Positron Annihilation Studies of Subsurface Zone Created during Friction in Pure Silver

Abstract

Positron lifetime research has been performed on well-annealed pure silver samples after dry sliding, compression, and sandblasting processes. These treatments affect the surface but also have been shown to significantly modify the region adjacent to the surface. A great number of dislocations decorated with monovacancies and jogs are observed in the layer up to 110 µm beneath the surface. In the deeper layer, which extends to a depth of 300 μm, their concentration decreases gradually. Unlike other metals, the total depth of the subsurface zone induced by dry sliding is almost independent of the applied load. A significant increase in the positron lifetime has been observed in the layer 15 µm below the worn surface in the sample exposed to a long duration sliding test. Large clusters that consist of about 12 vacancies are observed in this layer, which can be a tribolayer that occurs as a result of dynamic recrystallization near the surface. This shows that conventional positron techniques can be effective tools in the investigation of defects in tribolayers, whose thickness is about several dozen micrometers. For a sample exposed to sandblasting, the total depth of the subsurface zone is smaller—that is, about 120 µm; however, the depth dependency of the positron lifetime is very similar to that obtained for dry sliding.