Cubic boron nitride (c-BN) is the second hardest material next to diamond with high thermal conductivity but better chemical stability than diamond, therefore, it is widely used in the form of sintered polycrystalline cubic boron nitride (PCBN) for cutting tools in machining of hardened steel, cast irons and super alloys showing high chemical stability when abraded by ferrous materials at high temperature. Commercial PCBN cutting tools are classified into two types, firstly low c-BN content composites containing typically 40 -70 vol% c-BN with ceramic binders such as TiN and TiC, and secondly high c-BN content PCBN containing about 75–95 vol% of c-BN with Al and other metallic binders in addition. PCBN materials designed for mild to medium interrupted hard part turning (HPT) applications have been produced by using different raw materials and sintering conditions. These PCBN materials have been investigated by SEM and XRD for microstructure and phase information and XRF for the overall chemical composition. It was observed that, for some of the PCBN material, there was a discrepancy between the elements detected as well as their measured quantity by XRF and the phases identified by XRD. Advanced analytical STEM was applied for an in depth characterization of the complex microstructures formed in those PCBN materials with grain size ranging from sub-micrometer down to a few nanometers. The aim of this study was to increase the understanding of the relationship between raw material selection, processing conditions, and machining performance. The raw material selection, milling and sintering operations play a crucial role in the solid state phase diffusion processes which governs what phases are formed and how these constituents bond to each other and, thus, the mechanical behaviour. To be able to capture the light and heavy elements present in the microstructure, elemental maps were acquired in a CS corrected Titan3 60-300 kV (FEI Company) equipped with ChemiSTEM and DualEELS capabilities. Figure 1 depicts elemental maps of two PCBN materials produced by different methods of Al processing showing greatly differing microstructures. In Fig. 1a, the Al appears to have flowed well and coated the c-BN interphases forming a continuous binder phase, mainly consisting of AlN. It appears that almost all of the AlN phase is connected to c-BN. Whereas in Fig. 1b, the Al phases are much less continuous, but still coat most of the c-BN. Most of the Al appears to be in discrete aluminium oxide phase. To be able to determine all reaction products and phases present advanced TEM and STEM techniques had to be applied due to the small and overlapping grains. The results presented are of importance to further improve machining performance by tailoring the microstructure by carefully selecting the initial raw material and processing conditions.
To cite this abstract:Jacob Palmer, Martina Lattemann, Ernesto Coronel, Arno Meingast, Larry Dues, Rachel Shao, Gerold Weinl; Analytical STEM study of sintered polycrystalline c-BN materials for cutting tool applications. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/analytical-stem-study-of-sintered-polycrystalline-c-bn-materials-for-cutting-tool-applications/. Accessed: September 20, 2021
EMC Abstracts - https://emc-proceedings.com/abstract/analytical-stem-study-of-sintered-polycrystalline-c-bn-materials-for-cutting-tool-applications/