William Toreki

Polymer-derived silicon carbide fibers with low oxygen content and improved thermomechanical stability

Abstract Continuous silicon carbide fibers (UF fibers) with low oxygen content (∼1−2 wt%) were prepared in a range of diameters (∼8−50 μm) by the dry spinning of organosilicon polymer solutions and subsequent pyrolysis of the polymer fibers. Room-temperature mechanical properties were similar to commercially-available Nicalon TM fibers, as average tensile strengths ≈3 GPa were obtained for some batches with fiber diameters in the range of ∼10 μm. Furthermore, UF fibers showed significantly better thermomechanical stability compared to Nicalon, as indicated by lower weight losses, lower specific surface areas, and improved strength retention after heat treatment at temperatures up to 1700°C. The structure and composition of UF fibers were also characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Auger depth profiling analysis, neutron activation analysis, and atomic absorption.

Improved thermomechanical stability of polymer-derived silicon carbide fibers by decaborane incorporation

Decaborane was investigated as a precursor for boron-based densification aid to polymer-derived SiC fibers. By means of acid-base interaction, the infusibility of polycarbosilane-based polymers upon pyrolysis was enhanced significantly. The beneficial effect of decaborane toward improved thermomechanical stability took place when decaborane-doped SiC fibers were treated at 1800°C. With the decaborane content at 2–4 wt%, the strength retention after the 1800°C treatment was enhanced as high as 80%. Elastic modulus was improved as well, partly due to increased density and crystallinity. Improved densification by decaborane resulted in the fiber density as high as 2840 kg/m3, which corresponds to 89% densification. Decaborane also increased the Weibull modulus after the 1800°C treatment, indicating an enhancement in fiber reliability.