Nano-meter scale plasticity in KBr studied by nanoindenter and force microscopy
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1185-II07-08
Nano-meter scale plasticity in KBr studied by nanoindenter and force microscopy P. Manimunda1, T. Filleter2, P. Egberts2,3, V. Jayaram1, S.K. Biswas1, and R. Bennewitz2,3 1 Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India 2 Department of Physics, McGill University, Montreal, Canada 3 INM – Leibniz Institute for New Materials, Saarbrücken, Germany ABSTRACT The early stages of plasticity in KBr single crystals have been studied by means of nanometer-scale indentation in complementary experiments using both a nanoindenter and an atomic force microscope. Nanoindentation experiments precisely correlate indentation depth and forces, while force microscopy provides high-resolution force measurements and images of the surface revealing dislocation activity. The two methods provide very similar results for the onset of plasticity in KBr. Upon loading we observe yield of the surface in atomic layer units which we attribute to the nucleation of single dislocations. Unloading is accompanied by plastic recovery as evident from a non-linear force distance unloading curve and delayed discrete plasticity events. INTRODUCTION A prominent element of discrete plasticity is the pop-in event observed in nanoindentation experiments on many materials. It comprises the sudden creation of a multitude of dislocations and marks the transition from elastic to elasto-plastic response of the surface. Despite the singularity of the pop-in event, the resulting defect structure in the plastic zone can still be too complex for a full microscopic understanding. Various experimental strategies have been implemented to arrive at a simpler plastic response that allows for a detailed analysis. Woirgard and collaborators proceeded in their studies of MgO from a Berkovich indenter to a large-radius spherical indenter. The indentation experiments then resulted in a well-defined set of dislocation half-loops which could be quantitatively analyzed using a combination of etching techniques and atomic force microscopy imaging [1]. Gerberich and co-workers have followed nanometer-scale plasticity in great detail by studying isolated silicon nanoparticles and ultrathin metal films [2,3]. They found that few dislocations trapped in nanoparticles between the indenter and a rigid substrates cause a significant work hardening, and that the interactions between dislocations separated by only a few nanometers can drive reverse plasticity after unloading. Our approach to enter this regime attempts to limit the number of dislocations by employing very sharp tips and very low loads in indentation experiments on single crystal samples. This strategy allows the observation of single-dislocation pop-in events. In this paper we directly compare the two experimental methods, nanoindentation and atomic force microscopy (AFM) on the example of nanometer-scale indentation into KBr single crystals. EXPERIMENTAL METHODS Nanoindentation studies the local elastic and plastic response of a surface to a point-like load when a sharp tip i
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