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Spark Plasma Sintering (SPS) is an innovative sintering technique, whereby many of the beneficial effects of this process on sintering are still elusive. To allow for the detailed investigations of the SPS process, a custom experimental set-up and a corresponding finite element (FE) model was developed. The miniaturised setup allows for very high current intensities, custom pulse patterns, a wide pressure range and dilatometric measurements. The FE model was employed to calculate the temperature field in the set-up and the sintering specimen itself. A very good correlation of the temperature, current and voltage over the entire process was observed. Our investigations show that the contact conductivities have a significant impact on the process temperature. Also, the imperfect contacts at the interfaces between the graphite foil and the real specimen may lead to a significant variance of the currents necessary to obtain the desired sintering temperature. [ABSTRACT FROM AUTHOR]

The mechanisms of densification during spark plasma sintering (SPS) of spherical copper particles are investigated both experimentally and analytically. Experimentally measured densification rates are compared to expected contributions to densification coming from Coble creep, Nabarro-Herring creep, power-law creep, and pressure-assisted sintering described by the two-particle model. The results indicate densification by Coble creep or a low-temperature version of power-law creep in the investigated range of temperature and pressure at a relative density of 70%. The findings are supported by (i) activation energies obtained for various conditions using the Dorn-method as well as (ii) the pressure dependence of the densification rate. [ABSTRACT FROM AUTHOR]

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 particle–particle 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. [ABSTRACT FROM AUTHOR]

In this study, the evolution of the electrical resistivity of metal powders during densification and the resulting current flow through punch, powder compact, and die is investigated. The evaluation of the accompanying Joule heating identifies the graphite punches as main heating element providing more than 90% of the heat. The high electrical resistance of the punches and the low resistance of the graphite die as parallel electrical load to the specimen determine the current flow in the tool. For powder particles with intact oxide layers, virtually no current flows through the compact. On the other hand, for a powder resistivity below 10 − 3 Ωcm about 50% of the current flows through the compact. This fraction is constant despite further decreasing resistivity of the compact during densification. A constant current through the specimen has important implications for the microscopic temperature distribution and the understanding of the so-called 'spark plasma effects'. [ABSTRACT FROM AUTHOR]