Wear of Polycrystalline Cubic Boron Nitride Cutting Tools when Machining Nickel-based Superalloys

Cubic boron nitride is the second hardest material known, after diamond. Polycrystalline cubic boron nitride, PcBN, is a composite consisting of cubic boron nitride grains and either a metallic or ceramic binder phase. It is currently used as a cutting tool material for several difficult to machine alloys where diamond’s reactivity towards ferrous materials precludes its use. High tool wear rates are observed when using PcBN to machine Ni-based superalloys - in particular Inconel 718. While there have been other studies on the machining of this alloy using PcBN tools, and the types of wear that occur have been described, the underlying mechanisms are still not well understood. With a better understanding of the underlying processes, PcBN tools could be further refined so that they become an economical replacement for tungsten carbide cobalt-based tools, which are limited to low cutting speeds. Tungsten and cobalt are also subject to limited reserves and environmental concerns, while PcBN requires little more than energy to produce. Nine grades of PcBN with cBN concentrations from 30 % to 90% were characterised using a range of techniques including X-ray diffraction and electron microscopy. Of these, seven grades were used to machine Inconel 718. High pressure coolant was used in longitudinal finish turning tests at cutting speeds from 150m/min to 1200m/min. The wear rates and relative prevalence of flank, notch, and crater wear for the PcBN grades were measured using optical and electron microscopy and the cutting forces were analysed. While ion beam milling into the layer of adhered workpiece material on the tool showed some chemical reaction, the phases formed could not be adequately characterised in situ due to their size. In order to better understand the reactions occurring at the interface, static diffusion tests were used to simulate the diffusive and chemical wear processes. Diffusion couples were made from all grades of PcBN, and Inconel 718, as well as three other Ni-based superalloys with different minor elements. The samples were treated at 1200 ◦C in a low oxygen environment for 24 hours to encourage coarsening of the phases and therefore aid characterisation. The extent of the reactions at the interfaces as well as the identity of the new phases were analysed using energy dispersive X-ray spectroscopy. While the boron from the tool diffused hundreds of microns through the alloy before forming Nb, Mo, and Cr-rich phases, primarily along grain boundaries, the nitrogen from the cBN led to the formation near the interface of bands of cubic TiN precipitates containing other minor elements from the alloys. The morphology and quantity of these phases was dependent on both the PcBN grade and the alloy composition.