Structure-property relations for silicon nitride matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nicalon fibers

Si3N4 matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nica lon (SiC) fibers, were studied using tensile testing and transmission elect ron microscopy. Three types of samples were evaluated all with a nominal co ating thickness of 200 nm. The composites were densified by hot pressing at 1550 degrees C (type I and II) and at 1600 degrees C (type III). The fiber s were coated with pyrolytic carbon via CVD with identical (sample I) and o pposite (samples II and III) directions of the gas flow and of the fiber mo vement through the reactor. Tensile testing indicated for the three sample types respectively: brittle behaviour with huge pull out of the fibers, pse udo-plastic behaviour and brittle behaviour with little pull out. TEM indic ated for the three sample types debonding typically at the fiber/coating in terface, at the coating/matrix interface and in the coating, respectively. The relation between processing, structure, particularly of the coating and its interfaces with the matrix and the fibers and mechanical properties is addressed. (C) 1999 Kluwer Academic Publishers.