Functionally Graded Ultra-High Temperature Ceramics

Kerstin Stricker, Hans-Joachim Kleebe

Left: SEM-BSE Image showing a SiC fiber suffered from MoSi2attack. Right: TEM-BF Image of a SiC fiber.

One of the most promising materials for extreme temperatures in the UHTCs family is ZrB2, often in combination with 20-30 vol% SiC, which is known to improve mechanical properties and oxidation resistance. When SiC is added as fiber, it also notably improve the fracture toughness. Despite its good properties in aggressive environment, ZrB2-SiC materials show low sinterability, due to strong covalent bonds, and the operating temperature is limited to approx. 1650°C, since SiC severely suffers from active oxidation in the sub-scales. On the other side, MoSi2, with a high melting point, has been disclosed to promote densification and most of all it has been proven to positively affect the mechanical properties at high temperatures. However, when MoSi2 is combined with SiC fibers in a ZrB2 matrix and sintered above 1750°C, a progressive reaction of the SiC fibers occurs, with consequent degradation of the same. To preserve the beneficial effect of SiC fibers, the diffusion of Mo-compounds must be avoided. Therefore, a new concept of a Hf/SiCN-based thin buffer layer has been introduced, acting as a diffusion barrier to prevent the SiC fibers from MoSi2 attack. In this study, a layered structure of ZrB2-MoSi2 / buffer layer / ZrB2-SiCfiber-ZrSi2 was investigated, were the buffer layer is a ZrB2 powder encapsulated in a Hf/SiCN polymer-derived ceramic. After sintering, the material system has been oxidized in air for 15 min. at 1500°C and 1650°C. State-of-the-art scanning- and transmission electron microscopy (SEM/TEM) in combination with energy dispersive X-ray spectroscopy (EDS) are powerful tools to prove the functionality of the novel designed buffer layer. It sheds light on the existing phases and thus on ongoing diffusion processes. The effective functionality of the Hf/SiCN-based buffer layer has been proven by microstructural characterization. Molybdenum has not been found in- and below the buffer layer, whereas reference samples without a buffer layer revealed the overall presence of Mo, especially in the SiC fibers. Furthermore, it became apparent that the amount of available silicium has an influence on the severity of MoSi2 attack.