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Mechanical spectroscopy of rolling oil thin films on cold-rolled steel sheets

Fine traces of lubricating oil can be discovered in the form of fine debris left on the surface of cold-rolled steel sheets. High resolution mechanical spectroscopy is a sensitive technique for detecting such extremely fine oil traces. It is shown that a characteristic mechanical loss spectrum occurs in the low temperature range (from 180K up to 280K) only in the two following cases, namely (i) if traces of the rolling oil are left on the surface of sheets and (ii) if the clean sheets are covered with a thin film of the rolling oil (or any other natural or mineral oil). It is clearly demonstrated that similar mechanical spectra induced by the presence of oil can be observed in the sub-resonant mechanical spectroscopy if the oil film is deposited on a very soft cellulose neutral substrate. Therefore the behaviour of the oil alone can be obtained with a high resolution. It will be demonstrated that it is possible to assign the mechanical loss phenomena to the steel sheet (like the Snoek-Koster effect above 500K) and oil separately. In particular, it is shown that the low temperature relaxation phenomena do not originate from a specific interaction of the steel surface with oil, as frequently reported in the literature.