This work explores the additive manufacturing (AM) by digital light processing (DLP) of ultra-high-temperature ceramic (UHTC) materials. Both direct and indirect DLP are used in combination with pressureless Spark Plasma Sintering (SPS) as a means for producing complex UHTC 3D parts containing internal channels. For direct DLP, ceramic powders were mixed and stably suspended in photo-curable resins and, subsequently, used to produce the desired parts after a careful optimization of the printing parameters.
Firstly, a mixture of the selected UHTC, ZrB2, and MoSi2, which was used as a sintering additive, was prepared. The homogeneously mixed powders were then suspended in a commercial photocurable resin with the aid of an appropriate dispersant. The aim was to obtain a mixture containing as high a proportion of ceramic powders as possible, but with a low enough viscosity to enable additive manufacturing by DLP. However, the high refractive index and ultra-violet absorption coefficient of the ZrB2+MoSi2 mixture dramatically limited the maximum suitable solid loading attainable in the suspension and the maximum curing depth. Nonetheless, the parameters of the DLP process were optimized enabling the deposition of the desired part.
As an alternative process to be compared with the direct method just described, a castable photocurable resin was used to produce a negative mold of the desired part into which a high solid loading aqueous suspension of the ceramic powder mixture was poured.
After printing or casting the desired parts, debinding thermal treatment was optimized to ensure a complete and clean burn of the resin. Consolidation of the ceramic parts was carried out using a fast and energy-efficient sintering technology, namely, SPS, in order to maximize their densification and improve their thermo-mechanical performance. The use of a suitable graphite crucible avoided the application of undesired load onto the complex 3D parts and a pressureless sintering process.
Finally, a thorough microstructural and mechanical comparative analysis of the parts obtained through these AM processes was carried out.