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 CH_3SiCl_3/H_2 gas mixtures in a cylindrical cold-wall reactor. Similarly, Nextel fibers were wound onto A1_2O_3 substrates and infiltrated with α-Al_2O_3 from AlCl_3/H_2/CO_2/N_2 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 Al_2O_3-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