Non-destructive evaluation of TiN films with interface defects by surface acoustic waves

Abstract Surface acoustic wave (SAW) measurements are a fast and reliable way to obtain information about the elastic properties of thin film coatings. Measuring the surface wave phase velocity which is dependent on frequency and performing the inverse solution of dispersion relation for surface wave propagation in coated materials enable us to determine the elastic parameters of the film and substrate material. Measuring equipment is used where wide-band surface wave impulses are generated by short laser pulses and received with a piezoelectric transducer. The surface wave phase velocity is obtained by Fourier transforming the impulse form. In this work we are interested in Young's modulus of the film material and its relation to the defects in the interface region of the substrate. Young's modulus is a characteristic material parameter, depending on atomic composition and on the microscopic structure of the material. It is theoretically and experimentally established that, if defects such as flaws are incorporated, Young's modulus is reduced in comparison with that of bulk material. The examined samples were steel pieces (42CrMo4) coated with TiN by ion plating. During the deposition process we have only varied the duration of the ion etching process, while keeping all other technological parameters constant. The ion etching is responsible for the adhesion of the coating by removing residual contamination from the substrate surface. In this way a more or less non-defective interface zone is built up. The procedure of varying only the etching time allows us to obtain different interface properties while leaving film properties constant. Film properties were characterized by means of Auger, X-ray texture and dynamic hardness measurements. It was shown that there is a correlation between Young's modulus and conventional scratch test results. The acoustic emission was used to evaluate the coated material behaviour in the scratch test. From a Weibull diagram of the acoustic emission signals an initial crack load L i was determined. It could be proved that the initial crack load and the Young's modulus correlate with a coefficient R = −0.78.