Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Abstract

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 (4750K) sublimation to high-temperature (2467K) 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 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.