Advanced indentation technique for strength evaluation of hard thin films

Abstract

A new hybrid technique for studying the fracture mechanism and strength of hard thin film is developed. The technique utilizes acoustic emission (AE) data, corrosion potential fluctuation (CPF) data and finite element method (FEM) for Vickers indentation test. This method was applied to TiN film deposited by PVD method on four metallic substrates of austenitic stainless steel, carbon steel, forging steel and pure iron.We detected both the AE and CPF from equidistant ripple cracks or step-wise terraces produced outside of the indentation during loading in a buffer solution (H3PO4, pH 6.86). Characteristic potential fluctuations, termed as a RD-type CPF (rapid drop to active potential and gradual recovery) and strong AEs were simultaneously produced by Mode-I ripple cracks of the film. They were produced by film bending during the penetration of the pyramidal indenter. The number of ripple cracks observed during indentation agreed well with that of RD-type CPF. Detailed analysis indicated the generation of ripple cracks at a constant distance Lc. The value of Lc can be predicted by the RD-type CPF timing and the penetration rate of the indenter. Bending strength of the film was estimated by FEM using the Lc value. Intrinsic strength of the TiN film, determined by subtraction of residual stress, was measured as 0.9–1.4 GPa. These values are almost constant independent of the substrate material and consistent with the reported strength of 1.2 GPa.