Determination of the critical indentation depth for measuring coating-only mechanical property

Abstract

The critical indentation depth to coating thickness ratio below which the substrate has a negligible effect on the indentation response is identified for some coating-substrate systems by using finite element simulation or empirical hardness measurement. However conventional nanoindentation test generally controls its peak penetration depth shallower than 10 % of the coating thickness in order to measure the coating-only mechanical property without considering detailed information on the critical indentation depth for a given coating-substrate system. This uncertain testing condition makes hardness measurement more difficult. Thus, we proposed a new method for determining the critical indentation depth for general coating-substrate systems. A series of nanoindents were made on soft Au coating by applying various indentation loads and then observed with an atomic force microscope. The remnant indentation volumes were quantitatively calculated from the three-dimensional indent morphologies. In shallow indentation regime, a trend of the indentation volume depended on three-dimensional morphology of the Berkovich indenter. When the indenter penetration depth surpasses the targeted critical indentation depth, the remnant indentation volume deviated from its original trend due to severe elastic recovery generated by the hard Si substrate. The critical indentation depth of Au/Si system was measured by superposing two indentation volume trends from shallow and deep indentation regimes. Within the critical indentation depth, the hardness of the Au film was nearly constant 0.95±0.02 GPa and the validity of the proposed method was confirmed empirically.