Plastic Zone Development Around Nanoindentations
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PLASTIC ZONE DEVELOPMENT AROUND NANOINDENTATIONS C. L. Woodcock, D. F. Bahr, N. R. Moody* Mechanical and Materials Engineering, Washington State University, Pullman, WA; Sandia National Laboratories, Livermore, CA ABSTRACT Johnson’s cavity model relating indenter geometry and deformation resulting from elastic-plastic indentations is appropriate for a wide variety of materials. In the case of nanoindentations in single crystal BCC metals, limitations are reached when creep is not fully accounted for. Both the standard Berkovich and cube corner geometries show that the ratio of plastic zone radius to contact radius increases with the duration of time at the peak load. Indenter tip geometry is shown to play an important role in this phenomenon. Length scale phenomena, such as the indentation size effect, are also subject to various interpretations. The traditional definition of hardness does not produce similar trends with indentation length scale between the blunt Berkovich geometry and the sharper cube corner tip. However, the ratio of plastic zone radius to contact radius proves to be a tip geometry independent method of assessing the plasticity of these metals. INTRODUCTION As the field of nanoindentation continues to grow, the technique is being used for a wide variety of testing situations spanning multiple disciplines. One common trend, however, is the push to smaller and sharper tips for sampling near surface properties and small volumes. For example, extremely small tips must be used to study biological materials [1] and to test piezoelectric materials for microelectromechanical systems (MEMS) [2]. Unfortunately, several problems can arise with the use of these sharper tips. It has been shown [3] that sharp tips are difficult to calibrate correctly, especially at very shallow depths. This problem manifests itself as a disparity in the calculated hardness and modulus due to length scale issues in the tip geometry that are not accounted for in traditional indentation mechanics. One large-scale limitation of indentation techniques is that primarily blunt angle tips are used (such as the Berkovich or Vickers geometry). This leads to problems in achieving lateral resolutions better than 50 nm, as a 10 nm deep indentation has a contact radius of approximately 50 nm. For probing nanostructured features, it is also necessary to position the tip in relationship to the surface using non-optical imaging techniques. A common method in this case is to couple the indentation equipment with scanning probe microscopy (SPM), where the surface can be imaged prior to indentation. In SPM, tips 60° or sharper are used to improve the lateral resolution of the image, and are commercially available. Many researchers now using SPM techniques to perform nanoindentation are utilizing the same tip to both indent and image [4]. This creates a controversy between either using a tip that is robust and developed for indentation or one that is fragile and deforms with an additional “cutting” mode rather than the traditional indentation defor
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