Cracking behaviour of various aluminium alloys during fretting wear

Fretting wear tests were performed on several aluminium alloys (2024, 2091, 7075) in air. The normal load ranged from 200 to 1000 N and the imposed displacement from ± 15 to ± 100 μm. The tangential force was recorded during each cycle of the fretting test and plotted as a function of the displacement (F-D cycle). Three kinds of F-D cycle were observed depending on the fretting conditions: the closed, elliptic and parallelepipedic cycle. Friction logs (i.e. a three-dimensional representation of the F-D cycles for a given test) are used to describe fretting regimes and fretting transitions. Optical examination of the superficial layers of the samples was performed after each fretting test to characterize the material response. Two degradation modes, cracking and particle detachment, were observed. They were achieved generally simultaneously but, depending on the fretting regime, one or the other was predominant when comparing the behaviour of the three alloys. Results of crack initiation and propagation were found to fit well the classical fatigue properties of the three aluminium alloys. Wear debris originated from the transformed layer, whatever the metal alloy. The results are discussed in term of fretting maps: a running conditions fretting map which describes the fretting regime vs. the fretting conditions (load and displacement), and a material response fretting map which allows one to define the domains of degradation (no degradation, cracking, particle detachment) vs. the fretting conditions.