Preparation of high-temperature stable Si [sbnd]B [sbnd]C [sbnd]N fibers from tailored single source polyborosilazanes

Abstract

Bulk Si B C N ceramics derived from polyborosilazanes of the type [B(C 2H 4SiRNH) 3] n ( 1a, R = CH 3; 2a, R = H; C 2H 4 = CHCH 3, CH 2CH 2) exhibit an exceptional structural stability at high temperature. Therefore, such quaternary systems are of great scientific and technical interest as fibrous reinforcements intended for high-temperature applications. In this context, the design of novel polyborosilazanes, which display properties tailored for the preparation of Si B C N fibers, is studied. Boron-modified polysilazanes of the type [B(C 2H 4SiRNCH 3) 3] n ( 1b, R = CH 3; 2b, R = H) are prepared via aminolysis of the tris(dichlorosilylethyl)boranes B(C 2H 4SiRCl 2) 3 ( 1, R = CH 3; 2, R = H). It is shown that the functionalisation of the precursors with N CH 3 units improves their processability (i.e. solubility) compared to that of their ammonolysed analogs [B(C 2H 4SiRNH) 3] n ( 1a, R = CH 3; 2a, R = H). In addition to the influence of the N CH 3 units, the presence of the Si CH 3 functions in such polymers offers the best potential for the preparation of fibers by melt-spinning. As-spun fibers are then converted under controlled atmosphere into high-temperature stable Si B C N fibers according to the polymer-derived ceramic route.