A comparative study of the mechanical properties of TiN coatings using the non-destructive surface acoustic wave method, scratch test and four-point bending test

Abstract

Surface acoustic waves (SAWs) are an interesting non-destructive tool for thin film characterization. Measuring the surface wave phase velocity as a function of frequency enables the Young's modulus of the film material to be determined. In addition, the thickness and/or density of the film can be determined simultaneously, if the film thickness is larger than 1 urn and the bandwidth of the SAW measurement is greater than 100 MHz. The uncertainty of the Young's modulus determination is in the range ± 1%. The Young's modulus is a characteristic material parameter which is dependent on the atomic composition and microscopic structure of the material. The Young's modulus is also sensitive to defects. It has been established both theoretically and experimentally that a higher defect density in a material causes a decrease in the Young's modulus. Thus it is possible to investigate the adhesion by the SAW method, since a higher defect density within a brittle coating reduces the adhesion. Therefore TiN-coated steel (42CrMo4) samples with different states of adhesion were prepared. These samples were investigated by the SAW method, scratch test and four-point bending test. A significant correlation between the SAW results and the results of the destructive adhesion tests was found. The SAW method was shown to be complementary to the destructive adhesion tests. In addition, some results of the scratch test could only be explained by taking the SAW results into account.