Unsteady spallation of low-density carbon fiber ablators

Mechanical erosion of ablative heat shield materials, known as spallation, was investigated in supersonic air and nitrogen plasmas produced by an inductively coupled plasma wind tunnel at aerothermal conditions representative of atmospheric entry. Spalled particles from low-density carbon ablators were tracked using high-speed imaging, enabling time-resolved analysis of spallation events. Tests in nitrogen revealed high temporal variance in particle production over time, while tests in air exhibited steady particle release. Post-test microscopy and spectroscopy identified a disordered nitrogen-functionalized carbon precipitate that forms exclusively in nitrogen plasma. Under extreme conditions, this deposit decreases surface permeability, enabling subsurface pressure buildup that drives unsteady particle release. Spalled particle size was inferred from velocity data obtained via particle tracking, enabling estimation of spallation mass loss. Spallation was estimated to account for upwards of 45% of total mass loss for tests in nitrogen, underscoring its significance in anaerobic entry conditions. Results suggest that deposit formation, material orientation, and environment conditions collectively govern spallation behavior.