Investigation of the friction variation with sliding which is commonly observed in individual fretting test cycles

Many researchers that have carried out fretting wear tests have presented results which show a variation (usually an increase) in the friction force during the sliding phase of individual cycles. This phenomenon is contrary to the Amontons/Coulomb model of friction which predicts a constant friction force as sliding proceeds. An in-line fretting test involving an abrupt increase in amplitude was used to show that the effect is a result of wear scar interaction effects. A pair of rotational fretting tests were then carried out to determine whether these interaction effects originate from interaction of the wear scar ends, or whether such interaction occurs throughout the nominal contact area. A new experimental rig designed to adapt conventional in-line test machines for rotational fretting was designed and built for this purpose. Results show that the friction variation occurs whether or not wear scar ends are present. After testing, the worn surface topography of each fretting pair was scanned using a focus variation microscope, and these surface images revealed the existence (and size) of many distributed local peaks and troughs distributed throughout the contact area. In conclusion, the friction variation commonly observed in the literature arises predominantly from the interaction of local wear scar features distributed over the contact region.