The industrial steel tube in the cold rolling processes under liquid lubrication was simulated in a recovered cutting machine to study the tribological performances of an emulsion with four different concentrations of an emulsifier. The test machine was equipped with a device to measure the electrical contact resistance (ECR) between the rubbing surfaces of a steel tube and a roller. The ECR readings were useful to identify the existence of a surface absorption layer (a boundary layer film) between the contact surfaces and thus to realize the tribological effects. The tribological properties, including the friction coefficient, the electrical contact resistance, the wear loss of the tube and the worn surface area, were evaluated at these emulsifier concentrations. The displacement energy and the spraying coefficients for these emulsifier concentrations were applicable to interpret their relationships with the wear loss of the steel tube. The measures of the ECR are found to have a tendency more related to the values of the friction coefficient. Stribeck curves were also established for four emulsions; they are useful for assisting in the explanation for why a high friction coefficient in some cases does not necessarily produce a high wear loss. They were also provided to investigate the effects of the emulsifier concentrations on the friction coefficients at various lubrication stages. The emulsion with a dilute ratio of 50:1 proved to be the optimum choice of the four emulsifier concentrations because it leads to the lowest friction coefficient, the smallest wear loss and the worn surface as to the test time of 60 minutes.
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