Tensile and thermal properties of chemically vapor-infiltrated silicon carbide composites of various high-modulus fiber reinforcements

Chemically vapor-infiltrated (CVI) silicon carbide (SiC) matrix composites are candidate structural materials for proposed nuclear fusion and advanced fission applications due to their high temperature stability under neutron irradiation. To optimize the thermal stress properties for nuclear applications, CVI-SiC matrix composites were produced with three-dimensional (3D) fiber architectures with varied Z-fiber content, using the highly-crystalline and near-stoichiometric SiC fiber TyrannoTM-SA. In addition, hybrid SiC/SiC composites incorporating carbon fibers were fabricated to improve thermal conductivity. The purpose of this work is to obtain thermal and mechanical properties data on these developmental composites. Results show that the addition of small amount (>10 %) of TyrannoTM-SA fiber remarkably increases the composite thermal conductivity parallel to the fiber longitudinal direction, in particular the through-thickness thermal conductivity in the orthogonal three-dimensional composite system due to the excellent thermal conductivity of TyrannoTM-SA fiber itself. On the other hand, tensile properties were significantly dependent on the axial fiber volume fraction; 3D SiC/SiC composites with in-plane fiber content <15 % exhibited lower tensile strength and proportional limit failure stress. Results show that the composites with axial fiber volume >20 % exhibit improved axial strength. The carbon fiber was, in general, beneficial to obtain high thermal conductivity. However matrix cracks induced due to the mismatch of coefficients of thermal expansion (CTE) restricted heat transfer via matrix, limiting the improvement of thermal conductivity and reducing tensile proportional limit stress. (author)