Scanning Acoustic Microscopic and Brillouin Scattering Investigation of Surface Damage
Mechanical polishing involves progressive removal of material through repeated rubbing against an another material in presence of abrasives. Near the surface large strains are induced. The existence of surface layer that has been strained to different degree than the bulk has been demonstrated by several techniques like X-ray diffraction, electron microscopy, positron annihilation, hydrogen permeation and mechanical measurements[l]. It is expected that the mechanical properties of material near the surface might be different from the bulk interior. It has been shown that this top layer has large influence on stress-strain curves, creep, fatigue and stress corrosion resistance of metals. Although a variety of techniques have been used to investigate the effect of surface layer on the bulk material properties, the properties of the layer itself have not been investigated in detail, except for some micro hardness and flow stress measurements. These techniques measure the plastic properties of the material. The flow stress of the surface layer of single crystal copper has been found to be lower than the bulk, while in aluminum single crystals the opposite behavior has been reported. The micro hardness measurement of the surface layers has shown two distinctive behaviors. A few investigators have shown that the surface hardness is greater than the bulk, while several others have pointed out that it is low near the surface and increases with the depth and goes through a maximum before reaching the bulk hardness. A discussion about the controversies of the near surface mechanical properties can be found in Ref.. Theoretically both behaviors of flow stress and micro hardness have been explained [7,8,9]. The softness of the layer has been explained on the basis of image forces on dislocation, while the hard surface layer is attributed to the increase of dislocation density due to deformation.
KeywordsFatigue Nickel Anisotropy Acetone Argon
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