Effects of porosity in graphite materials on ablation in arc-heated jets

The ablation performance of chemically vapor-deposited carbon on a carbon felt made from polyacrylonitrile fiber, ATJ-S, and ATJ-S percursor graphites was studied in an arc jet at stagnation pressures and bulk gas enthalpies of 2.2 to 10.3 MPa and 4700 to 5800 J/g, respectively. The initial nonlinear ablation kinetics is associated with the attainment of thermal and chemical equilibrium. In the subsequent steady-state period, the ablation rate is constant. Ablation was modeled with an exponential rate relationship and the steady-state rate constants were shown to be related exponentially to porosity. A scanning electron microscope examination of ablated surfaces showed that nonuniform surface ablation can be correlated with porosity, fiber density, and the fiber-matrix interfacial spacing.