Ceraset™ H
Hafnium Modified
Similar to Si(B)CN, ceramic composites based on Si(Hf)CN show promising resistance to oxidation, but at much higher temperatures than Si(B)CN. Mujib et al. reported excellent oxidative stability for Si(Hf)CN/Cf composites (Cf = carbon fiber) to temperatures up to >2,000 deg-C with the formation of HfO2 and HfxSiOy. The HfxSiOy liquid phase at temperatures above 1650 deg-C limited the diffusion of oxygen, and the formation of HfO2 acted as a “grip” to hold the HfxSiOy in place, thereby preventing further oxidation.
It has recently, now, also been shown that single-source precursors comprising polysilazane modified with both B and Hf can also be prepared and pyrolyzed to Si(Hf)BCN ceramics that combine the lower temperature oxidation inhibition of B2O3 and SiO2 with the higher temperature oxidation inhibition of HfxSiOy / HfSiO4. Si(Hf)BCN from a Hf-modified poly(methylvinyl)silazane exhibited only 10.5% volume shrinkage and 5.9% porosity upon annealing in nitrogen at 1300 deg-C, values similar to SiC ceramics. In addition, the incorporation of both Hf and B into SiCN ceramic results in outstanding high temperature stability with respect to decomposition. The mass losses of Si(Hf)BCN annealed to 1700 deg-C in nitrogen were found to be below 5 wt%, representing a significant improvement compared to SiCN annealed under identical conditions, which exhibits a mass loss of approximately 30%.
Furthermore, Si(Hf)BCN/Cf composites exhibited enhanced hydrothermal corrosion resistance than similar, SiCN/Cf composites because of their improved corrosion kinetics due to Hf and B incorporation. Additionally, a tight carbon fiber/matrix interface was found to be of further benefit in improving the corrosion behavior of SiHfBCN/Cf composites.
In such ceramics, the presence of Hf and B within the molecular structure of the single-source precursor leads to low-temperature phase separation, as indicated by the formation of SiC4 units at 800 deg-C, which is not common in B- and Hf-free SiCN ceramics at the same temperature. Furthermore, incorporation of both Hf and B seems to facilitate crystallization upon annealing at higher temperatures and thus allows for the preparation of ultra-high temperature nanocomposites (UHTCs) with phase compositions suitable for applications at ultra-high temperatures under harsh conditions.