Surface Morphology, Contact Size and Contact Geometry Effects on Grease-Lubricated Fretting Contacts

The fretting behaviour of AISI 52100 steel and low alloy steel under grease lubricated conditions is investigated. The friction behaviour is characterized by a high friction regime for the smallest displacement amplitudes below and next to the partial-slip transition whereas a low friction regime is stabilized above a threshold sliding amplitude when a lubricating tribofilm is generated in the interface. In order to quantify the influence of different contact parameters like surface roughness, contact size and contact geometry, a new fast simplified fretting methodology is introduced. This method allows scanning the entire fretting regime through a single experiment. An experimental comparison of smooth and rough surfaces shows that the effect of surface roughness influences the stability and the performance of the lubricating tribofilm. Experimental results obtained through changing the contact size of the cylinder-on-flat configuration permit identifying two contact size regimes. For small interfaces, the effective fretting sliding amplitude triggering the tribofilm activation is proportional to the contact size, whereas for large contacts this amplitude appears to be constant. The comparison with a punch-on-flat interface suggests that the lubrication activation is better captured by considering the peak pressure width rather than the nominal contact size.