Temporal development of indentation plasticity on the atomic scale revealed by force microscopy

Time-dependent indentation plasticity experiments have been conducted with single-dislocation resolution on KBr(100) surfaces using atomic force microscopy (AFM) in ultrahigh vacuum. Discontinuous displacements of the the tip (pop-ins) with a typical distance on the order of 1 Å or less indicate the nucleation and glide of single dislocations within the sample. Pop-in events were observed to occur repeatedly for as long as 4 min while holding the indentation at constant load. These observations indicate that nucleation of dislocations below the indenting AFM tip is stress assisted and thermally activated. The rate of pop-in events decays with time in a power-law dependence with an exponent of −0.8. The characteristic decay of indentation creep in AFM indentation is much slower than in instrumented nanoindentation for comparable experimental conditions. Closed-loop load controlled and open-loop indentations result in the same pop-in displacement and rate, proving that in AFM-based indentation the influence of instrumental inertia is small compared to most instrumented nanoindentation experiments. A comparison between indentation with sharp silicon tips and with blunter diamond tips demonstrates the importance of the tip radius even at the nanometer length scale; sharper tips activate additional glide systems.