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(C2H4−SiR1R2−C2H4−SiHNH)3]n (C2H4 = CHCH3, CH2CH2; 5a: R1, R2 = H; 5b: R1 = H, R2 = CH3; 5c: R1, R2 = CH3) 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 [(H2CCH)SiH−NH]n (4) with tris(hydridosilylethyl)boranes of general type B(C2H4SiHR1R2)3 (C2H4 = CHCH3, CH2CH2; 3a: R1, R2 = H; 3b: R1 = H, R2 = CH3; 3c: R1, R2 = CH3) 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.