Ceramic materials have exhibited great potential for applications as structural materials in harsh environments. However, their functionality is limited by poor fracture toughness and reliability. Therefore, researchers have been looking for effective methods for strengthening and toughening ceramics and have identified ceramic composites as a promising solution. This includes imitating the microstructures of biomaterials to realize superior composite properties.
Presently, fibrous monolithic ceramic with bamboo-like structures has been identified as a promising method for enhancing the mechanical properties of ceramics through extrinsic reinforcement. Their boundaries are generally selected according to the ceramic matrix fibers. These fiber cells and boundaries are composed of different materials with different chemical and physical properties. Therefore, heterogeneous boundaries get oxidized at high temperatures which limits their long-term use in harsh environments.
To this note, Dr. Yunfeng Su, Dr. Hengzhong Fan, Prof. Yongsheng Zhang, Dr. Junjie Song, Dr. Shuna Chen and Prof. Litian Hu from the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences fabricated the fibrous monolithic ceramics containing a single alumina phase to realize a low-cost and high oxidation resistance composite. Their work is currently published in Journal of The American Ceramic Society.
In brief, the fibrous monolithic ceramic architecture was prepared using nano-sized Al2O3 as polycrystalline fiber cells and micro-sized Al2O3 as boundaries to form the microstructures. Next, hierarchical architectures derived from the difference between the grain sizes of the fibers and boundaries were introduced to enhance the self-toughening and reliability of the Al2O3. Furthermore, the fracture behaviors and tribological properties of the resulting ceramics were investigated.
The authors observed that the single-component and complex structure fibrous monolithic ceramics demonstrated high resistance to fracture and reliability. This was attributed to the extrinsic reinforcement derived from hierarchical architectures as well as the influence of the grain boundaries on the mechanical properties. Additionally, the fibrous monolithic ceramic was presented in three distinct surfaces induced by the anisotropic pressing direction and fiber alignment during preparation. Whereas the first two surfaces exhibited laminated structures, the fibers on the last surface comprised of oval cross-sections.
It was necessary to conduct the fracture toughness tests. Deformation occurred before fracture failure in the fibrous monolithic ceramic at about 0.038 mm as compared to that of 0.023 mm of the monolithic Al2O3. The material also exhibited the load redistribution behavior and higher fracture toughness. On the other hand, the fibrous monolithic ceramic recorded a lower loading rate during the stable crack propagation stage as compared to the monolithic Al2O3. This justified the high fracture resistance observed in the fibrous monolithic ceramic.
Furthermore, the friction coefficients of the material coupled with C/C were investigated at room temperature, 800°C, and alternating temperature environments. For the two materials: fibrous monolithic ceramic and monolithic Al2O3, the friction coefficients were recorded to be 0.30 and 0.35 at room temperature and 800°C respectively. Despite their structural difference, they exhibited similar tribological properties since they were composed of identical materials.
Based on the findings, Dr. Su the first author in a statement to Advances in Engineering highlighted that the fibrous monolithic ceramics with single alumina exhibited the potential to work in alternating temperature environments which makes them ahead of others in terms of material choice in such applications.
Su, Y., Fan, H., Zhang, Y., Song, J., Chen, S., & Hu, L. (2019). Fibrous monolithic ceramic with a single alumina phase: Fracture and high‐temperature tribological behaviors. Journal of The American Ceramic Society, 102(8), 4399-4404.