Effects of negative skewness of surface roughness on the contact and lubrication of nominally flat metallic surfaces

This article studies the effects of negative skewness of the surface roughness on the state of contact and lubrication between nominally flat metallic surfaces. An asperity-contact model with a Weibull distribution of asperity heights is used in the study. Analyses are carried out for both frictionless and frictional contacts of surfaces of a range of plasticity index and roughness skewness in light and heavy loading. The state of contact is examined in terms of the asperity pressure and the mode of asperity deformation. Analyses are also carried out for the contacts with fatty-acids boundary lubrication. The state of contact and lubrication is evaluated in terms of the system friction coefficient and the proportion of the real area of contact with deteriorated boundary film. The results obtained suggest that, for sufficiently smooth surfaces or surfaces with a sufficiently low plasticity index, the system can perform satisfactorily under various conditions of practical interest and the negative skewness of the surface roughness can greatly help to enhance the system performance. For problems where the performance is marginally satisfactory, the negative skewness can help to improve the contact and lubrication conditions to further run-in the surfaces toward a lower plasticity index. The results also suggest that, when the surfaces are significantly rough or with plasticity index significantly above unity, the system may operate in a severe state of contact and lubrication, particularly under heavy loading and with high friction. Under these conditions, the negative skewness of the surface roughness is no longer effective to sufficiently bring down the degree of severity.