Physical, thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si₃N₄ whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.