Analysis of ceramics toughened by non-conventional fiber reinforcement

Many refractory materials that are not readily available in fiber form, including ultra-high temperature ceramics (UHTC), are attractive candidates for use in high temperature structural components. This work explores the possibility of using non-conventional fiber forms that can be fabricated by “fibrous monolith” techniques to design composites with high strength and toughness along with 2D isotropy. The use of low-aspect ratio bone-shaped short fibers (BSSF) to improve fracture toughness and the use of composition tailoring to increase fiber strength were analyzed and it was found that both concepts need to be used in combination to achieve significant toughening. Computational models using UHTC as model materials indicate, for example, that significant improvements in fracture toughness can be realized with an aspect ratio of just 15, but only if the fiber strengths can be raised to 1.5 GPa. The use of a single outer layer of lower thermal expansivity composition is predicted to increase low temperature strength by a factor of 2, while multilayers of reasonable thickness (10 μm) result in strengthening by a factor of 3. For UHTCs, processing improvements that reduce flaw sizes will be necessary to take advantage of these results, but considerable improvement in properties can result from such progress.