Rapid processing of tailored ceramic components – CERAPIDE

After having obtained the processing conditions for monolithic samples of simple shape, we are working on increasingly complicated components, monomaterials of complex shape, possibly including porous zones, bimaterials of more or less complex geometry, possibly including zones with composition gradient, and finally, some very complex parts intended to validate the methodology developed and the experience acquired. The model materials chosen are alumina and zirconia. At each step, from simple parts to target parts, we study the influence of the parameters of each process (composition and rheology of the paste, printing strategy, thermal cycle, and configuration of the microwave cavity) on the dimensional and microstructural characteristics of the final component and we optimize these parameters to achieve the targeted characteristics (geometry, composition, density, gradients, healthy interfaces). Multiphysical modeling (electromagnetic/thermal/mechanical) of microwave sintering is being developed to assist in understanding and optimizing sintering experiments, with the objective of obtaining homogeneous or selective heating and to limit internal stresses. The microstructure of raw and sintered materials is characterized by different methods: scanning electron microscopy, combined with FIB preparation and EDX composition analysis, X-ray diffraction to estimate residual stresses, X-ray microtomography for 3D imaging of architectures and possible defects.

Single-material parts of different geometries and sizes were produced by Robocasting and sent to the partners after thermal treatment for consolidation. The formulation and shaping of ceramic pasts have been modified to increase the homogeneity and reduce the proportion of organic elements that may cause defects to appear during drying. In addition, the percentage in the volume of dry matter (ceramic powder) has been increased to obtain increasingly dense and flawless parts. For the moment, pastes with 44% ceramic volume are printable and parts with a density greater than 95% can be produced in several geometries.
With regard to sintering, work was devoted to optimizing the operation of SIMAP 2.45 GHz microwave oven and studying the direct microwave heating (without susceptor) of various single-material parts of alumina, zirconia or alumina-zirconia mixtures, obtained by matrix compression or printed by robocasting. It appeared that the behavior of each material was strongly related to its dielectric properties, which were very different between alumina and zirconia, and that direct heating was problematic in both cases. Then, the effect of the size and shape of the parts was studied. In particular, it appeared that hollow cones could be sintered homogeneously, provided that their position and environment in the microwave cavity were optimized.
Numerical modeling of sintering combines three «physics«, electromagnetism, thermal transfer, and mechanics (including free sintering deformation). The many necessary material parameters were collected in the literature or measured. In terms of microstructure characterization, we have implemented experimental protocols to analyze the surfaces and volume of materials.

Various designs of single-material parts have been made by robocasting thanks to the improvement of the rheological properties of ceramic pastes and the increase in the dry matter content. The work carried out on the printing parameters has also made it possible to avoid problems such as paste accumulation causing the defect, or crack aparition in the parts at the interface between the support and the printed parts. We will continue to work on the formulation of pastes to improve their homogeneity and behavior during printing and drying, particularly for bimaterial parts.
The work carried out on microwave sintering showed that, except for samples of simple geometry and precise composition (in particular mixtures of the two powders in certain proportions), direct sintering posed many problems (thermal runaway and thermal gradient for zirconia, absence of coupling at room temperature for alumina). In the following, we will move towards indirect or hybrid heating by using susceptors with their geometry and position optimized thanks to modeling.

The first results of the project have been presented in several national and international conferences:
- Forum Young Ceramists Additive Manufacturing, Mons, April 2019
- National Symposium POUDRES2019, Grenoble, May 2019
- Conference XVI ECerS, Turin, June 2019
- Symposium Biomat Santé 2019, La Grande Motte, June 2019
- Conference EUROMAT 2019, Stockholm, September 2019

CERAPIDE project will investigate the feasibility of fabricating ceramic components with complex shape, composition and architecture by coupling two processes : forming by robocasting-type additive manufacturing and consolidation by microwave sintering. Robocasting is a 3D printing technique that can provide complex multimaterials parts by successive deposition of layers of powder-filled pastes. Microwave sintering uses the coupling of ceramic materials with an oscillating electro-magnetic field. It is attractive because it is quick, it has a low energy consumption and it may create specific microstructures. We will first find the process conditions of monomaterial parts with simple shape. Next we will work with more and more elaborate components, monomaterial with complex shape with possibly porous zones, biomaterials with more or less complex geometries and possibly composition-graded zones. Finally, we will fabricate very complex target parts so as to validate the developed methodology and value the gained experience. Model materials such as alumina and zirconia have been selected. At every step, from the simple parts to the target parts, we will study the influence of the parameters of each process (paste composition and rheology, printing strategy, thermal cycle and arrangement of microwave cavity) on dimensional and microstructural features of the final component and we will optimize these parameters to reach targeted characteristics (geometry, composition, densities, gradients, sound interfaces). Fabricated architectures will use as much as possible the capabilities of robocasting and they will benefit from selective heating that can be obtained with microwave heating. A multiphysics model (electromagnetic/thermal/mechanical) will be worked out to help in understanding and optimizing sintering experiments with the objective of obtaining homogeneous or selective heating, according to the targeted architecture, and of limiting internal stresses. The microstructure of green and sintered materials will be characterized by miscellaneous methods : scanning electron microscopy with possible sample preparation by FIB technique and composition analysis by EDX, X-ray diffraction for estimating residual stresses and X-ray microtomography for 3D imaging of architectures and defects. Already identified applications are in the field of energy (SOFC cells, piezo-electrics), biomedicine (bone implants, tooth prostheses), chemistry (reactor membranes) and, more generally, any area where mono- or multi-materials ceramic components with complex architecture are required.