The hardness of a material is very useful in predicting how a material will perform while interacting with another and also in understanding why a material performed the way it did. Many times, especially in micro/nano-scale systems, it is beneficial to know the hardness near the surface as it is most likely different from the bulk of the material. This can be accomplished with nano-indentation which involves indenting a material with a diamond tip at a very low load and sensing the depth of the indentation. Images of indentations on a thin wire produced with a scanning electron microscope are shown below.
This is an image four consecutive indentations spaced 20 µm apart. Note that a human hair is roughly 50-80 µm in diameter. (Yu et al., 2004)
A closer look at an indentation (Yu et al., 2004)
The hardness is determined by dividing the maximum load by the projected area of the indentation. Nano-indentation is also used to determine the elastic modulus of materials.