Effects of a predeposited pyrocarbon layer on the microstructure and mechanical properties of carbon/carbon composites

Carbon/carbon (C/C) composites with multiple matrixes were fabricated by chemical vapor infiltration (CVI) and an impregnation and carbonization step. To study the effects of predeposited pyrocarbon on microstructure and the mechanical properties of C/C composites, different predeposition times (3 h, 5 h, 7 h and 9 h) were tested. This caused the predeposited pyrocarbon layer to have a large effect on the mechanical properties of C/C composites. C/C composites with 3 h predeposition exhibited the worst mechanical properties because the predeposited pyrocarbon layer is thin, which does not prevent deformation of the preform during impregnation with liquid pitch. For composites with a 5 h predeposition time, a pyrocarbon layer with proper thickness played an important role in protecting the preform; this sample possessed better mechanical properties. However, as the predeposition time was increased to 7 h and 9 h, due to the increased thickness of the deposited pyrocarbon, several pores were generated in the C/C composites that decreased their density and worsened their mechanical properties.