The multiple "pop-in" events were observed in the load-displacement (P h) curve and appeared to occur randomly by increasing the indentation load.
The curve exhibits irregularities during loading characterized by small jumps in the penetration depth (see the arrows), referred to as the multiple "pop-in" events in what follows.
Since the multiple "pop-in" events are usually observed after the permanent plastic deformation has occurred and two of the possible mechanisms, the deformation-induced phase transition [14] and fracture of thin films [23], were basically ruled out, the most likely mechanisms responsible for the multiple "pop-in" events appear to be associated with the activation of dislocations sources [24].
Returning to the multiple "pop-in" events, it should be noted that the phenomena have been observed previously in hexagonal structured sapphire [19], single-crystal bulk ZnO [20], and GaN thin films [21].
The fact that multiple "pop-in" events are observable over such a wide range of indentation load and penetration depth indicates the close relations to the plastic deformation of GaN thin films.
In this scenario, the plastic deformation prior to the multiple "pop-in" events is associated with the individual movement of a small number of newly nucleated and pre-existing dislocations.
In addition, CL imaging of indents displays to be a sensitive measure of the onset of slip since CL contrast of indents correlates well with the observation of multiple "pop-in" events.
Results show the clear multiple "pop-in" events (indicated by the arrows) during loading, while no "pop-out" event is evident in the unloading segments, indicating that no phase transition is involved.
Herein, in this study, Berkovich nanoindentation-induced multiple pop-in behaviors were observed and the mechanical properties of the hexagonal GaSe thin films were obtained by analyzing the nanoindentation load displacement curves.
In addition, according to the previously studies [10, 17], we note here that the critical applied indentation load for direct identification of the multiple "pop-in" events in P h curve is not only dependent on the type of indenters used, but also even very much dependent on the test systems and the maximum indentation loads applied.
Nevertheless, the above discussions do suggest that multiple "pop-in" events indeed are specific features of materials with the hexagonal lattice structure and the geometry of the indenter tip may play an important role in determining the nanoindentation-induced mechanical responses.
