Microstructure of Cu–Be alloy triboxidative wear debris

The wear mechanism in disk-on-disk tests of a Cu–Be alloy against AISI D2 steel counterparts changes from metallic to oxidative for increasing loads. At low load the wear debris are made of the Cu–Be alloy, whereas above 50 N the main constituents are copper oxides, with a few residual particles of the copper-base alloy. Despite the lower oxidation state, cuprite (Cu2O) is the main fraction of the high-load debris, whereas tenorite (CuO) is less than 10%. An analysis of the X-ray diffraction line profile, supported by high-resolution transmission electron microscopy (TEM), shows that cuprite domains are nanocrystalline, with domain sizes distributed about a mean value of 12 ÷ 13 nm, and contain a high density of dislocations (∼5 × 1016 m−2). The small domain size is considered as a possible stabilization mechanism of cuprite against the higher oxidation state oxide tenorite, whereas the large dislocation content is a consequence of the heavy plastic deformation in the contact area. As a further support to the size-stabilization mechanism, a diffraction measurement repeated on the wear debris after a 6 month aging shows a marked increase in the tenorite fraction. According to line profile analysis, the remaining cuprite fraction is made up of nanocrystalline domains (∼6 nm) smaller than in the as-produced debris, thus supporting the hypothesis that small cuprite grains are more stable than larger ones, which more easily transform to tenorite.