Design of Polymeric Si−B−C−N Ceramic Precursors for Application in Fiber-Reinforced Composite Materials

The synthesis, detailed spectroscopic characterization, polymer-to-ceramic conversion, and high-temperature behavior of a new class of polymeric precursors for Si−B−C−N composites are discussed. The title compounds [B(C 2 H 4 −SiR 1 R 2 −C 2 H 4 −SiHNH) 3 ] n (C 2 H 4 = CHCH 3, CH 2 CH 2; 5a: R 1, R 2 = H; 5b: R 1 = H, R 2 = CH 3; 5c: R 1, R 2 = CH 3 ) were designed especially for the preparation of ceramic films and fiber-reinforced ceramic composite matrixes. They are obtained in quantitative yields by the reaction of oligovinylsilazane [(H 2 C CH)SiH−NH] n ( 4 ) with tris(hydridosilylethyl)boranes of general type B(C 2 H 4 SiHR 1 R 2 ) 3 (C 2 H 4 = CHCH 3, CH 2 CH 2; 3a: R 1, R 2 = H; 3b: R 1 = H, R 2 = CH 3; 3c: R 1, R 2 = CH 3 ) in a thermally induced hydrosilylation reaction without catalyst and/or solvent and without the formation of byproducts. Ceramic yields are 83% for 5a, 82% for 5b, and 63% for 5c as shown by thermogravimetric analysis (TGA). High-temperature TGA of the as-obtained amorphous ceramic materials, carried out in an argon atmosphere, reveals a thermal stability toward degradation of the 5b -derived material 6b up to 2000 °C. In contrast, the 6a material, which was obtained from 5a, decomposes around 1850 °C. The least stable is the 6c ceramic, which decomposes at 1450 °C. The microstructure development of 6a − 6c was investigated in the temperature range of 1400−2000 °C by X-ray diffraction (XRD), indicating that preferentially crystalline α-silicon carbide is formed at 1700 °C for 6a, 1500 °C for 6b, and 1600 °C for 6c . In addition, there are less intensive reflections observed in the XRD patterns of 6a, caused by the formation of β-silicon nitride.