High-temperature behavior of graphite under laser irradiation

Understanding the high-temperature ablative behavior of rocket nozzle materials is crucial for future space propulsion applications. In this investigation, the high-temperature ablative behavior of G-90 grade graphite was studied under CO₂ laser irradiation in a helium gas environment with and without hydrogen addition. The objective of this study was to determine if the high-density graphite experiences considerable mass loss at temperatures significantly below its boiling point of 3,915 K. Additionally, understanding the mechanisms that cause the ablation is desired. Mass loss was observed at surface temperatures in excess of 2,100 K for both conditions. The mass loss was minor until the surface temperature reached a value of approximately 2,650-2,700 K. Beyond this point, increased laser energy flux causes an exponential increase of mass loss rate while the surface temperature remained constant. When hydrogen was introduced to the heated sample, acetylene, methane, and ethane were detected by a flame ionization detector. The high measured concentration of acetylene at elevated mass loss rates suggests that the hydrogen is reacting primarily with the pyrolyzed carbonaceous material generated from the sample surface rather than through heterogeneous reactions. The finding of significant mass loss by ablation suggests a new mechanism for nozzle erosion in addition to thermochemical and mechanical erosion.