Conclusion

Nanoindentation is a powerful technique to determine the surface mechanical properties in the nanoscale regime in practically all kinds of solid materials. This technique is especially important for new materials developed by nano-science and nanotechnology applications. The method is based on the analysis of the load-unload curve, which allows us to obtain information about the elastic-plastic surface behavior in a well-controlled way.

Hardness and elastic modulus values are measured using known analysis methods which are widely accepted nowadays. However, certain additional care is necessary in measurements performed under extreme conditions, such as when the elastic-plastic behavior is strongly dependent on the ratio H/E, or when soft surfaces show pile up around the indenter and very hard surfaces present a sink-in effect. In these situations, the measured values of the hardness and elastic modulus may show deviations from the actual values. Additional care is also recommended for measurements on thin films where the substrate influence is an important factor.

The more common indenters used in nanoindentation are the Berkovich and Vickers tips, but additional information about the mechanical surface properties may be accessed by other kinds of indenters:

• Surface fracture toughness for brittle materials can be studied by using a cube-corner indenter.

• Viscoelastic properties, typical for polymer materials, can be obtained by using cylindrical or flat punch inden-ters.

• Surface adhesion quality between the coating substrate and residual stress at the surface can be studied by using spherical indenters.

A scratch test performed by using a modified nano-indentation machine can also give extra information about the mechanical surface properties, especially for coatings. However, it is not very clear because of their complexity, and mainly due to the frictional force. The obtained results are qualitatively correct, but more theoretical work is needed in order to compare the different results.

Despite the great number of published papers relating mechanical surface properties acquired by using nano-indentation, supplementary efforts still need to be performed on the theoretical approach in order to quantify and better understand the results in the nanoscale regime.

As a future challenge, it is necessary to develop nanoindentation devices that permit us to perform indentations at higher temperatures. In some materials, temperature dependence on their ductile-brittle behavior occurs, and it may be better investigated if high-temperature nano-indentation is available in the future.

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