Ceramic composites with multilayer interface coatings

Silicon carbide matrix composites have been fabricated from either ceramic- grade Nicalon((TM)) or Hi-Nicalon((TM)) fibers coated with an interface mat erial consisting of six alternating carbon and silicon carbide layers. Init ial efforts involved the use of chemical vapor infiltration to produce mini composites (single toms of fibers). In subsequent work, forced-flow thermal -gradient chemical vapor infiltration was used to produce a single composit e plate with a multilayer interface from ceramic-grade Nicalon fabric and t wo plates from Hi-Nicalon fabric, one with a single carbon layer and one wi th a multilayer interface. Tensile testing of the minicomposites and of spe cimens cut from the plates revealed typical composite behavior and strength s for the as processed samples. Exposure of tensile specimens to 950 degree sC air for 100 h resulted in large losses in strength and strain tolerance regardless of the interface coating. The results demonstrate that forced-fl ow thermal-gradient chemical vapor infiltration can be used to prepare mult ilayer interface material. The results also verified that relatively thick (>100 nm) single or multiple carbon layers are susceptible to oxidation tha t causes the loss of composite properties.