Radial fretting tests with a 52100 steel ball-on-flat contact have been carried out under different normal loads. TiN, MoS2 and TiN+MoS2 coatings on a 1045 steel flat were examined. The normal loads amplitude used were 200, 400 and 800N at speeds of 12 and 1.2 mm/min. Dynamic analysis in combination with microscopic examinations by SEM and EDX have been performed. It was observed that the vertical stiffness increased with the increase of loading speed and number of cycles. The metallographic examinations showed that little damage was observed for the MoS2 coating, which exhibited excellent radial fretting fatigue resistance. For the TiN coating, micro-cracks appeared at the lower load while delamination occurred at the higher load. For the TiN + MoS2 composite coating, the vertical stiffness increased but accompanied by some micro-cracks. As a result of the study, the radial fretting test is proposed as one possible new method to evaluate coating life.

Lubrication is one of the important methods of preventing fretting damage owing to a remarkable reduction in the coefficient of friction. This review attempts to bring up-to-date the known facts concerning the lubrication in fretting situations. Results obtained by the authors in combination with results from literature are examined in detail. The effects of many lubricants on fretting behaviour have been investigated and some general features have been found although the majority of experimental data was obtained under quite differing conditions. In order to select successfully the right lubricant for a specific fretting application, representative lubricants, oil, grease, polymeric film and molybdenum disulphide MoS2.with different physical states were used. Results indicate that the palliative effect on fretting should be analyzed using the fretting regimes concept. In particular, solid lubricant is most efficient in the partial slip regime while grease and oil, particularly the latter, are more suitable in the gross slip regime. Nevertheless, it has been noted that the lubrication mechanism in fretting is poorly understood and the need for further work is discussed. q1999 Elsevier Science S.A. All rights reserved.

The mixed fretting regime (MFR) has been proven to be the most dangerous regime for crack nucleation and service failure. It can be identified from the evolution of the "tangential toad-displacement" loops, i.e. when there is a variation in the shape of the loops. Usually elliptical (partial slip) and quasi-rectangular (gross slip) loops are encountered in MFR. This regime has been obtained in fretting wear and fretting fatigue tests on numerous metal-base alloys. In contrast with the classic stick-slip condition in Mindlin's elastic theory, MFR depends strongly on the normal load, the imposed amplitude and material properties such as elongation. During fretting testing under MFR, the material can fail due to overstraining or overstressing and the location of the maximum varies. For ductile materials, a competition exists between the formation of the tribologically transformed structure, particle detachment and the nucleation of fatigue cracks. In the case of brittle materials, MFR is generally avoided due to the rapid formation of debris and the establishment of a third body layer in which easier velocity accommodation favours the gross slip condition. A mechanical and material approach is used to analyse the specific behaviour of materials submitted to MFR. The study of fretting crack nucleation and propagation is emphasized.

An experimental investigation was conducted to find the associated changes in characteristics of wear before and after the transition between fretting and reciprocating sliding wear. A set of experiments were carried out using a AISI 52100 steel ball rubbing against a plate specimen made from the same steel under dry condition. Wear coefficient, wear volume, coefficient of friction, profile of the scars and wear debris were analyzed. The results displayed that there were significant differences in wear coefficient, wear volume, profile of the wear scars and wear debris before and after the transition. Wear coefficient and wear volume at a constant sliding distance were found to be the most appropriate for identifying the transition amplitude between fretting and reciprocating sliding wear.

Fretting wear tests with a large range of displacement amplitudes have been carried out using molybdenum disulfideŽMoS2. bonded solid lubricant coatings. Dynamic analyses in combination with microscopic examinations through SEM, EDX and XPS have been performed. Experimental results showed that such MoS2 coatings influenced greatly the fretting regimes and reduced the coefficient of friction. Further studies on comparisons between MoS2 bonded solid lubricant and magnetron-sputtered MoS2 coatings have been made in fretting conditions. MoS2 bonded solid lubricant, as one of the simple and cheap modification techniques is also considered as an effective palliative for fretting.