Polymer-Derived Si-Al-C-N-O Ceramics for High Temperature Applications

Polymer-derived ceramics are a new class of materials synthesized by thermal decomposition of polymeric precursors. The resultant materials are amorphous alloys of silicon, carbon, and nitrogen, which can be converted to crystalline materials by annealing at higher temperatures. This novel chemical-to-ceramic route offers a unique opportunity to tailor the structures and compositions, therefore the properties, of the resultant materials by designing the chemistry of the precursors. In this paper we report the studies on synthesis and oxidation behavior of polymer-derived SiAlCN materials. The precursor was synthesized by mixing a polysilazane and aluminum isopropoxide. The mixture was then pyrolyzed at 1000°C in Ar/N₂ to convert to SiAlCNO ceramics. The oxidation studies revealed that the SiAlCNO possesses a lower oxidation rate than SiCN. It is believed that the better oxidation resistance exhibited by SiAlCN is due to the oxide layer containing Al, which makes oxygen diffusion more difficult than in pure SiO₂. The materials are promising for many high temperature applications, e.g. environmental barrier coatings, high temperature fibers, matrixes for composites, and even monolithic components.