Study of the Effects of Surface Coating on Magnetic Barkhausen Noise in Grain-Oriented Electrical Steel

Magnetic Barkhausen Noise (BN) is a useful parameter for analysing physical and micro structural properties of electrical steel. It is believed to be due to microscopic discontinuities in domain wall motion due to the presence of defects. Grain-oriented electrical steel has an insulating surface coating which provides a beneficial stress in the steel. BN is sensitive to changes in the surface condition of steels. Measurements have been carried out on strips of high permeability grain-oriented (HGO) and conventional grain-oriented (CGO) 3% silicon steels, at 50 Hz in the peak flux density range 8.0 mmmT to 1.0 T before and after chemical removal of the coatings. Coating removal from HGO and CGO materials increases BN at high and low fields. This can be explained in terms of widening of 180° domains due to the release of coating stresses so that domain walls move further between pinning sites thereby generating higher rates of change of magnetisation which result in higher BN. In the coated materials, BN was found to be higher in HGO than in CGO at flux densities above around 0.2 T but the trend changes at lower flux densities because of grain size/misorientation effects. However, it is interesting to note that in the decoated material the BN is higher in HGO at all flux densities. Application of a 3 MPa tensile stress to the decoated samples caused the BN below 0.2 T to become higher in CGO than in HGO demonstrating that domain refinement due to coating stress and externally applied stress produce similar effects on BN. A significant correlation was found between the average velocities of domain wall movement and changes in BN at all the peak flux densities in the test materials.