Introduction

Load and depth-sensing indentation, also known as the nanoindentation technique, started to be developed in the 1980s, and nowadays is being continuously improved. The experimental device is based on recording load, depth, and time during the indentation process. The physical models and experimental systems are based on contact mechanics adapted to conditions of very low applied loads and indenter displacements. This means sensitivity of the applied load and tip penetration on the order of micronewtons and nanometers, respectively.

The determination of the mechanical properties of nano-structured materials is a new and very interesting area. Nanostructured materials are characterized by nanoscale structures with a significant volume comprised of surfaces, interfaces, and grain boundaries, giving properties vastly different and often greatly superior to their bulk-metallic, ceramic, and polymeric counterparts. They can be produced by different techniques to form a myriad of engineered structures. These advances will provide better sensors, medical diagnostics, displays, data storage technologies, and surface protection coverings. However, the determination of their mechanical properties and how these properties are measured in nanostructured materials are in constant revision. As a result, an intense effort has been made in recent years to adapt and develop methods in order to measure deformation and fracture properties at the nanoscale. At the same time, experimental and theoretical work is being done to determine and understand the properties and processes influencing the mechanical behavior and fracture in small volumes. Advanced and novel characterization and testing methods, as well as analytical, continuum, and atomistic simulations, are being continuously developed. Of particular interest are those studies that extend over length scales from atomistic to nanometer or from nanometer to submicron, and thus provide insight regarding length-scale effects.

The characterization of mechanical properties at nanoscale includes: structure-property relationships at the nanoscale in nanostructured materials, composites, films, multilayers, and functionally graded materials; mechanical properties of bulk materials at the nanoscale; deformation and fracture processes at the submicron scale; dislocation emission at probes and crack tips at the nanoscale; effects of intrinsic stresses on properties and fracture; creep measurement and mechanism determination; tribological properties of nanostructured material; comparative results for the modeling of submicron-scale indentation, scratch, and wear response; and development of standards tests to measure mechanical properties at the submicron scale.

The purpose of this review is to show the reader the basic concepts of nanoindentation, and how it can be used to determine mechanical surface properties in the nano-scale regime. These concepts include the basic equations, a brief discussion about theoretical models, the equipment calibrations, and the correct choice of an appropriate inden-ter. In the following, focus will be given to a series of application results for modified surfaces and thin films (coatings),

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 7: Pages (1-20)

tailoring the use of this technique. The further tendencies about using load and depth-sensing indentation are also shown.

This chapter consists of four parts. First, a definition of nanoindentation and some comments with respect to how this technique can be useful for nanoscience and nano-technology are given. Historical aspects, fundamental equations, and experimental methodology follow. A third part concerns the most important applications and principal results that can be found in the literature. Finally, some discussions about new applications, recent developments in the technique and in the theoretical approach, as well as future perspectives are also taken into consideration.

Nanoindentation is a term used to designate indentation experiments where very low loads are used to press a hard indenter into a surface in a controlled way at submicron-meter penetration depths. As an important feature of the process, the applied load, the indenter displacement, and the time are continuously recorded during the experiment. Due to its versatility, nanoindentation is one of the most important methods to obtain the mechanical properties in the near-surface regions, and it can be applied to practically all types of solid materials.

The Vickers and Brinnel conventional tests use the indentation image to measure the contact area which is necessary to calculate hardness. Unlike this, in nanoindentation, the contact area is obtained from the load and displacement data recorded in a complete load-unload cycle, without the necessity of imaging methods. If more elaborate analyses are performed, other mechanical properties, such as elastic modulus, residual stresses at the surface, strain rate sensitivity, toughness of brittle materials, and adhesion quality of thin films, can be determined by applying the nanoindentation technique.

The determination of nanomechanical properties is a new field, as well as practically all areas in nanoscience and nano-technology. For this reason, there is a considerable gap to link the mechanical properties from a continuum mechanics approach to an atomistic point of view. This atomistic approach takes into account the forces and movements of individual atoms or groups of atoms that are not considered in continuous mechanics. Atomic force microscopy (AFM) gives us a better understanding of the weak mechanical forces that govern the behavior of atoms in the surface of the materials. However, AFM is not well suited to determine the behavior of the material in a region deeper than a few atomic layers under the surface. Nanoindentation that presents high depth and load resolution is very useful to correlate the nanoscale mechanical behavior with the material structure. In the near future, an improved modeling and interpretation of load versus displacement data is expected since a considerable effort is being developed in the nanoindentation area. The new incoming equipment will probably become better. For these reasons, nanoindentation will continue to play an important role in nanotechnology development.

Surface mechanical properties of several different materials, like metal nanocomposites, polymer films, metal surfaces, coatings, and microelectromechanical systems (MEMS), are being determined by applying the nano-indentation technique. Tailoring modified surfaces to improve mechanical and tribological applications requires a good knowledge of the mechanical properties at the material surface. Knowledge of the hardness, elastic modulus, and wear resistance is very important to specify the correct application for newly developed materials used in the electronic, automotive, or aerospace industry. Then, nanoindentation has an important role in the analysis of nanoscale mechanical properties of materials, and its application is continuously increasing. Finally, despite the fact that results will be more consistent, a great effort must yet be made to adequately interpret the depth and load-sensing indentation data because the deformation process under the indenter is very complex.

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