The ZrB2–SiC composition shows the best performance of the three materials studied, both in terms of strength and oxidation resistance. This material presents an exceptional room temperature compressive strength of 3.1 ± 0.2 GPa and a high temperature strength of 0.9 ± 0.1 GPa at 1400 °C, and can withstand exposures up to 24 h in air at 1400 °C before its compressive strength falls significantly when compared to the as-fabricated material.
It has been shown that the addition of C to a ZrB2–SiC composite is detrimental to the high temperature mechanical properties, since the degradation in strength and oxidation resistance observed counters any possible improvement to the thermal shock resistance. The ZrB2–SiC–C composition is probably not suitable for application in air if no protective coating is applied. The oxidation process has been shown to be diffusion controlled and the application of a diffusional barrier is probably needed.
As-fabricated SCS-9a SiC fiber reinforced ZrB2 presents significant matrix microcracking due to residual thermal stresses, and therefore shows poor mechanical properties and oxidation resistance.