The hardness and strength of metal tribofilms: An apparent contradiction between nanoindentation and pillar compression

After sliding contact of a hard spherical counterface on a metal surface, the resulting wear scar possesses a complex microstructure consisting of dislocations, dislocation cells, ultrafine or nanocrystalline grains, and material that has undergone dynamic recovery. There remains a controversy as to the mechanical properties of the tribolayer formed in this wear scar. To investigate the properties of this thin layer of damaged material in single crystal nickel, we employed two complementary techniques: pillar compression and nanoindentation. In both techniques, the tests were tailored to characterize the near surface properties associated with the top 500 nm of material, where the wear-induced damage was most extensive. Pillar compression indicated that the worn material was substantially softer than neighboring unworn base metal. However, nanoindentation showed that the wear track was substantially harder than the base metal. These apparently contradictory results are explained on the basis of source limited deformation. The worn pillars are softer than unworn pillars due to a pre-straining effect: undefected pillars are nearly free of dislocations, whereas worn pillars have pre-existing dislocations built in. Nanoindentation in worn material behaves harder than unworn single crystal nickel due to source length reduction from the fine-grained wear structure.