Gelcasting is a net-shape forming process that was developed in the 90s at the ORNL’s lab to promote the formation of homogeneous green ceramic parts that can be of complex shape and/or large size parts. It consists in preparing a homogeneous powder suspension, containing a solvent, a dispersant, a plasticizer, a monomer, a cross-linking agent and a ceramic powder. Gelcasting suspensions have the advantage of being highly charged (> 50 %vol) while remaining sufficiently fluid to be poured into moulds of complex shape. The slurry is then "gelled" with an initiator. The polymeric network formed around the ceramic particles allows to "freeze" the suspension and to guarantee a granular stacking of quality (green density of 55-60 % for an initial powder volumic content of 50 %).

Many parameters can be adjusted to optimize gelcasting: monomer concentration, monomer/crosslinker ratio, … Among these parameters, the nature of the monomer is certainly one that can have a strong effect on the process. The first to be used in the development of gelcasting was acrylamide (AM) and since then, many other types of monomer have been tested. But until now, few works have been done on the effect of these parameters on the microstructure of the obtained green samples and their sintering behavior.

For our part, we are interested in the sintering of alumina parts with complex shapes, while seeking to optimize the sintering conditions. This is why we investigated the effect of gel casting parameters on the sintering behaviour of a reference alumina. More specifically, we aim at presenting in this communication the influence of the chemical nature of the monomer on the properties of the green and sintered samples. We considered the acrylamide family by testing 3 of them. which differ in their molecular structure and affinity with their environment (solvent and powder) : dimethylacrylamide (DMAA), methacrylamide (MAM) and acrylamide (AM)

In this study, we will present the results of the thermal evolution of the samples (dilatometry, SEM), according to the chemical nature of the monomer (AM, MAM or DMAA). Significant shrinkage differences were observed during the densification of the pre-sintered samples and very different sintered densities were obtained after sintering at 1530°C (from 88% to 98%). These different sintering behaviours were interpreted in terms of green pellet homogeneity. X-ray tomography analysis of the green pellets  revealed not only density values which depend on the used monomer but also slightly different widths of grey level distribution, which reflect differences in grain packing, linked to the nature of the polymer. These microstructural variations at the scale of a few grains (agglomerates) have likely a strong impact on the densification of these samples. Thanks to this study, we have shown that the chemical nature of the monomer actually influences the final densities and the properties of gelcasted alumina samples.

We will finally present a parametric study that allowed us to test the influence of others parameters such as the monomer content or the monomer/crosslinker ratio on the sintering behaviour.