On the nature of displacement bursts during nanoindentation of ultrathin Ni films on sapphire

Quasi-static nanoindentation tests were performed on polycrystalline Ni films sputter deposited on basal plane-oriented sapphire substrates. In the majority of tests a combined elasto-plastic response of the film was observed, without detectable displacement bursts during loading. In some of the tests a single large displacement burst was observed slightly below or at the maximal load (75 or 50 μN). The residual plastic depth of the corresponding indents was close to the film thickness. These large displacement bursts were interpreted as manifestations of intergranular brittle fracture in the indented region. The strength of the disclination-type defect at the triple junctions leading to brittle fracture at the grain boundaries was estimated based on the theory of strain gradient plasticity.