review on spark plasma sintering of copper and its alloys

“…It is believed that in works that have proposed excessive local overheating, a fundamental factor, which is heat dissipation by conduction inside the particles, has not been considered and the calculations have been performed in adiabatic conditions. Taking into account this factor in the calculations, the amount of temperature distribution within the particles in copper [104], TiAl [105], bismuth telluride [106], and steel particles [74] was evaluated. It was found that although the current density at the contact points of the particles can reach up to several thousand times higher than its average value in the sample, the heat conduction phenomenon is high enough to prevent a significant temperature gradient inside a particle [105].…”

“…In addition, temperature inhomogeneity inside the particle is directly proportioned to grain size, which means the finer the grain, the smaller the temperature difference between particle contact and particle center, and vice versa. Figure 6 demonstrates the effect of particle size and contact radius on the temperature inhomogeneity inside the Cu particle, which is called supertemperature [104]. As shown in Fig.…”

“…This process is repeated intermittently until the compact achieves full densification. As a result, it causes homogeneous grain size distribution, as well as restricts grain Semenov et al [104,[134][135][136] modeled the SPS process by thermo-electro-mechanical approach. Two limiting conditions have been investigated (i) low mechanical pressure and (ii) high mechanical pressure.…”

B4C Composites with a TiB2-C Core–Shell Microstructure Produced by Self-Propagating High-Temperature Synthesis-Assisted Spark Plasma Sintering

“…Nevertheless, the use of pulsed DC current is believed to facilitate electric discharges at the points of powder contacts and grain boundaries, making any gaps in the compact act very similar to capacitors and hence fusing them into a monolithic solid at a faster rate when compared to conventional sintering methods . However, this is a topic of debate, and there is evidence as for negligible influence of pulsing rates and sparks . Furthermore, one must be very cautious when referring to sintering temperatures in SPS systems, given that it is not possible to measure the exact temperature of the powder compact at the time of sintering through direct measurements by a pyrometer or a thermocouple, especially at higher temperatures.…”