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.