Microstructure development and fracture toughness of acid-treated carbon nanofibers/alumina composites

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In this study, carbon nanofibers (CNFs) having different amounts of defect were prepared by acid-treating for 0.5, 1 and 5 h and were combined with alumina. Then, the influence of the amount of defect on the CNFs on microstructure development of the CNFs/alumina composites and relationship between the fracture toughness and the average alumina grain size was investigated. The intensity ratio of D-band to G-band (D/G) in Raman spectra of the CNFs increased from 0.34 for pristine CNFs to 0.95 for the CNFs acid-treated for 5 h with the acid-treatment time, which indicates that the amount of defect on the CNFs increased with the acid-treatment time. The alumina grain growth in the dense composites sintered at 1200–1300°C was not influenced by the amount of defect on the CNFs, however, the composite containing CNFs having the moderate amount of defect (D/G = 0.56) showed the slowest alumina grain growth rate at 1350–1450°C. The fracture toughness of the composites containing the CNFs acid-treated for 0.5 h increased with a decrease in average alumina grain size and reached 5.6 MPa·m0.5 at the average alumina grain size of 0.84 µm, which was 60% higher value compared to monolithic alumina (3.5 MPa·m0.5). However, fracture toughness of the composites containing CNFs acid-treated for 1 and 5 h increased with a decrease in average alumina grain size, showed the maximum values of 5.0 and 4.5 MPa·m0.5 at average alumina grain sizes of 1.3 and 1.6 µm, respectively, and decreased as the average alumina grain size decreased further. The maximum fracture toughness of the composite containing the CNFs acid-treated for 5 h was lower than that of the composite containing the CNFs acid-treated for 1 h.

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