Formation and Impact of Microcracks in Plasma Erosion of M26 Boron Nitride

Abstract

This Paper investigates the role of microcracks in Hall thruster wall erosion. The formation and growth of microcracks on the surface of M26 grade boron nitride composite due to repeated thermal shock was quantified, and the subsequent impact of microcracks on plasma erosion was assessed. Thermal shock cycles (20 → 800 → 20°C) were provided by a radiation oven to induce thermal stresses similar to those incurred by a Hall thruster wall. The average ratio of crack area to total area was observed to grow as a power law with subunity exponential from 4–5% before thermal cycling to 15–18% after 20 thermal shock cycles. Cycled and control samples were simultaneously exposed to argon plasma with average ion energy of 130 eV. All samples were observed to preferentially retain boron nitride relative to silica, and microcracks were not observed to significantly impact surface composition or feature development.