Rethinking the role that the “step” in the load-displacement curves can play in measurement of fracture toughness for hard coatings

The “step” in the load-displacement curves during nanoindentation of hard coatings has been regarded as a representation of the energy dissipated during fracture of the coating thus is used in measuring the fracture toughness of this coating. This paper scrutinizes the “step” and finds that the “step” is a gap artificially bridged up with a straight line and that the “step” size corresponds with the thickness of the coating in testing. It is realized that the gap is formed due to loss of contact of the indenter with the sample. Upon catastrophic fracture of the coating, the indenter undergoes a freefall of a distance about the thickness of the coating. The size of such a “step” has no logic relationship with the energy dissipation that fractures the coating before the freefall of the indenter takes place.