Microwave sintering of architectured ceramics shaped by additive manufacturing
Résumé
The production of ceramics involves three main steps: preparation of raw matter (powder synthesis for technical ceramics and mixtures of raw materials for traditional ones), shaping and sintering. Conventional shaping processes such as pressing of powders or casting of suspensions have drawbacks such as use of tools or limits of accessible geometries. Sintering is the thermal treatment carried out at high temperatures (> 1000 °C) aiming at densifying the materials. This last step is generally carried out in resistive furnaces and is long (due to low heating rates and long dwell times at high temperatures). Therefore, more advanced processes such as additive manufacturing and rapid sintering processes are of particular interest to produce ceramics with reduced duration and energy consumption. Additive manufacturing enables to overcome different issues of conventional shaping processes, especially fabricating complex geometries. Among them, robocasting and stereolithography are the most studied processes for ceramics. Rapid sintering such as microwave (MW) sintering is interesting to reduce the duration of thermal treatments. MW sintering is a fast volumetric heating of the samples by direct MW/material interaction. The combination of additive manufacturing and MW sintering offers new perspectives for the minimization of material, energy and time waste. This presentation will propose an overview of different studies carried out in our laboratory about MW sintering of ceramics shaped by different AM processes (robocasting, stereolithography and fused deposition modeling). Technical ceramics (zirconia) and traditional ceramics (porcelain) were printed and sintered in an instrumented MW cavity. During sintering, shrinkage of samples and MW powers were monitored. Different geometries of samples were tested (bulk and lattice types) with millimeter-sized pellets. Then, sintering of larger samples was also investigated to study the possible upscaling of the processes. The results showed the possibility to obtain highly dense samples with fine and homogeneous microstructures. The evolution of MW powers during the thermal cycle also highlighted an influence of the architecture of the materials on the MW/material interactions. The specificity of the MW heating (heating by direct MW/material interactions) has to be taken into account in the case of sintering of an architectured material (interactions between MW and the heated object with an influence of the material and the architecture).