Cyclic loading as an extended nanoindentation technique

Abstract

Conventional depth sensing nanoindentation test which produces a load–depth curve for a monotonically increasing load can be used to determine many mechanical properties. Alternatively, if the specimen is loaded to a specific value, unloaded and immediately reloaded, a cyclic nanoindentation curve is produced. The unloading–reloading paths do not necessarily overlap in all materials. The reason for this is thought to be the different response of the dislocations generated under the indentation, in different materials. The dislocation behaviour will be controlled by the chemistry and microstructure of the material. Therefore, sensitive dislocation–microstructure interactions may be detectable in the unloading–reloading curves during cyclic indentation. This paper reports such cyclic nanoindentation investigation done in calcium-doped gold used in wire bonding. It is shown that calcium, even in ppm levels, can influence the cyclic load–depth curves to measurable extents. The loading and unloading profiles obtained are reported and the effect of increasing dopant level is evident in them. This is attributed to dislocation–solute interactions. The microstructure of the material consists of elongated (drawn) grains of diameter of about 200 nm and length of a few micrometers. It is concluded that cyclic nanoindentation could be used as an extended technique to extract sensitive material information that are not reflected in the conventional test.