Microstructural damage of α-Al2O3 by high energy density plasma

Abstract One limitation on plasma thruster lifetime for space propulsion is plasma erosion of ceramic insulators. The underlying plasma erosion mechanism remains scarcely understood. In this work, polycrystalline α-Al2O3 was exposed to a high energy density plasma for up to 30 min using Cu and W electrodes. The damaged structures were studied by scanning and transmission electron microscopies. It was found that plasma-induced damage includes not only surface erosion but also damage down to a depth of ∼1 μm. The surface damage including cracking, formation of surface nanostructures, γ-Al2O3 and epitaxial intergrowths of α-Al2O3, is caused mainly by ion-based sputtering, rapid surface melting and quenching. Deeper damage, i.e. formation of Al nanoparticles and O2 bubbles, is caused by the electron-induced decomposition of both α- and γ-Al2O3 via electronic excitation. Electrode impurities, especially the W impurities sunken in the molten Al2O3, act as a local heat source and cause expansion of γ-Al2O3 and Al regions. Furthermore, the impurities react with Al to form Al2Cu and Al5(W,Cu) particles. The Al2Cu are formed as surface dendrites and nanoparticles within the Al matrix. The Al5(W,Cu) having Al- and W-rich nano domains is determined to be a rhombohedral structure with lattice parameters of a = 0.490(3) nm, c = 2.632(4) nm and a space group of R 3 ¯ c. This study indicates that besides surface erosion, deeper damage caused by electrons and impurities should be considered when evaluating thruster plasma damage on ceramics.