Preparation and characterization of HfOC/SiOC composite powders and fibermats <i>via</i> the polymer pyrolysis route

We report on the synthesis and characterization of HfOC/SiOC ceramic composite powders and electrospun fibermats, which integrate the high-temperature resilience of HfOC with the oxidation resistance of silicon oxycarbide (SiOC). The composites were fabricated through a polymer-pyrolysis route by integrating 1,3,5,7-tetramethyl, 1,3,5,7-tetravinyl cyclotetrasiloxane (4-TTCS), a precursor source for SiOC, and a commercial HfC precursor in a 1 : 1 ratio by mass. First, the HfC precursor was heated to 70 °C to drive off water molecules, followed by its blending with the liquid phase 4-TTCS and cross-linking at a moderate temperature (160-400 °C). This was followed by pyrolysis at three different temperatures - 800, 1000, and 1200 °C in an inert argon atmosphere. The composite ceramic was comprehensively characterized by the use of electron microscopy for particle and fiber morphology, X-ray diffraction for the evolution of various ceramic phases, and a range of spectroscopies to document the change in molecular vibrations or the evolution of the functional groups and molecular bonding in preceramic polymer during cross-linking and ceramization. The crosslinked polymer-to-ceramic yield for powder samples was observed to be as high as approximately 78 wt% when pyrolyzed at 800 °C, and 74 wt% when pyrolyzed at 1200 °C. The oxidation test performed at 800 °C in stagnant air for the fibermat pyrolyzed at 1000 °C indicated a linear shrinkage of 6% for the HfOC/SiOC composite. This represents an improvement over the carbon rich-SiOC fibermat which exhibited a mass loss of 71 wt% and a linear shrinkage of nearly 19%, while the neat carbon fibermat was completely burned off under similar conditions.