Damage Tolerance of Silicon Carbide- and Alumina-Matrix Surface Composites

A method for the fabrication of a ceramic-matrix composite (CMC) layer on the surface of a monolithic substrate via chemical vapor infiltration (CVI) is described. Preforms consisted of tows of fibers wound onto the surface of monolithic cylindrical tubes. Nicalon fibers were wound onto mullite substrates and infiltrated with β-SiC from CH₃SiCl₃/H₂ gas mixtures in a cylindrical cold-wall reactor. Similarly, Nextel fibers were wound onto A1₂O₃ substrates and infiltrated with α-Al₂O₃ from AlCl₃/H₂/CO₂/N₂ gas mixtures. Composites with densities as high as 88% of the theoretical value were fabricated in 8 h. The effective fracture strength of the SiC- and Al₂O₃-matrix surface composites, as determined from diametral compression tests of C-ring specimens, was found to be insensitive to damage caused to the outer diameter by a Vickers indentation. The tolerance of the SiC-matrix surface composites to surface damage was retained in specimens subjected to oxidation at 1000°C for 6 h.