Microstructure and mechanical behavior of magnetron sputtered Ta, Nb and Mo thin films on NiTi and Si substratesFreitag (07.07.2017) 11:10 - 11:30 Uhr Bestandteil von:
Superelastic Ni-49.2 at.% Ti alloy and Si(100) substrates were coated with 10 µm thick Ta, Nb and Mo films, respectively, by DC magnetron sputtering at room temperature. The films were characterized with regard to their microstructure and selected mechanical properties. The microstructure of the films was determined by X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) analyses. XRD results show, that all films consist only of the equilibrium phase at room temperature. SEM micrographs show a columnar structure of all films, in agreement with the Thornton diagram.
The intrinsic stresses of the films were measured by the substrate curvature technique, applying the Stoney equation. This evaluation reveals different stress states for the various coating materials. The compressive stress level for Ta films (-567 MPa) is significantly higher than for Mo (-224 MPa) and Nb (-118 MPa) films. These stress levels were determined on Si(100)-beams. Indentation experiments were performed using both a nanoindenter and a microindenter. These results show that mechanical properties of the films (i.e. indentation hardness and elastic modulus), deposited on NiTi and Si substrates, cannot be determined according to classical procedures. Indentation results show, that the substrates influence the measurement at indentation depths less than 10 % of the film thickness. Taking this into account, values for both hardness and modulus were very similar to those known for the bulk materials. Progressive and multi-pass scratch tests were used to evaluate the adhesion and fatigue behavior of the films. The scratch tests suggest, that the metallic coatings fail by plastic deformation independent if they are deposited on NiTi or Si substrates. These mechanical film properties will probably not match those of the superelastic NiTi substrates when these composites are subjected to macroscopic deformation. It will be necessary to investigate the deformation behavior of such composites in more detail, i.e. with tensile tests.