Multiscale microshaping of functionally graded polymer-derived SiCN ceramics

Polymer-derived ceramics (PDCs) exhibit excellent properties and are compatible with many shaping techniques due to the liquid character of the preceramic polymers (PCPs). Furthermore, their properties are adjustable by modifying the precursor composition and controlling the processing conditions. In this thesis, poly(methylvinylsilazane) (PMVSz) is chosen as the precursor basis and varied amounts of divinylbenzene (DVB) are added to increase the carbon content, which significantly increases the electrical conductivity of the silicon carbonitride (SiCN) obtained after pyrolysis. Based on molding and pressureless processing, samples are manufactured with varied homogeneous composition and functional properties as well as with engineered property contrasts, so-called functionally graded ceramics. Starting with shaping aspects, several mold materials, mold fabrication techniques, and the casting and thermal processing are discussed in the first part. The liquid precursor formulation is directly pipetted into the molds, either CNC-machined PTFE for bulk samples or anisotropically etched Si for microsized parts. Thermally cured bulk green bodies (GBs) are removed from the PTFE molds before pyrolysis whereas the monolithic and resilient character of Si allows for an in-mold pyrolysis which eliminates the need for mold release of fragile GBs. While the Si molds define the parts' general shape, sacrificial 3D microstructures are optionally added onto the mold bottom by two-photon polymerization (2PP). After pyrolysis, the PDC parts are obtained loose in the mold with sub-µm resolution features replicated as imprints. The second part focuses on the processing conditions and the tuning of the PDCs' bulk properties. Thanks to a thermal initiator, the PMVSz-based precursors are cured without pressurization within 30 min at 200°C. The pore-free GBs, filled with 0-60 wt.% DVB, are converted to SiCN with varied carbon (C) concentration by pyrolysis at 800-1400°C under Ar atmosphere. Dense and defect-free PDCs are obtained whose free carbon content is adjusted in the range 15-25 wt.%. The influence of the pyrolysis temperature on the ceramics' microstructure is analyzed and correlated to the electrical conductivity, tunable across 10 orders of magnitude up to 0.4 S/cm. Very high characteristic flexural strength of up to 1.65 GPa is observed and the SiCNs' cytocompatibility demonstrated. Thirdly, PCPs with and without DVB-filling are combined by consecutive casting to fabricate functionally graded SiCN parts. The dissimilar precursors are joined in vertical or lateral arrangement in dedicated PTFE molds, typically with an intermediate thermal curing step. Monolithic and defect-free property-contrast PDC parts are obtained after pyrolysis which exhibit seamless transitions and no signs of delamination at the interface. The casting order and extent of intermediate curing are varied, showing that composition gradients are adjustable from sub-µm to mm-range transition lengths. Significant contrasts in C-concentration and consequently electrical conductivity are observed within monolithic samples entirely consisting of SiCN. Finally, a cm-scale plate with two seamlessly integrated contrast regions is designed and fabricated. The all-SiCN parts have an insulating matrix and two conductive feedthroughs. An LED connected to the areas with higher C-concentration is powered through the ceramic and lit up