Application of Positron Annihilation Studies of the Subsurface Zones Beneath the Surface Exposed to Normal Loading

This paper presents an experimental study of the subsurface zones in copper induced by normal loading of a spherical indenter, detected by the positron annihilation technique. The obtained depth dependencies of the S-parameter were compared with the principal stress distributions beneath the ball predicted by the elastic theory. Our studies show that the depth-concentration profile of open-volume defects is not uniform and exhibits a local minimum. Nevertheless the concentration profile decays with increasing depth, reaching a value for residual defect concentration at a depth where the von Mises criterion for yield is fulfilled. For comparison, we performed studies of subsurface zones which occurred in compressed copper samples and a sample whose surface was blasted by silicon carbide particles. The detected zones exhibit quite different features in both cases. A similarity between the dependency of the S-parameter on the depth obtained for the case when the surface was blasted and when the surface was exposed to sliding contact during the friction process was found.