Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750 K) sublimation to high-temperature (2467 K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173 K and later on became fairly flat till 1523 K and onwards. In the compression test, 4750 K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467 K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young's modulus.