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.