Lithography-based additive manufacturing of ceramics from siloxane preceramic polymers

Schmidt, Johanna Eva Maria

PhD Tesis (2018)

Additive manufacturing is a fabrication approach which offers the possibility to build complex 3D structures from a virtual model without requiring moulds or costly post-processing steps to accom-plish the final structures. Digital Light Processing (DLP) and 2-Photon-Lithography (2PL), two lithog-raphy-based techniques, represent additive manufacturing processes, which offer the highest de-gree of achievable complexity and resolution in their printed structures. Both techniques print their 3D structures by using light to polymerise photosensitive materials. Photocurable preceramic poly-mer resins offer the possibility to be shaped by both DLP and 2PL printing and are subsequently transformed into ceramic material through pyrolysis, while maintaining their predetermined printed structure. This work is divided into four parts and presents complementary approaches at the material and production level to build highly complex 3D ceramic macro- and micro-structures, all based on the printing of a photosensitive siloxane preceramic polymer. In the first part the photosensitive polysiloxane is blended with other preceramic siloxane resins, of-fering no photosensitivity but a high ceramic yield upon pyrolysis. Complicated structures with cm-sized dimensions and resolution as low as 30 µm are shaped via DLP printing and turned into SiOC macro-structures with complete shape maintenance. The blending of two siloxanes offers the pos-sibility to control and alter the ceramic yield, shrinkage, resolution and free-carbon content of the structures, while at the same time exhibiting no diminished printing capability. Detailed sinter- and mechanical properties of one of the blends was investigated in detail and at all scales and demon-strated that, while the overall shape of ceramic structures are preserved during pyrolysis, different shrinkages as well as a change in aspect ratio depending on the structural configuration can occur and has to be taken into consideration. The photosensitive polysiloxane, already used for macro-fabrication to gain SiOC structures, was al-so used in 2PL printing to fabricate structures of the same complexity at the microscale. SiOC ce-ramics with homogenous shrinkage and feature sizes as low as 800 nm were built with the help of a new printing configuration and printed support structures. The third part of this work describes a complementary approach at the processing level, when SiOC ceramic structures are fabricated with a new hybrid additive manufacturing approach, combining DLP and 2PL printing. The advantages of DLP, the free standing and easy handling of macro-dimensional structures, are joined with the resolution capability of 2PL printing. Precisely positioned 3D structures with sub-µm sized features on top of cm-sized structured components were printed. In the final part the polymer processing capability of preceramic polymers and their transformation into a reactive ceramic phase upon pyrolysis is exploited. Instead of producing pure SiOC ceramics, the photocurable siloxane preceramic polymer is combined with alumina powders to develop a new ceramic phase, mullite, upon sintering. The phase transformation at low sintering temperatures de-veloped the new mullite phase within the 3D structure, fabricated due to the photosensitive capabil-ities of the siloxane via DLP printing. Due to the complementary approach in this work, 3D ceramic structures have been fabricated at the macroscale (DLP), microscale (2PL) and multi-scale (Hybrid additive manufacturing; DLP + 2PL) on basis of a photosensitive preceramic polymer. Different ceramic materials, SiOC and mullite, have been produced from the polysiloxane thanks to its transformation capability into SiOC ceramic and reactive SiO2 phase at high temperatures. Through the addition of passive and active fillers complex, dense, pore- and crack-free ceramic structures with no sign of delamination and complete mainte-nance of shape have been developed with varying properties.