Multiscale microshaping of functionally graded polymer-derived SiCN ceramics

Polymer-derived ceramics are a unique class of materials known for their outstanding properties and versatile applicability. The liquid nature of the precursors allows easy addition of fillers in order to alter the resulting ceramics’ properties. We use a polysilazane with a thermal initiator and 0 – 60wt.% divinyl benzene to vary the final free carbon content and tune the electrical conductivity. Casting of mm-sized parts is performed in PTFE molds whereas high-resolution microparts are made in KOH-etched silicon molds. Furthermore, due to silicon’s high melting temperature, pyrolysis of the preceramic material can be conducted at 1000°C directly inside the mold. This brings the additional advantage that the part is conveniently released due to its shrinkage and the typical flat and smooth surfaces of KOH-etched monocrystalline silicon. By means of direct laser writing, we have added sacrificial polymeric 3D microstructures at the mold bottom which decompose during pyrolysis and replicate into the cast PDC part at sub-micrometer resolution. Parts are manufactured with flexural strength up to 1.7 GPa and an electrical conductivity ranging from ~10-11 to 1 S/cm. Additionally, monolithic parts with a seamless composition and conductivity contrast have been obtained by subsequent casting and crosslinking of precursor formulations with dissimilar DVB concentrations. Different levels of carbon concentration profiles across the interface are achieved by varied degrees of first phase pre-crosslinking. The interfaces are visualized and their transition lengths quantified by means of synchrotron-microtomography. Our new approach of heterogeneous PDCs opens novel opportunities towards all-ceramic functionality integration for harsh environment applications.