Instrumented Nanoindentation 411 Principle and Equipment

Conventional hardness measurement is insufficient to estimate the properties of, for example, thin nanostructured films, because of its low accuracy in the low-load indentation region that is due to limitations of the resolution of the optical systems attached to the hardness testers of conventional type. The development of the testing equipment has enabled the users to continuously monitor the load experienced by the indenter and the depth of penetration. This made it possible to overcome the limitations of the optical system and allowed us to derive new information on the complex mechanical behavior of materials deformed in a very small area. Thus, the nanoindentation appears as a unique method for studying the mechanical properties of solids with a tiny volume.

The idea of depth-sensing indentation measurements was already realized for the first time more than two decades ago [144], while the first nanoindentation testers were designed ten years later [145-148]. Further, the pioneering results of ultra low-load indentation reported by Pethica et al. [146] resulted in a rapid development of this new area of research that recently has found its important application in a case of nanostructured materials.

The nanoindentation experiments achieved a "new level," owing to the new phenomena available for study as a consequence of recent developments in this particular research equipment. This has stimulated an urgent need for a general theory of indentation-induced deformation, which would yield a sound basis for interpreting experimental results. Considerable effort has already been undertaken to analyze the data of depth-sensing tests and to relate them to the observed phenomena [146, 148-152].

The significant progress has been achieved in this area through numerical modeling based on the finite element method [113, 153, 154]. However, the critical issue of this approach lies in the formulation of the pertinent constitutive equations as well as in the difficulty in estimating the elastic/plastic stress-state in the vicinity of the contact. The only case solved analytically is the pure elastic contact of spherical (Hertzian indentation [155, 156]) and axisymmetric sharp indenters (cone indentation—Boussinesq stress field [157]).

Nanoindentation devices which allow us to use loads as low as fraction of millinewtons have recently been developed in response to an increasing number of requirements for the mechanical testing of crystalline thin films designed for electronics. Some commercial instruments operate automatically after initial setup and measure independently force and displacement experienced by the indenter tip. The successful examination of nanomaterials is in a part due to the selected tester. Indeed, the precision of measurements is of primary importance when one deals with new, advanced materials, which have never been characterized so far.

Indentation techniques may be broadly classified according to the shape of the indenter used in experiments. It was found that sharp indenters, for example, the Berkovich pyramid, are useful in the investigation of mechanical properties in the smallest possible region, while the spherical tip is recommended for crystalline material with remarkable anisotropy.

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