Accuracy and repeatability of coating thickness measurements on non-oriented electrical steels using the magnetic induction method

The insulation coating often applied on non-oriented electrical steels is of importance in many ways. One of the most critical aspects of the coating is its thickness. It is therefore very important for good control of the coating thickness, to measure this parameter accurately and with good repeatability. This paper concentrates on the practical aspects of the magnetic induction method for coating thickness measurements, since this is the most common method with users of electrical steel. The investigation demonstrates that it is important to calibrate a magnetic induction instrument using a calibration standard with the same base metal permeability, thickness and surface roughness as the sample to be measured. Calibration foils used and wear of the measuring tip are other important factors. In view of the accuracy and repeatability of the method, the allowed range of measured coating thickness needs to be about ±0,5 μm for an average coating thickness of 1 μm.