Identification of critical load for scratch adhesion strength of nitride-based thin films using wavelet analysis and a proposed analytical model

• Wavelet Analysis (WA) of scratch test in thin film adherence
• Micro-scratch testing for TiSiN coating on high speed steel
• Wavelet Analysis establishes indication of scratch adhesion strength.

The use of Acoustic Emission (AE) signal of scratch test to evaluate the thin films' adherence led to unavoidable large uncertainties due to the problematic determination of the frequency band and the improper installation of the acoustic sensor, especially for coatings thinner than 100 μm. In this paper, thin film–substrate adherence in high-speed steel is evaluated by Wavelet Analysis (WA) of the force–displacement curve in micro-scratch tests performed on the samples. The measured load displacement signals of deposited thin films were used as inputs for the wavelet module. We propose an analytical model combined with wavelet analysis, which is based on experimental data. Scratch adhesion strength could be identified by applying wavelet technique to the measured force displacement data to detect the location of the critical load during micro-scratch testing for TiSiN coating on high-speed steel. An analytical model is also proposed to predict the adhesion stress and to support the wavelet analysis technique where the stress is difficult to be assessed. Thin films were studied by optical microscopy, micro scratch testing machine and X-ray diffraction. The WA results are compared with model predictions in order to establish indication of scratch adhesion strength aimed at improving the manufacturing process.