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 divided by 13 nm, and contain a high density of dislocations (similar to 5 x 10(16) 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 (similar to 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. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.