Measurement of insulating coatings on electrical sheet

The oil film thickness measurement systems EMG SOLID® IR and LIF solve the problem

Electrical sheets - together with magnets and laquered copper wires - are key components for the production of electric drives. The projected growth of e-mobility will increase the demand for electrical sheets for the automotive industry from 100 thousand tonnes in 2018 to around 1.2 million tonnes in 2030 in Germany (ZVEI 2020). Quality assurance and the control of quality costs for electrical sheets thus determine the profitability of motor manufacturers to a large extent. 

Today, with the increasing development of cold strip production in reversing or tandem cold rolling and continuous annealing lines, electrical steel is supplied to customers mainly in the form of wide strip or slit strip, mostly with insulating surface coatings. 

The main function of pigment-based electrical steel coatings is to insulate the electrical sheets to prevent the flow of electrical current and reduce eddy currents. In addition to corrosion protection, modern electrical steel coatings also help to extend the service life of the required stamping tools and offer the possibility to specifically adjust important properties such as weldability or annealing resistance.

The thickness of the insulating layers typically varies between 0.5 µm and 10 µm, depending on the specific type of coating and the end application. Both the thickness and the homogeneity of the insulating layers are decisive quality factors for electrical sheets.

Limitation of current solutions
Beta transmission through insulating coatings, which has so far been used to determine the area weight and thickness of these insulating layers, will soon reach the limit of its availability. Different isotopes (krypton, strontium, promethium) are used for different weight ranges. In addition to the fundamental problem of handling radioactive isotopes (radiation protection), there is the even more critical problem with promethium that it is practically no longer available on the market today and can thus only be used for two to three years at the most. 

EMG SOLID® IR and LIF as a perfect substitute
The EMG SOLID® systems, which have so far been used for the quantitative determination of lubricant layers on sheet metal in the lower micrometre range (g/m²), offer an ideal substitute here. Both infrared absorption (IR) and laser-induced fluorescence (LIF) can be used to reliably determine the desired measurement data online. Depending on the type of coating and material surface, the methods offer their specific advantages. 

Test measurements
In an initial feasibility study for an electrical steel manufacturer in 2021, samples of electrical steel sheets coated with 6 different, commonly used lacquers were examined using the methods of infrared absorption (EMG SOLID® IR) and laser-induced fluorescence (EMG SOLID® LIF). It was shown that coating thicknesses between 0,5 µm and 10 µm depending on the type of lacquers could be clearly detected on the sample sheets. A clear separation from the background signals was possible even with low coating thicknesses and without optimal adjustment of the system parameters.  

Further results
Further feasibility studies and on-site tests covered a wide range of insulating coatings used in production (90 %). 

The figure shows the comparison (relative deviation) between a measurement with EMG SOLID® LIF (blue) and a beta backscatter measurement (red) used as standard today for a specific type of coating. The time course over several bands of a total of 1935 observations is shown.

The graph shows the high relative accuracy of the EMG SOLID® LIF measurement technique compared to the very strongly varying beta backscatter measurement. The course of the layer thickness over time (and thus the strip length) can only be conclusively evaluated with the EMG SOLID® LIF measurement.

The feasibility studies covered a wide range of insulating coatings used in production (90 %). The results of laboratory measurements and on-site tests clearly showed that coating thicknesses between 0.5 - 10 µm can be successfully determined. This also applies to very dark and opaque grey coatings. Both methods are designed for inline or online measurement operation, even in traversing mode with typical travel speeds of up to 1 m/s.

Conclusion
The results of these feasibility studies clearly show that quantitative detection of the investigated insulating coatings on electrical sheet in the micrometre layer thickness range is very well possible using both infrared absorption measurement and laser-induced fluorescence analysis.

Both methods are already routinely used today for quantitative measurement of the oil layer thickness in steel and aluminium mills. The use in electrical steel production is the logical next step for the widespread use of EMG SOLID® technology, especially with a view to the medium-term inevitable replacement of measurement methods based on beta emitters.

For more information on EMG SOLID® IR and LIF, please visit the EMG homepage at: https://www.emg.elexis.group/en/metal/quality-assurance-systems/emg-solidr/ 
 

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