Microstructure and mechanical properties of carbon/carbon composites doped with LaCl₃

Carbon fiber felts with different LaCl₃ contents were densified to produce carbon/carbon composites by film boiling chemical vapor infiltration from xylene pyrolysis at 1223 to 1323K. Microstructure and mechanical properties of the composites were studied by polarized light microscopy, scanning electron microscopy, and three-point flexural test. Results showed that, the interbundle matrix of these composites was dominated by rough laminar (RL) pyrocarbon; whereas the intrabundle matrix may vary depended on the LaCl₃ content. RL, smooth laminar (SL), and isotropic pyrocarbon were formed mainly when the LaCl₃ content was 0, 5-10, and 15wt%, respectively. The composites produced from the preforms with 5 and 10wt% LaCl₃ exhibited a pseudo-plastic fracture behavior combined with high flexural strength and toughness, which was attributed to the presence of lanthanum compound particles, the proper bonding of fiber-matrix interface, and the formation of SL pyrocarbon.