Grain oriented silicon steel is the most widely used soft magnetic material. Core loss of grain oriented silicon steel is mainly dependent on many metallurgical factors such as silicon contents, impurities, degree of (110)[001] texture and surface conditions. In the present study, we have studied effects of surface treatments of 0.3mm grain oriented silicon steel on core loss. The core loss of the processed silicon steel sheets are mainly dependent on residual stress imposed by the coating and on surface roughness. For this purpose highly grain oriented 3% silicon steel sheets produced by POSCO were used. Forsterite layer and tension coating in fully processed specimens were removed by dipping into NaOH and HCl solution. Domain wall spacing and core loss increased after removing the coating layers due to absence of tension introduced by the forsterite and the tension coating. To study effect of surface roughness, the coating layer removed specimens were chemically polished by HF+$H_2O_2$ solution. Domain wall spacing of the polished sample is the same as that of the uncoated specimens but core loss of the polished sample decreased about 30% compared to the unpolished specimens. The sample with rough surface shows higher core loss because the rough surface hinders domain wall motion as domain pinning sites.
A sol-gel method was employed to coat an insulation layer of magnesium oxide(MgO) on the polished grain oriented silicon steel sheets. The main purpose of coating MgO layer is to introduce tensile stress to the sample by thermal expansion coefficient difference between the coating layer and the grain oriented silicon steel. The MgO coated grain oriented silicon steel was annealed at the different temperatures and cooled down to room temperature. Domain refinement was obtained only at the specimens annealed above 500℃. A maximum reduction of about 15% in core loss was obtained at the annealing temperature of 500℃. The core loss reduction was associated with formation forsterite at the specimens annealed above 500℃. The specimens annealed at 600℃ showed domain refinement but core loss of the samples did not change or even increased. The reason of this is as follows. At the higher temperature, the thicker forsterite forms but forsterite grains become bigger and the surface roughness increases and the reduction of core loss due to higher tensile stress is overshadowed by loss increase due to the surface roughness.
TiN films were deposited on the grain oriented silicon steel by a plasma enhanced CVD method because TiN with very small thermal expansion coefficient can introduce higher tensile stress to the grain oriented silicon steel. However, the expected core loss reduction was not obtained because the oxide layer formed during the polishing process on the surface of the sample prohibited hetero epitaxial growth of (111) TiN on (110) silicon steel surface.
Therefore it is concluded that epitaxial growth is necessary condition for higher residual stress at TiN and silicon steel interfaces.