Microstructure and sputtering properties of graphite, disordered, and isotopically enriched carbon

During ground testing of electric propulsion (EP) thrusters, carbon sputtered from both thruster components and facility surfaces redeposits on thruster surfaces, obscuring erosion measurements and complicating lifetime predictions. One possible approach to differentiate thruster and facility carbon is to use ¹³ C as an isotopic tracer. This work presents the synthesis and characterization of a bulk hard carbon enriched in ¹³ C ( ¹³ C/C ≈ 90%). Isotopic composition measurements made with both bulk elemental analyzer isotope-ratio mass spectrometry and surface time-of-flight secondary ion mass spectrometry agree to within ± 3.3%. Microstructure and sputtering properties of the ¹³ C material were compared with a highly disordered ¹² C material and polycrystalline high-density graphite. Raman spectroscopy confirmed a clear hierarchy of structural disorder across the three materials, with D-to-G band intensity ratios of 0.3, 1.25, and 1.35 for HDG, ¹³ C, and ¹² C, respectively. Krypton-ion total sputter yields of all three materials were determined from mass loss measurements following simultaneous ion bombardment under nominally identical conditions. The yields of the three materials were statistically indistinguishable within this single experiment, and the absolute yield was 0.109 ± 0.003 atoms/ion, in reasonable agreement with literature. These results establish isotopically enriched hard carbon as a surrogate sputtering source for graphite with respect to total sputter yield under the conditions investigated.