Designable density defect influencing the mechanical property of 3D needled C/SiC composites

Abstract

Density defects, which refer to the uneven distribution of density in materials, are widely present in C/SiC composites prepared by CVI method, causing a degradation in mechanical properties. A facile route was designed, which masked graphite sheets on the preform and then generated density defects to investigate mechanical properties of 3D needled C/SiC composites containing different Pd (Pd, namely proportion of defective region length Ld to whole sample length Ls). Tensile strength and modulus of the specimen steadily declined until the Pd reaches 30% and tended to be stable over 72.1% and 71.6% as Pd exceeded 30%, respectively compared to the initial data. Density defects of 3D needled C/SiC composites have a greater impact on compressive properties than tensile properties. Compressive strength dropped by 11.2% when the Pd equaled to 10%, which was 1.5 times that of tensile strength. When the Pd reached 30%, the compressive strength was exposed to a greater reduction and then dropped slightly as the Pd further increased. The modulus of tensile and compressive specimens dropped by almost identical degree with the increase of Pd, ultimately falling to 73.1% of the origin value, indicating that the density defects have little effect on the stiffness of the specimen.