Effects of boundary films on the frictional behaviour of rough-surface contacts in incipient sliding

Research has shown that the interfacial shear strength in an asperity contact is generally a linear function of the asperity pressure with a maximum value below the shear strength of the substrate material. Research further suggests that the properties of the surface-film materials in the micro-contact largely govern the shear-strength - pressure relation and the maximum attainable interfacial shear strength. This paper studies the effect of boundary films on the frictional behaviour of rough-surface contacts in incipient sliding. Two parameters are used to describe the shearing properties of the boundary film. One is the shear-strength - pressure proportionality constant and the other, the ratio of the maximum interfacial shear strength to the substrate shear strength. The study develops and uses an asperity-based mathematical model for frictional sliding-contact of nominally flat elastic-plastic rough surfaces incorporating these interfacial shearing properties in the asperity contacts. A sequence of parametric studies is carried out to study the frictional behaviour of the contact system. The parameters include surface plasticity index, contact load, and boundary film properties. Results are presented showing some insights into the system frictional behaviour.