Degradation behavior of 4D carbon/carbon composites under supersonic oxidative air plasma

摘要

Four-directional (4D) carbon/carbon (C/C) composites were prepared using an intermediate modulus carbon fiber as the reinforce ment and two kinds of coal tar pitches as the source for carbon matrix. After repeated high-pressure impregnation, carbonization and graphitization at 2250℃, a high density of 1.86 g/cm~3 was achieved. A plasma arc jet was applied to the composite for studying its degradation behavior. The oxidative supersonic jet was generated by an air plasma torch. For studying various stages of degradation during a long exposure time of 90-s, the intermittent ablation and erosion rates were measured for the C/C composite at time intervals of 70, 50 and 30 s. The mass ablation rate was increased rapidly with the time, while the erosion showed a bilinear response with a decreasing trend. The results were discussed based on the machined and eroded surfaces of the test samples. For studying the residual ablative performance, the samples of 30-70 s were then re-ablated and re-eroded in steps of 20-s so that all the samples are equal in total exposure time. The samples showed decreasing trends in the re-ablation and re-erosion rates and ultimately became zero for the last 20-s exposure time. There was a progressive increase in the char layer on the samples with the re-ablation. The porous char layer protected the samples against chemical and mechanical actions of the plasma stream. Compressive tests were carried out, and the decrease in the modulus was due to the presence of microcracks resulting from the mismatch in thermal expansion of the fiber and the matrix. The SEM micrographs reveal that inter- and intra-bundle cracks tend to grow through the weaker matrix pockets.