Porosity Characterization and Associated Mechanical Degradation of Heat Treated Carbon Phenolic Ablative Composites

ABSTRACT Thermal protection systems (TPS) in aerospace applications commonly employ carbon phenolic and silica phenolic‐based composites as ablative materials. Pyrolysis, gas evolution, and char production with porosity growth occur in the ablative matrix, changing the thermal and mechanical characteristics. Therefore, heat‐treatment‐induced porosity in carbon phenolic composites is investigated herein. The samples are exposed to controlled thermal treatments at temperatures of 500, 700, and 950 K. Consequently, the generated porosity was examined and compared with that of the virgin material at room temperature (RT = 300 K). Synchrotron X‐ray Micro‐Computed Tomography (S‐micro‐CT) is employed as a nondestructive, high‐resolution technique with voxel sizes in the micrometer range to capture the microstructures in terms of segregation of porosities, fiber and matrix. This level of resolution makes the technique uniquely attractive for imaging the porosity of ablatives used in re‐entry space vehicles. 3D reconstructions and analyses of S‐micro‐CT images are performed to quantify porosity parameters, viz., total porosity and pore size distribution, as a function of heat treatment. The results indicate a progressive increase in porosity (from 0.01% to 0.6692%) with increasing temperature for a 20 s exposure. While an increase in porosity leads to a strength reduction of 3.5% up to 700 K, a much larger decrease in strength to 62.3% is observed at 950 K, which is above the glass transition temperature. Thus, the findings establish the S‐micro‐CT as a robust and quantitative characterization tool for ablative materials, enabling direct correlation between porosity evolution and thermally induced mechanical strength degradation.