Influence of Surface Microcracks on Mass Change in Hf and B Modified SiCN‐Carbon Fiber Minicomposites

Polymer‐derived ceramic (PDC) coatings on carbon fibers (CFs) protect against high‐temperature oxidation; however, thermomechanical stress and cracking during precursor pyrolysis can expose fiber surfaces, creating pathways for oxygen ingress. In this study, a boron‐ and hafnium‐modified polysilazane precursor is used to prepare PDC coatings on CF single plies via a modified polymer infiltration and pyrolysis (PIP) process. Rather than studying high‐temperature stability limits of SiHfBCN‐coated CF composites, we track macroscale mass changes and localized crack evolution. Up to three PIP cycles for fabrication and an oxidation temperature of 550°C were used as a model system to investigate defects in PDC coating, oxygen ingress, and CF degradation. The relationship among coating layers, crack width, crack spacing, ceramic mass loading, and fiber mass retention is investigated using electron microscopy. After oxidation at 550°C for over an hour, SiHfBCN‐coated samples exhibited the lowest average crack width of 5.6 ± 1.5 µm and the highest crack spacing of 38.5 ± 10.9 µm compared to SiBCN and SiCN coatings. At 36% ceramic mass loading, SiHfBCN retained 25.7% of the fiber mass, compared with 12% for SiCN, 9.2% for SiBCN, and 2.9% for uncoated CFs. These findings provide mechanistic insights into coating defects, microcrack evolution, and CF degradation in PDC‐CF composites.