The yield strength level of modern hard metal grades is about twice as high as that of high-speed steels and can be tuned by the appropriate selection of binder content and carbide grain size. They have an enormous potential for use in components subjected to high cyclic-mechanical loads, which is largely determined by the existence of microscopic defects and material inhomogeneities as well as their size and frequency. In microtools such as PCB drills with 200 μm diameter the volumes subjected to the highest tensile loads are usually very small. The capability of a tool to withstand such tensile stresses increases with decreasing loaded volume. The limits of this increase in tensile strength, which can be described by the well-known size effect, have so far not been investigated for hard metals.
Specimens of different sizes were subjected to various strength tests to highlight the enormous tensile strength potential of ultra-fine grain WC-Co hard metal. The loaded volume ranged from 100 mm3 for large-volume tools to a few μm3 for extremely small components and tools. The low probability of a fracture-inducing defect resulting from the reduction in the tested volume led to extremely high tensile strength values. Tensile strength was found to increase from about 2000 MPa in the macroscopic specimens to 6000 MPa in the smallest specimens. The fracture surfaces of the micrometer sized specimens showed no material inhomogeneities, indicating that cracks originate from natural stress concentrations at the phase boundaries between WC and the cobalt binder. The immense inherent tensile strength of the material is a great stimulus for the project team to further improve the manufacturing technology in order to reduce the number and size of inhomogeneities in the material and thus exploit its full potential.
The project was carried out in cooperation with AT&S Austria Technologie & Systemtechnik Aktiengesellschaft, Ceratizit Österreich, HPTec GmbH, the Erich Schmid Institute of the Austrian Academy of Sciences and the Institut für Struktur- und Funktionskeramik of the University of Leoben.