Elevated temperature flexure behavior of continuous carbon fiber reinforced ZrB ₂ –ZrSi ₂ ultrahigh temperature ceramic matrix composites

Ultrahigh temperature ceramic matrix composites (UHTCMCs) were fabricated from unidirectional prepreg tapes consisting of a matrix of ZrB 2 with 5, 10, and 15 vol.% ZrSi 2 additions and continuous polyacrylonitrile carbon fibers and were densified at 1600°C in a hot press. The relative matrix densities ranged from 88% to 93% with interlayer spacings of ∼72 µm and fiber volume fractions between 30% and 36%. Phenolic resin additions were utilized to react with ZrSi 2 acting as a transient sintering aid to form ZrC and SiC phases. Elastic moduli of the UHTCMCs decreased with increasing temperature during 4‐pt flexure testing. The elastic moduli values were 95–122 GPa at room temperature and ∼26 GPa for all three compositions at 1800°C. Strength increased as test temperature increased for each UHTCMC with varying rollover temperatures for each composition. Strength started decreasing above 1400°C for the 5 vol.% ZrSi 2 composition, above 1600°C for the 10 vol.% ZrSi 2 composition and did not decrease for the 15 vol.% ZrSi 2 composition over the range of temperatures tested. The work of fracture increased from 0.3 to 1.9 kJ/m 2 at room temperature to 18 to 20 kJ/m 2 for each UHTCMC at 1800°C. All UHTCMCs displayed graceful failure beyond the initial fracture event.