| Popis: |
Carbon-Carbon (C/C) composites have been shown to be a preferred material for high temperature applications as they retain their properties and performance at temperatures in excess of 2000°C. This study shows that High Temperature Treatment (HTT) at 2400°C for 4 hrs followed by two subsequent Polymer Infiltration and Pyrolysis (PIP) cycles using SC1008 phenolic resin changes the failure mechanism of 2D C/C which has been subject to directional ablation prior to flexural testing. The study observes that prior directional ablation of the non-HTT C/C condition decreases flexural strength by 50.2%, whereas negligible change for the HTT C/C condition was observed (6.6%). This is attributed to the significant degradation of the tensile surface of the non-HTT C/C during ablation corresponding to an average linear thickness loss of 0.321mm (Std Dev = 0.223mm) and average mass loss of 0.364g (Std Dev = 0.196g) while the HTT recorded 0.033mm (Std Dev = 0.005mm) and 0.032g (Std Dev = 0.008g) respectively. The difference in degradation is attributed to the microstructure which was characterised through X-Ray Diffraction and Scanning Electron Microscopy. It is shown that HTT transformed the carbon matrix from a glassy/amorphous matrix to a layered matrix with an indicative increased degree of graphitisation (from 0.52 to 0.69). This not only increased the average density from 1.511 g/cm3 (Std Dev = 0.002 g/cm3) to 1.652 g/cm3 (Std Dev = 0.003 g/cm3) but also increased the average thermal conductivity from 9.1 W/mK (Std Dev = 1.06 W/mK) to 13.3 W/mK (Std Dev = 1.32 W/mK). This ultimately contributed to a reduction in available sites for the oxidation reaction to occur, while also allowing for thermal energy to be conducted away from the ablation surface reducing the amount of heat related damage. For conditions without and with prior ablation damage, the non-HTT C/C is found to fail in matrix dominated tension with the fibres and matrix breaking in a single plane originating at the tensile surface and propagating towards the neutral axis whereas the HTT C/C is found to fail in shear at the neutral axis with the fibres-matrix debonding being the primary failure mechanism. The non-HTT C/C is found to have an average flexural strength of 88.8 MPa (Std Dev = 13.7 MPa) and flexural modulus 81.0 GPa (Std Dev = 10.5 GPa), where the HTT C/C has 196.7 MPa (Std Dev = 31.4 MPa) and 115.2 GPa (Std Dev = 3.3 GPa) respectively. Lastly, this study found that a square notch in the non-HTT C/C condition resulted in a 23.9% and 26.4% reduction in flexural strength for conditions without and with prior ablation damage, respectively. No change in the failure mechanism was observed for notched specimens compared to un-notched specimens, and the debit in strength was attributed to broken fibers created by the notch. |
