Bulk specimens of precursor‐derived silicon carbide (SiC) suitable for mechanical‐property measurements were prepared from allylhydridopolycarbosilane (AHPCS), which is a commercially available, hyperbranched polycarbosilane. Crack‐free pellets were obtained by cold‐pressing mixtures of finely ground, 1000°C pyrolyzed, “AHPCS‐SiC” with neat AHPCS, followed by pyrolysis to 1000°C and ten subsequent reinfiltration/pyrolysis steps with the neat liquid AHPCS. Then, these pellets were heat‐treated to 1200°, 1400°, and 1600°C, followed by additional reinfiltration/pyrolysis cycles to the final respective maximum temperatures. This fabrication process simulated the production of the matrix phase for ceramic‐matrix composites via successive infiltration/pyrolysis cycles. The density of the material processed at these temperatures, measured via the Archimedes method, was 2.3, 2.5, 2.6, and 2.9 g/cm3, respectively, and the average open porosities of the samples were 2, 0.2, 1, and 9 vol%, respectively. The fracture toughness was measured using the single‐edge V‐notched‐beam method, and the hardness was measured via Vickers indentation. The samples had an average toughness of 1.40 ± 0.08, 1.65 ± 0.09, 1.67 ± 0.07, and 1.46 ± 0.08 MPa·m1/2 for the samples that were treated at 1000°, 1200°, 1400°, and 1600°C, respectively. The Vickers hardness for these samples, measured at a load of 1000 g, was 12 ± 1, 13 ± 2, 11 ± 1, and 9 ± 1 GPa, respectively.

A hyperbranched polycarbosilane of the type [R3SiCH2−]x[−SiR2CH2−]y[−SiR(CH2−)1.5]z[−Si(CH2−)2]l (R = H, –CH2CH=CH2, or OR) has been prepared, which was used as a source of inorganic/organic hybrid materials and, through pyrolysis, as a precursor to inorganic solids with unusual microstructures and properties. A partially allyl-substituted derivative “AHPCS”, nominally ['Si(allyl)0.1H0.9CH2']n, has been extensively studied as a precursor to silicon carbide (SiC) and is now used commercially as a SiC matrix source for C- and SiC-fiber-reinforced composites and binder for particulate ceramics. The alkoxy derivatives, ['Si(OR)2CH2'], (R = Me, Et) yield, after hydrolysis and condensation, carbosilane/siloxane gels with unusually high surface areas (700–900 m2/g) and microporosity that is retained in the resultant SiOxCy ceramics formed after pyrolysis to 1000 °C. The fully condensed ['Si(O)CH2'] gel in the latter case was obtained as thin, adherent films on Si surfaces by spin coating and was found to exhibit dielectric constants as low as 2.0 after heating to 400 °C. The SiC precursor, AHPCS, has also been used recently, along with other polymeric precursors, to make two-phase (SiC/C and SiC/BN) amorphous ceramics that exhibit unusual microstructures and thermal/mechanical properties. These microstructures are formed during the mixing and thermosetting of the constituent polymers, which undergo phase separation due to their immiscibility. Certain of the SiC/C composites, which have the C phase uniformly distributed as ca.1-µm droplets in a SiCx matrix, exhibit high oxidation resistance, and microindentation tests on the SiC/BN system suggest unusual toughness.

A 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.

No abstract