Microstructure and Indentation Fracture Behavior of SiC-BN Composites Derived from Blended Precursors of AHPCS and Polyborazylene

novel method of preparing multiphase ceramic materials from mixtures of preceramic polymers is described. This method employs the typical immiscibility of polymer blends to prepare two-phase SiC-BN ceramic composites by co-pyrolysis of a mixture of two polymeric precursors, allylhydridopolycarbosilane (AHPCS) (nominally, [Si (CH2CH=CH2)2CH2]0.05[SiH2CH2]0.95) and polyborazylene (PBz) [B3N3H4-x]n, which serve as the sources for the SiC and BN phases, respectively. Two different proportions of these polymers (80:20 and 50:50 wt %) were mixed in glyme solution, and, after removal of the solvent, heated to 1000 C to obtain amorphous SiC -BN ceramic composites whose microstructure resembles that of an immiscible polymer blend. Studies of the composition (by electron microprobe analysis), microstructure (by SEM), and Vickers hardness of these samples were carried out as a function of annealing temperature up to 1600-1700 C. The hardness of the SiC-BN composite was found to decrease with increasing PBz addition, with both mixtures exhibiting a lower hardness than that of the AHPCS-derived SiC alone. Particularly in case of the 50:50 SiC-BN sample prepared at 1600 C, evidence was obtained for debonding and crack deviation at the interface between the two phases, suggesting possible interest in this system as a tough two-phase ceramic composite or as a weak interphase material in fiber-reinforced ceramic matrix composites