Validation of basic friction equation based on nano properties of tribofilm on real contact area

Abstract

The formation of tribofilm by lubricant additives plays an important role in improving boundary lubrication performance. In previous studies, it has been reported that the frictional force due to adhesion can be expressed as a product of the real contact area and the shear strength of the contact junction. Therefore, to evaluate the friction mechanism dominated by the tribofilm in detail, it is important to identify the effects of real contact area and the shear strength. In addition, tribofilms are covered on a part of sliding area, where metal/metal contact occurs. However, few studies have verified the effects of coverage of tribofilm on both real contact area and shear strength related to macro friction coefficient with experimental data. In this study, to investigate the coverage of tribofilm on macro friction coefficient with sliding tester, shear strength and real contact area of tribofilm were investigated using nanoindentation equipment and contact analysis. The results showed that the shear strength of a substrate and tribofilms were correlated with the macro friction coefficients. Using both the shear strength and the real contact area, macro friction coefficients calculated from the basic friction equation coincided with those measured by the macro friction test. Finally, we obtained coverage ratio β using the real contact area and the shear strength for various types of tribofilms.