Ablation and oxidation resistance properties of C/C composites densified by xylene pyrocarbon using LaCl₃ as a catalyst and resin carbon

Carbon/carbon composites with densities of 1.72-1.73 g/cm³ were prepared by densification with pyrocarbon produced from xylene using a LaCl₃-catalyzed film boiling chemical vapor infiltration method, followed by resin impregnation, carbonization and graphitization. The ablation and oxidation resistance of the composites were respectively tested by an oxyacetylene torch and air oxidation methods. The phase composition and morphology of the ablated and oxidized surfaces were studied by XRD and SEM, respectively. Results show that the ablation rate and oxidation loss exhibit minima at catalyst contents from 0 to 15 wt%. During the ablation and oxidation, the La₂O₃ film formed from LaC₂ oxidation protects the composites from further oxidation. Catalytically-formed carbon nanofilaments prevent mechanical separation of the carbon fibers from the matrix. As a result, mass ablation and volume ablation rates and the weight loss of the composites due to oxidation are decreased by 7.6%-15.2%, 10.7%-20.0% and 17.7%-38.5%, respectively compared with those formed without the catalyst. The composites produced at catalyst contents of 6 and 10 wt% have higher ablation and oxidation resistance than samples with 0, 3 and 15 wt% contents. Both the thicker isotropic pyrocarbon when the catalyst content is more than 10 wt% and the lower fraction of carbon nanofilaments when the catalyst content is less than 6 wt% are unfavorable for the improvement of the ablation and oxidation resistance properties. The ablation and oxidation resistance of the composites are improved by graphitization from 1 800℃ to 2 250℃.