The Role of Ultra-High Temperature vs High Entropy Phases in Polymer Derived Ceramics

Materials capable of withstanding ultra-high temperatures are becoming increasingly important for various aerospace applications, ranging from atmospheric re-entry shielding to aircraft brakes. While silicon-based ceramics have proven to be a key class of materials for these harsh applications, introduction of transition metals into these ceramics is required for ultra-high temperatures conditions. Preparation of ultra-high temperature ceramics (UHTCs) using traditional inorganic powder sintering methods limits the structural/compositional designs possible and is challenging for preparing composite components. Preceramic polymers (PCPs) have been used to address these challenges via the synthetic tunability and processing properties of polymers prior to pyrolysis. Following pyrolysis of the PCP, a polymer derived ceramic (PDC) is obtained. By tuning the PCP structure, curing/pyrolysis conditions, and pyrolysis atmosphere, the composition of the PDC can be manipulated for different properties.

Group IV-VI transition metals paired with C and N anions can produce ultra-high temperature ceramics (UHTCs) suitable for a range of applications. Use of multiple cations and/or anions can result in compositionally complex ceramic (CCC) or high-entropy ceramic (HEC) phases, with some compositions showing unique or enhanced properties (e.g. retained strength and hardness at elevated temperatures) relative to the individual binary ceramics. Here we report that by functionalizing a commercially available polysilazane with transition metal complexes, we have prepared polymer-metal complexes that convert to dense ceramic monoliths when pyrolyzed via spark plasma sintering (SPS).

A series of materials were prepared that, following pyrolysis, produces SiC (a high temperature ceramic), ZrCN (a UHTC), TiZrHfCN (a CCC), and TiZrHfNbTaCN (a HEC) nanocomposites. Monoliths of these systems were subjected to torch testing and controlled oxidation experiments to understand the comparative rate of oxidative degradation and the resulting compositional changes. Additionally, advanced synchrotron experiments (e.g., hard X-ray photoelectron spectroscopy) were employed to further understand the oxidation mechanism. Ultimately, we report a route to polymer derived compositionally complex UHTC nanocomposites and their behavior under extreme thermal and oxidative conditions.