Microstructure of a cubic boron nitride tool material and its degradation during hard turning operations

Abstract. Polycrystalline cubic boron nitride (PCBN) materials are widely used in turning operations of hardened steels due to their beneficial mechanical properties like maintained high hardness at elevated temperatures, adequate toughness and for being more chemically stable towards iron compared to diamond at temperatures typical for turning operations. With the increase of commercially available grades, hard turning is constantly growing as are the new areas of applications. The wear of PCBN tools is influenced by several factors, such as machining parameters, different tool geometries and varying composition of the PCBN tool as well as of the workpiece material. To take full advantage of the beneficial properties of PCBN, the knowledge of the microstructure of unworn materials as well as of how wear progresses and which mechanisms that dominate during machining operations needs to improve. This study reports on several suitable microstructure investigation techniques for PCBN materials. A commercial low content PCBN tool material was used to study the unworn microstructure and its degradation during hard turning operations. The tool material was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) including methods such as energy dispersive X-ray spectrometry, electron energy-loss spectroscopy (EELS) as well as energy filtered TEM and finally atom probe tomography (APT). Cubic boron nitride (cBN) is the major phase in these materials. Quantitative investigations (with both EELS and APT), within the experimental errors, show values corresponding to stoichiometry with no detected impurities. The major matrix phase, Ti(C,N), was found to have a varying C/N ratio, typically between TiC0.5N0.5 and TiC0.7N0.3, in different grains. An Al additive in this material was shown to start a chain of reactions during the high-pressure, high-temperature sintering where several reaction phases form: α-Al2O3, TiB2 and AlN. Alumina networks are formed by Al and surface oxides, while the other phases form by reactions with cBN and Ti(C,N). In-depth microscopy investigations (with EELS) have shown small amounts of hexagonal BN and in rare cases also B2O3, which demonstrates how cBN can participate in reactions. Investigations also showed that the Ti(C,N) phase contains several dissolved elements such as O (~ 2.5 %), Al, B and W (~ 0.2-0.3 at%) and thus acts as a transport medium for elements so that shape adjustments and reactions can take place. The worn PCBN tool exhibited the typical crater wear on the rake face and flank wear, and both faces were covered by an adherent layer containing elements originating from the workpiece material. The degradation behaviour in the crater region of PCBN tools varied with machining parameters such as chemical composition of the workpiece material (a case hardened steel with high and low sulphur content) and cutting speed (150 m/min and 200 m/min). A protective layer of sulphides was found in the crater region when the workpiece material contained a high amount of sulphur. This layer was shown to protect the tool from degradation by iron-rich material. If the layer was missing, iron-rich material was found to make its way into the tool material microstructure by chemically dissolving phases. cBN grains were preferentially worn compared to Ti(C,N) grains, both at the wear surface and below. The strength of the protective sulphides was concluded to be weakened at higher cutting speeds, hence with decreased protection towards the iron-rich material degrading the microstructure