The effects of addition of La₂O₃ on the microstructure and mechanical properties of carbon/carbon composites

For the purpose of comparison, C/C composites were prepared with and without lanthanum oxide (La₂O₃) by thermal gradient chemical vapour infiltration (TCVI). Natural gas was used as the carbon source, and N₂ was selected as the diluent gas and protected gas in the experiments. The effects of percentage composition of La₂O₃ on the microstructure and mechanical properties were investigated. The microstructure was investigated via polarized light microscope (PLM) and scanning electron microscope (SEM). The mechanical properties were studied via three-point flexural testing. The results indicate that the addition of La₂O₃ has a considerable effect on pyrocarbon texture, with the matrix of the pure C/C composite being composed of ISO and smooth laminar (SL) pyrocarbon, whereas the matrix of composites with La₂O₃ was mainly dominated by SL pyrocarbon. The differences are the refinement of pyrocarbon texture and increase of the extinction angle with the increasing content of La₂O₃. Moreover, it was observed that the ultimate flexural strength of C/C composites with La₂O₃ was significantly improved, from which it can be deduced that La₂O₃ improves the interface roughness and interface bonding strength between carbon fibres and the pyrocarbon matrix by interfacial chemical reaction.