Pop-ins in Nanoindentations – the Initial Yield Point

The load – displacement curves as obtained by nanoindentations on most metallic and intermetallic materials show discontinuities or pop-ins at load levels below or around 1 mN. These pop-ins mark a sharp transition from pure elastic loading to a plastic deformation of the specimen surface, thus correspond to an initial yield point. On smooth surfaces pop-ins are observed frequently, but not on surfaces with a high roughness. Step edges on the surface are discussed as dislocation sources for the initial yield events. Since the pop-ins mark the transition from elastic to plastic deformation, the elastic loading part can be described by the Hertzian contact model. Discontinuities in the load –displacement curves are observed on nearly all materials, examples are given for c-BN, TiAl, Ti Al, Cu, and Mo SiB . On Mo samples, discontinuities are found sometimes only in the unloading part of the load – displacement curves. The origin of the different pop-in effects are discussed.