In situ analysis of PCBN cutting tool materials during thermo-mechanical loading using synchrotron radiation

Abstract

Polycrystalline cubic boron nitride (PCBN) has outstanding properties in terms of hardness and chemical stability at elevated temperatures. Therefore, PCBN is used in cutting tool materials for hard machining applications e.g. hard turning of hardened steels. Due to the hardness of the workpiece, high forces act on a low contact area between tool and workpiece. Hence, severe thermo-mechanical loadings occur in such applications causing enhanced tool wear. Fundamental knowledge about the material behavior of PCBN-cutting-materials under thermo-mechanical loading is valuable as a basis for a better understanding of tool wear and finally for improvement of tool wear behavior. PCBN-materials are polycrystalline multi-phase compounds with strongly deviating material properties. In order to investigate the phase selective thermo-mechanical behavior of such materials lattice strain measurements are conducted under thermo-mechanical load using in situ X-ray diffraction with high energy synchrotron radiation. A four point bending test set-up and ceramic thermal heaters are used for the application of thermo-mechanical loading. Three different materials are investigated: a solid low PCBN-content material, a low PCBN-content material on a cemented carbide (CC) substrate and a high PCBN-content material on a CC-substrate. The low PCBN-content material exhibits a single phase binder material whereas the high PCBN-content material exhibits a multi-phase binder with up to five phases. Residual stresses are found in the samples with CC-substrate, only. Different phases of one material show different strains but nearly same stresses upon loading. Thus, thermo-mechanical loading can be seen as superposition of the respective mechanical and thermal loads. The space-resolved experimental data is used to validate an analytical model for the calculation of macroscopic stresses. The phase selective space-resolved strain and stress analysis presented in this paper provides a valuable method for the investigation and optimization of hard cutting tool materials and coatings under real cutting conditions.