Pop-ins in nanoindentations - the initial yield point

The load-displacement curves as obtained by nanoindentations on most metall ic and intermetallic materials show discontinuities or pop-ins at load leve ls below or around 1 mN. These pop-ins mark a sharp transition from pure el astic loading to a plastic deformation of the specimen surface, thus corres pond to an initial yield point. On smooth surfaces pop-ins are observed fre quently, but not on surfaces with a high roughness. Step edges on the surfa ce are discussed as dislocation sources for the initial yield events. Since the pop-ins mark the transition from elastic to plastic deformation, the e lastic loading part can be described by the Hertzian contact model. Discont inuities in the load-displacement curves are observed on nearly all materia ls, examples are given for c-BN, TiAl, Ti3Al, Cu, and Mo5SiB2. On Mo sample s, discontinuities are found sometimes only in the unloading part of the lo ad-displacement curves. The origin of the different pop-in effects are disc ussed.