Mechanically induced surface damage and resulting thermal instability in polycrystalline 0.94Na1/2Bi1/2TiO3-0.06BaTiO3

Surface layers exhibiting distinct physical properties compared to the bulk have been observed in ferroelectric materials, e.g., BiFeO₃, SrTiO₃, Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃, and more recently, lead-free 0.94Na_1/2Bi_1/2TiO₃-0.06BaTiO₃ (NBT-6BT). However, the underlying origin of these surface layers, or “skins”, remains unclear. In this work, the appearance of a distorted skin in relaxor ferroelectric NBT-6BT was investigated, after the removal of the sinter surface during grinding procedures. A sequence of different surface finishing media with decreasing particle size (15 μm silicon carbide abrasive paper, 9, 6, 3, and 1 μm diamond polishing suspensions, and finally 0.04 μm oxide polishing suspension) was used to investigate the influence of mechanical damage on ferroelectricity, thermal and chemical stability, and mechanical field modulation of the crystal structure at the surface. Laboratory X-ray diffractometry, using both Bragg-Brentano and grazing incidence geometries, revealed the emergence of phases with distorted lattice structure far from the expected pseudo-cubic symmetry. In addition, the phase degradation of NBT-6BT into the metastable Bi₂Ti₂O₇-like pyrochlore phase was observed due to a likely sharp increase in the local temperature in the contact points between the grinding particles and the ceramic sample. These structural findings are also linked to the observed increased phase instabilities in ceramics with severe mechanical damage. This enables the formation of a Ba-rich secondary phase at the surface upon thermal treatments at 850 °C for 10 h under different atmospheres, such as oxygen and nitrogen.