Development of robust methods for residual stress measurement on PVD coated cutting tools

• Applicants: Professorin Dr.-Ing. Kirsten Bobzin; Professor Dr.-Ing. Hans Jürgen Maier
• Subject Area: Metal-Cutting and Abrasive Manufacturing Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541063095

In order to increase the cutting performance of carbide inserts, nitride hard coatings deposited by means of physical vapor deposition (PVD) are state of the art. During deposition, compressive residual stresses are induced into the coating. These exhibit a geometry-dependent distribution on the inserts, which leads to locally influenced crack resistance and fracture toughness of the coating. The overall objective of the research project is the identification and validation of reliable methods for residual stress measurement and to gain knowledge about compressive residual stresses and crack resistance, with particular regard to this local distribution on cutting inserts. For this purpose, knowledge about the accuracy of the residual stress measurement will be obtained. Furthermore, a better understanding of the distribution of residual stresses is gained. In addition to the two-dimensional distribution on the surface of the insert, the spatial distribution of the residual stresses along the coating thickness is investigated. Furthermore, knowledge is achieved about the relationship between local residual stresses and local crack behavior. Overall, this will contribute to building a better understanding of residual stresses in PVD coatings. In order to achieve the desired objectives, the samples are investigated using combined methods. TiAlN coatings with varying coating thickness and bias voltage will be deposited at Surface Engineering Institute (IOT), RWTH Aachen University, in order to adjust different residual stress states. Measurements on Si wafers with known residual stresses are used at the Institute of Materials Science (IW) of Leibniz Universität Hannover to determine the measurement reliability of the residual stress measurement method consisting of focus ion beam (FIB) and digital image correlation (DIC). By means of nanoindentation, the geometry-dependent distribution of residual stresses across the surface will be investigated at IOT and spatially resolved mappings are generated. For this purpose, the residual stresses will be determined as a function of the distance to the cutting edge. To investigate the three-dimensional distribution of residual stresses within the composite of coating and substrate, an X-ray microscope (XRM) for three-dimensional imaging and a digital volume correlation (DVC) software for determining three-dimensional displacements and strains will be used at IW. To analyze local crack resistance, nanoindentation will be used to apply increased maximum forces. Further, confocal laser scanning microscopy and scanning electron microscopy will be used to study the resulting cracking behavior. For correlation with the local residual stresses, the same grid is used as for the residual stress measurements by nanoindentation. In application, increased temperatures occur. In a possible third year of research, an analysis of the influence of thermal stress on residual stresses and cracking behavior is planned.

• DFG Programme: Research Grants