With our instrument, the penetration depth of a three-faced indenter is continuously measured during both indentation and unloading. Useful loads are about 10 μN to 30 mN (1 to 3000 mg). The specimen can be transferred between two locations (test and microscopic observation); a particular area can be relocated by means of a computer-controlled three-dimensional movement. Scratch hardness testing with measurement of frictional force can be performed at speeds as low as 30 nm/s. The general shape of the depth/load graphs, together with measurement of hysteresis and slope, gives an initial characterization of the specimen. Further analysis involves various theoretical topics, which we review in the light of recent literature. These include the way in which a variation of yield stress with depth will affect the measurements and the surface effects, such as the contribution of surface energy to the effective applied load. A controversial topic is the extent to which the depth of indentation in metals, relative to the mean distance between preexisting dislocations or grain boundaries, can lead to a true size effect, or even a critical load below which no plastic deformation occurs. Indentation creep may be significant even for metals such as steel and nickel at ambient temperature. Accordingly, the concept of microindentation hardness as a material property can be of doubtful value for depths in the 10 to 100-nm range; the apparent mean pressure can be characteristic of a depth much greater than that of the plastic indentation. However, it is still possible to obtain measurements within this depth range, to deduce the type of regimen (for example, elastoplastic or fully plastic), and to derive alternative semiempirical indexes of the extent of plastic deformation, elastic recovery, and time-dependent behavior.

Pollock, H., Maugis, D., & Barquins, M. (1985). Characterization of Submicrometre Surface Layers by Indentation. Microindentation Techniques in Materials Science and Engineering, 47–71.