Eddy currents can be used to characterize the conductivity and thickness of coatings on metals. However, when the same techniques were applied to magnetic metals, some uncertainties were found. We have discovered that the broadband behavior of eddy current coils in proximity to ferromagnetic surfaces depends dramatically upon very thin surface layers. For nickel, we found a 10∼100 micrometers thick dead layer at the surface that reduces the apparent relative magnetic permeability substantially [1]. Conversely, this extreme sensitivity to surface conditions means that measurement methods can be devised that will be sensitive to very thin surface coatings, on the order of a few micrometers thick or less.
展开▼
