Effect of a Coating Induced Residual Stress on Magnetic Domain Structure in Non-Oriented Electrical Steels

To produce the magnetic core of the electric motors, non-oriented electrical steels (NOES) are used. Recently much attention has been focused on optimizing the efficiency of electric motors and, as such, various materials aspects of NOES are being re-examined. Typically, NOES are used in the form of laminations (~200-1000 μm thick) with an electrically insulating coating applied to the surface. Residual stress is induced during the coating process, which will alter magnetic domain structures of the steel substrate. Variations of the resultant magnetic domains due to coating have been characterized mainly in grain oriented electrical steels [1-3], and only limited numbers of studies are available on non-oriented electrical steels [4]. In this study, the effect of the coating is examined, specifically, its role in creating a residual stress near the coating/steel interface. The stress was investigated by nanoindentation technique. With this method, a tensile stress of ~200 MPa was calculated based on hardness values in the steel substrate, which was linked to the variations in near surface hardness profile, and corresponding magnetic domain structure. The magnetic domain structures were imaged using a field emission scanning electron microscope (FE-SEM).