Effect of CVI-induced porosity on elastic properties and mechanical behaviour of 2.5D and 3D Cf/SiC composites with multilayered interphase

The effect of CVI-induced porosity on mechanical behaviour of Cf/SiC composites possessing 2.5D (woven) and 3D (NOOBed) architectures with multilayered (PyC/SiC)n=4 interphase has been investigated. X-ray computed micro-tomography has shown that inter-bundle and inter-fibre pore distribution is influenced by fibre architecture. Elastic constants measured by ultrasound phase spectroscopy have revealed greater anisotropy in 2.5D Cf/SiC composite than 3D Cf/SiC composite. Owing to negligible Cf fraction and flattened inter-bundle pores distributed in thickness direction of 2.5D Cf/SiC composite, the elastic constant, C11 (∼15±0.03 GPa) is <1/6th of C22 and C33 obtained along fibre directions. Despite the flexural strength of 2.5D Cf/SiC composite (∼335±8 MPa) being ∼24% higher than 3D Cf/SiC composite, a more equitable distribution of Cf along three mutually orthogonal directions leads to higher fracture toughness of 3D Cf/SiC composites (∼19±1 MPa-m1/2) and greater damage tolerance. Porosity distributions having influence on crack paths, also affect both strength and fracture behaviour.