Despite the current strong worldwide interest on the Spark Plasma Sintering process (SPS), the mechanisms at the origin of its high efficiency were still poorly understood and documented. SPS consists in densifying a powder by the simultaneous application of a pressure and a DC pulsed current. The aims of the MF2 project were to determine the physical mechanisms activated during the SPS densification and to understand the intrinsic role of the current on these mechanisms.
This project was restricted to metals and alloys getting a FCC structure (or close): the copper and the intermetallic alloy TiAl which exhibits a brittle/ductile transition at high temperature. Moreover, this latter system allows using the lamellar microstructure as a “marker” of the temperature history within the sample, which is a key point for the control of SPS. <br />Our final goal was to improve the SPS process understanding and control, and to build synthetic tools helping engineers to select suitable SPS conditions. Two exploratory studies of materials with industrial interests were scheduled at the end of the project. <br />
The project was based on complementary, specific, dedicated and up to date experimental investigations. Complementary Finite Element Modelling was performed to evaluate the temperature fields in the powder particles. Three methodological principles were used: i) systematic comparison with materials sintered by SPS and Hot Pressing, i. e. without the application of a current, ii) specific and interrupted experiments aiming at isolating the mechanisms inside SPS or dedicated experiments for their deep understanding, iii) fine microstructure analysis with adapted tools (TEM, EBSD, 3D-FIB/SEM...). Investigations were performed from atomic to macroscopic scale.
The MF2 consortium was built with laboratories, which had not really previous experiences of common collaborations. They appear to be very complementary on the following points: SPS and Hot Pressing, macroscopic approach and microscopic studies, knowledge of copper and TiAl alloys, knowledge of sintering and plasticity.
All the results obtained on Cu and TiAl indicate that the SPS densification occurs by classical mechanisms of sintering controlled by stress distribution and temperature. The rapidity of the SPS is due to the heating process by Joule effect. No specific mechanisms resulting from the current application were detected.
In fact the densification occurs by a heavy plastic deformation localized at the necks between particles. Recrystallization and solid diffusion phenomena are also activated.
In perspective, it would be interesting to work on the very first moments of the densification cycle, when the contacts between the powder particles are almost reduced to contact points. When the contact area between the powder particles tends to zero, the electric field and the current density mathematically diverge, and are therefore dramatically intensified. Thus, electrically-induced mechanisms, still very poorly described, are expected locally, at the contacts between the particles. To maintain quasi point contacts, the major idea of this project is to use the SPS in a non-conventional way, i. e., with very low loads, in order to limit the plastic deformation of the contacts. Although a large number of studies which have been devoted to the investigation of the SPS mechanisms, in most cases they have not been carried out in the suitable conditions we propose. A model (Chaim, 2013) in the literature has defined a set of conditions for the intensification of the electric field in ceramics, which have been verified in a limited number of cases, mainly oxides (MgO, Al2O3, YAG) and on LiF. Regarding metals, only indirect proofs of plasma formation or of local overheating (as observation of melted zones), are provided in the literature. The intensification of the electric field on one hand, and of the current density on the other hand, could then be investigated, with ceramic (alumina-zirconia, WC) in the first case, and metallic (W, Ni) powders in the second. It would be useful to precise the conditions for the apparition of these phenomena, to study whether they are localized at only few contacts or generalized at the entire powder, to find the conditions which favor them, and finally to master them for establishing new SPS cycles, allowing the development of original materials. Regarding applications, HfB2 and lamellar TiAl, which are important materials in the aerospace industry, could be developed, based on the proposed ideas.
Since these studies correspond to the launch of new experimental activities in our laboratories, it took about one year to achieve the first results. Consequently, the main parts of the results have yet to be published. Joined publications between the var
Spark Plasma Sintering (SPS) is one of the most attractive techniques for producing innovative materials with a moderate cost and, for some cases, enhanced properties. It consists in simultaneous applying a pulsed current of high intensity and a uniaxial pressure to a powder, and allows producing materials 10 to 100 times faster as compared to classical powder metallurgy techniques. Nevertheless, the elementary and underlying mechanisms involved in the SPS are still poorly documented and understood.
What are the physical mechanisms activated during the SPS densification?
Why is SPS sintering so fast?
What is the intrinsic role of the current on these mechanisms?
This project is thus aimed at understanding the effect of the current and at explaining why the SPS is so rapid and efficient. It is focussed on the neck formation and growth, on the grain coarsening and on the densification stage. Particular attention will be paid to the intrinsic role of the DC pulsed current and to its induced effect resulting in fast heating. In addition to the mechanisms quoted in the literature, like plasma formation and mass transport (diffusion, electro-migration), metallurgic transformations, including plasticity and recrystallisation, will be considered. Our final goal is to improve the SPS process control and to build synthetic tools helping engineers to select suitable SPS conditions.
In this project, we have decided to work only on metals and alloys getting a FCC structure (or close to for TiAl). Two metallic systems, Ti/Al and Cu/Ni, will be investigated. Works on Cu and Ni pure metals will be privileged and the use of TiAl and CuNi alloys will be restricted to specific studies.
The validity and the pertinence of the acquired knowledge will be checked through the fabrication of a TiAl alloy suitable for the low pressure stages of aircraft engines and through the densification of nanostructured Ni large samples. For reasons of project size, this study will not involve any systematic analysis of the processing conditions, and will be restricted to metallic materials.
The project is built on:
• the comparison between materials fabricated by SPS and by conventional routes (High Pressure and Dilatometry).
• the comparison of “classical” SPS specimens with SPS specimens electrically protected using alumina liners.
• the study of the pulsed current effect.
• the realisation of interrupted and specific experiments and of sintering under extreme conditions.
• the development of dedicated experiments for the study of mechanisms.
• the characterisation of the involved mechanisms at various scales.
• the coupling of modelling and experimental studies.
The project clusters four laboratories, which possess three complementary SPS machines, two conventional sintering techniques (Dilatometry and High Pressure), all the necessary characterisation facilities and recognised expertises in relevant fields.
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) - DELEGATION REGIONALE MIDI-PYRENEES (Divers public)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) - DELEGATION REGIONALE MIDI-PYRENEES
CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE (CNRS) - DELEGATION REGIONALE CENTRE-EST
INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON
INSTITUT NATIONAL POLYTECHNIQUE DE GRENOBLE
Help of the ANR 649,896 euros
Beginning and duration of the scientific project: December 2011 - 48 Months