High-frequency Eddy Current conductivity measurement for near-surface metal layer characterization

Modifications on surfaces in mechanical engineering or electronic industry are used to enhance performance or lifetime of technical systems. Surfaces and surface coatings are specifically processed to adjust required properties depending on the component's task. There are several processes, e.g. coating of conductive or non-conductive layers, or special mechanical treatments. In many applications it is essential to control the surface layer properties, because the thickness and quality of these layers define future performance of the entire product and may change during fabrication or operation. This paper presents a new Eddy Current device, which is able to measure at very high frequencies up to 100 MHz. By using high frequencies, we are able to characterize thin conducting layers, near to the surface with minor influences of the substrate. Due to the skin effect depending penetration depth, the use of frequency sweeps in the range of 3 decades allows to obtain information on electrical conductivity from different depths below the surface. The information about the conductivity can be used to distinguish parameters of mono and multi layer structures, such as the material itself, thickness, hardness, stress, states of cold work, and microstructure properties. The paper presents results from samples with different conductive coating on conductive substrates and the analysis of different mechanically treated surfaces like shot peened surfaces.