The traditional method of using the continuous variation of the friction coefficient with sliding distance to monitor the tribological properties between the contacts of soft metal films is generally low in sensitivity. This paper proposed the novel method of using instead the continuous variation of tribo-electrification voltage. This method was investigated experimentally for the dry friction sliding of iron on copper coated with a thin film of tin and was shown to be much superior to the traditional method in terms of sensitivity. The method has the added advantage of the ability to assess the solid to film lubrication of the soft metal film. Finally, a continuous model to represent the wear mechanisms for iron sliding against copper coated with a thin film of tin was proposed.