Evaluation of Graphite/h-BN Bimaterials for Electric Propulsion

The performance of an innovative dielectric bimaterial is assessed for use in Hall-effect thrusters. The bimaterial consists of a structural body of graphite with a converted surface layer of hexagonal boron nitride (h-BN). The bimaterial couples the dielectric behavior and low emissivity of h-BN at its surface, with the thermal shock resistance and machinability of graphite at its core. In this paper, the performance of graphite/h-BN bimaterials synthesized from liquid-phase and vapor-phase carbothermic reactions of B2O3 in nitrogen is compared and evaluated against the state-of-the-art wall material, bulk h-BN. Graphite/h-BN bimaterials synthesized through vapor-phase carbothermic reactions are shown to perform comparatively better than bimaterials synthesized through liquid-phase carbothermic reactions. The erosion rate of vapor-phase grown h-BN layers is similar to that of bulk h-BN with a sputtering yield of 0.021 mm3/C for xenon ions at 300 V.