Effects of Al2O3 coating induced structural and mechanical changes of C/C composites

Abstract Different C/C composites, identified as Cf/C, CAl2O3/C, C1600/C and C1600-Al2O3/C, were fabricated by condensation of different treated 3D needle-punched carbon fiber preforms through a thermal gradient chemical vapor infiltration (TG-CVI) method. Effects of high temperature treatment and Al2O3 coating of the carbon fiber preforms on matrix texture and mechanical properties of C/C composites were investigated. SL PyC is obtained for Cf/C and C1600/C, which preforms are without Al2O3 coating. Whereas the texture of carbon matrix for CAl2O3/C, and C1600-Al2O3/C changes to RL under the same deposition conditions, indicating that Al2O3 coating layer significantly affects the texture of carbon matrix. Heat treatment of the carbon fiber preforms at 1600 °C for 4 h obviously increase the toughness of the C/C composites. The fracture toughness increased by 226% from 0.938 MPa/m2 for Cf/C to 3.061 MPa/m2 for C1600-Al2O3/C. The significant toughness increasing is attributed to the unique fracture mode for C1600-Al2O3/C, where a hollow “cylindrical” PyC matrix is pulled out and a corresponding big hole is left with a central fiber in it. The flexural strength, however, is mainly affected by the final density of the C/C composites.