The role of debris in the fretting wear of a SiC reinforced aluminium alloy matrix composite

The addition of 20 vol.% of 3 μm SiC particles to a 2124 Al–Cu–Mg alloy matrix can achieve appreciable improvements in fatigue strength, by 50%, and fretting wear behaviour against a 0.4% carbon steel, over sliding distances up to 80 m. Over greater distances, the fretting performance is inferior to that of the matrix alloy. However, anodising of the composite, to a thickness of 10 μm, gives more than a factor of three reduction in wear, at least for sliding distances up to 800 m. In this paper, the role of debris in the fretting wear of the composite, for strokes of 40, 80 and 120 μm, is examined. Tests were carried out using a crossed flat-against-flat specimen arrangement. Localised adhesive transfer, from the composite to the steel counterface, occurred during the early stages of fretting, particularly at the lowest value of stroke. It was sufficient to separate the contacting surfaces, so that wear was initially patchy. As fretting continued, layers of compacted debris were formed. Cross-sectional scanning electron microscopy/X-ray analysis revealed the presence of Al, Fe, Si, C and O, but an absence of atomic contrast in the majority of layers, suggesting that, at a minimum, intimate mixing of composite and counterface debris had occurred. Debris particles ejected from the contact area increased in size with increasing fretting stroke. Transmission electron microscopy of these particles, after tests at all values of stroke, showed them to be agglomerations of sub-micron particles containing iron and possibly a spinel, FeAl2O4 or Fe3O4, suggesting that a degree of mechanical alloying had occurred. The matrix alloy, Al2O3 and Fe2O3 were not discernable, although SiC particle remnants were occasionally found.