Characterisation of TiN and carbon-doped chromium thin film coatings by acoustic microscopy

Acoustic microscopy has been used as a non-destructive analytical tool for characterising bulk materials (fused silica, steel) and thin films (carbon-doped chromium, titanium nitride). In the acoustic signature mode (quantitative mode), elastic parameters such as Young's modulus and Poisson ratio local modulus are extracted. In the case of coatings, the variation of wave velocity as a function of operating frequencies (30 MHz-2 GHz) is modelled by an open system whose parameters (longitudinal and transversal velocity of the coating and the substrate, thickness of the coating) are adjusted. The numerical results are compared to those obtained by impact excitation and to depth-sensing instrumented indentation. In the imaging mode (qualitative mode), this technique allows a clear distinction of the surface grain structure from the stress gradients at grain boundaries and from the mechanical anisotropy of differently oriented grains, without etching. Moreover, tomographic imaging of subsurface flaws is shown. For example, hidden scratch-induced cracks are revealed by comparison with standard metallographic microscope observations. (C) 1999 Elsevier Science S.A. All rights reserved.