Our research aims at understanding sintering mechanisms in both conventional and field assisted sintering techniques. A major challenge in the recent years has been to develop an experience and specific devices for a comprehensive study of various field assisted sintering techniques. Besides their ability to ensure very fast heating, these techniques enable heating by direct interaction between the electromagnetic field and the powder compact. This offers opportunities for enhancement or inhibition of internal sintering mechanisms. Exploring these possibilities was one of our major concerns.
A midterm objective is to apply these techniques, in particular their selectivity, to develop the co-sintering of multimaterial parts in metal-metal, ceramic-ceramic and metal-ceramic systems.
Pore localization in Cu-Cr sintered composites and contact (left), and local grain boundaries and interfaces in WC-Co composites (right).
Experimental investigations uses classic and optical dilatometry, completed by other techniques such as differential thermal analysis and thermos-gravimetry for conventional sintering. Part of the research is driven by industrial needs, often throughout long term partnership. Among them: liquid phase sintered WC-Co, silver base and Cu base composites for electrical contacts and NdFeB magnets. We also work on the processing of materials with controlled porosity for various applications (fuel cells, heat exchangers, bone implants). Conventional sintering data provide references for identifying distinctive effects of field assisted sintering techniques. Modelling efforts include finite element analysis to predict dimensional changes and internal stresses, discrete element simulation to investigate particle interactions and analytical models to identify sintering mechanisms.
Direct induction sintering (left) and microwave sintering (right) devices
A specific device for direct induction heating of compacts has been developed, enabling heating rates up to 1000°C/min. It allowed finding out original effects due to electric current in Ni and to high heating rate in Ag base alloys.
Microwave sinterin is particularly adapted to ceramics, which dissipate heat from microwaves through dielectric losses. The device was recently equipped to perform optical dilatometry and modified to allow direct heating (without “susceptor”) of ceramics and even metals. Sinter forging is also possible. Recent results include the direct heating of pure alumina, while all previous works needed a susceptor or a significant amount of additive elements, the evidence of specific microwave effects in alumina sintering in early stages of densification (150°C shift in starting temperature), and the strong effect of microwaves on the g à a phase transition. Using MgO doped alumina, we showed that microwaves mainly impact mechanisms at grain boundaries.
FEM modelling is used to calculate the electrical field in the cavity as function of process parameters (sample geometry and properties, insulating parts, susceptor, if any) and thus optimize the process. Coupling with thermal and mechanical (shrinkage) phenomena is in progress.
Effect of microwave on the gàa transition during sintering of g alumina (left); time incubation vs. Joule power E2seff in flash sintering (right)
Flash Sintering, which is observed in ionic conductive ceramics, leads to abrupt increase in current with densification in a few seconds when a field is applied to a preheated ceramic sample. Associated with LEPMI lab and using yttria doped zirconia and alumina-zirconia microcomposites, we showed that effective conductivity seff and electric field E, through the Joule power E²seff, drives the phenomenon, which can mainly be attributed to electro-thermal runaway. Estimated temperatures were confirmed by microstructural observation.
Written by Charles Josserond
Date of update June 28, 2018