Nanoindentation
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Indentation tests, sometimes called hardness tests, are perhaps the most commonly applied means of testing the mechanical properties of materials. The technique has its origins in the Mohs scale of mineral hardness, in which materials are ranked according to what they can scratch and are, in turn, scratched by. The characterization of solids in this way takes place on an essentially discrete scale, so much effort has been expended in order to develop techniques for evaluating material hardness over a continuous range. Hence, the adoption of the Meyer, Knoop, Brinell, Rockwell, and Vickers hardness tests. More recently (ca. 1975), the nanoindentation technique has been established as the primary tool for investigating the hardness of small volumes of material.
In an indentation test, a hard tip whose mechanical properties are known (frequently made of diamond) is pressed into a sample whose properties are unknown. The load placed on the indenter tip is increased as the tip penetrates further into the specimen and soon reaches a user-defined value. At this point, the load may be held constant for a period or removed. The area of the residual indentation in the sample is measured and the hardness, H, is defined as the maximum load, Pmax, divided by the residual indentation area, Ar , or
<math>H=\frac{P_{max}} {A_{r}}</math>.
For most techniques, the projected area may be measured directly using light microscopy. In nanoindentation, however, this area may only be a few square micrometres and so presents problems in determining the hardness. Atomic force microscopy or scanning electron microscopy techniques may be utilized, but can be quite cumbersome. Instead, an indenter with a geometry known to high precision (usually a berkovich tip) is employed. If a record of the depth of penetration is made, then it becomes possible to determine the unknown area through an application of geometry. In this fashion, the important parameters to be measured during a nanoindentation test are the load and the depth of penetration. A record of these values, plotted on the y and x-axes, respectively, of a chart is called a load-displacement curve and is the most important data associated with a given experiment. These curves can often be used todetermine other mechanical properties of the material, such as the modulus of elasticity, by means of analysis of the unloading slope, dP/dh.
The construction of a depth-sensing indentation system is made possible by the inclusion of very sensitive displacement and load sensing systems. Load transducers must be capable of measuring forces in the micronewton range and displacement sensors are very frequently capable of sub-nanometer resolution.
Conventional nanoindentation methods for calculation of Modulus of elasticity or Young's Modulus (based on the unloading curve) are limited to isotropic materials that show no time-dependent behaviour (creep or damping) such as metals. Attempts are being made to extend the technique to the characterisation of thin films. Problems associated with the pile-up and sink-in of the material during the indentation process remain a problem that is still under discussion.
[edit] References
Fischer-Cripps, A.C. Nanoindentation. (Springer: New York), 2004.
W.C. Oliver, G.M. Pharr J. Mater. Res. 7 (1992) 1564.
Y.-T. Cheng, C.-M. Cheng, Scaling, dimensional analysis, and indentation measurements, Mater. Sci. Eng. R, 44 (2004) 91.
J. Malzbender, J.M.J. den Toonder, A.R. Balkenende, G. de With, A Methodology to Determine the Mechanical Properties of Thin Films, with Application to Nano-Particle Filled Methyltrimethoxysilane Sol-Gel Coatings,Mater. Sci. Eng. Reports 36 (2002) 47.
