Grain-Orientation related surface effects on polycrystalline tungsten caused by mechanical polishing and etching

The problem of obtaining a smoothly polished planar surface on polycrystalline tungsten is important for its applications as substrate in electronics as well as for preparing surfaces for research related to fusion reactor applications. In the latter case the classical metallographic polishing methods, using diamond abrasives, are generally used to prepare surfaces for the study of the interaction of the fusion plasma with tungsten. During a short study of the mechanical polishing of pure tungsten, it was observed that the material removal was different from grain to grain, giving rise to a non-planar surface. By polishing with 3 μm diamond grit as an intermediate step a dot pattern develops which outlines the grain structure. The dots were shown to consist of small cracks. On further polishing with 1 μm diamond the dot pattern disappears. However, it re-appears when polishing again with 3 μm grit. Apparently these effects are caused by the orientation dependent mechanical interaction between the 3 μm diamond particles and the tungsten crystal lattice. The coincidence of the surface dot patterns and the underlying grain structure could be clearly demonstrated by etching. Etching also demonstrated the presence of plastic deformation to a depth of the order of 4 μm. It is advised to further examine the sub-surface deformation layer in view of its effect on deuterium and tritium storage.