Synthesis and Thermal Behavior of Novel Si−B−C−N Ceramic Precursors

Several boron-modified polysilazanes of general type {B[C 2 H 4 Si(R)NH] 3 } n (C 2 H 4 = CHCH 3 or CH 2 CH 2 ) were synthesized and their thermal behavior studied. In contrast to the known derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties R are substituted with lower weight groups and/or reactive entities. Reactive units enable further cross-linking of the polymeric framework and therefore minimize depolymerization during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was monitored by thermogravimetric analysis. Both low molecular weight substituents and/or cross-linking units increase the ceramic yield from 50% (R = CH 3 ) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out that, with increasing temperature, formation of α-SiC or α-SiC/β-Si 3 N 4 crystalline phases occurs at 1550−1750 °C, depending on the material's composition. The resistance of these novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.