Three-dimensional microstructural characterization of a polycrystalline cubic boron nitride composite by means of focused ion beam (FIB) tomography

Abstract

The focused ion beam (FIB) tomography technique has been used to conduct a detailed study of the microstructural assemblage of a polycrystalline cubic boron nitride (PcBN) composite. In doing so, ∼400 slices (i.e. two-dimensional (2D) field emission scanning electron microscope images) have been sequentially milled and reconstructed, resulting in a cube of 10.8 × 4.3 × 8.1 μm³. The three-dimensional (3D) microstructural characteristics of a high cBN content and metallic binder PcBN composite have been assessed and compared to those determined using a conventional 2D – linear intercept method (LIM). Results demonstrated FIB tomography to be a suitable and powerful technique for gaining in-depth knowledge and understanding of the microstructural assemblage of PcBN composite materials. In general, a satisfactory agreement is found between 2D and 3D characterization techniques. Nevertheless, 3D reconstruction provides information impossible to gather with conventional LIM. First, analysis of the phase content distribution in 3D permits to determine a homogeneous distribution of the cBN particles along the bulk material. Second, 3D tomography allows for assessment of geometrical aspects while also considering a greater number of cBN particles. As a consequence, they are found to exhibit a bimodal distribution rather than a monomodal one, with larger and finer particles showing a tetrahedral-like or a close-to-sphere geometry, respectively. Finally, binder reconstruction reveals the metallic matrix to be a contiguous and interconnected network through the cBN skeleton. In this regard, some discrepancies in the binder mean free path calculations between 2D and 3D methodologies are discerned.