Microstructure of a cubic boron nitride tool material before and after hard turning
Polycrystalline cubic boron nitride (PCBN) materials are widely used in turning of hardened steels due to their beneficial mechanical properties like high hardness at elevated temperatures and, compared to diamond, for being more chemically stable towards iron at temperatures typical for turning operations (800-1000˚C). The wear of PCBN tools is influenced by several factors, like machining parameters, different tool geometries and varying composition of PCBN tools as well as workpiece material. The knowledge about how wear progresses and which mechanisms that are dominating is still limited.
This study reports on the microstructure of a commercial PCBN tool material with ~55vol% cBN and the degradation of the microstructure after turning. The tool was used for turning a case hardened steel using cutting parameters typical for hard part turning. The tool material has been characterised by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) including spectroscopy methods like energy dispersive X-ray spectroscopy, electron energy-loss spectroscopy and energy-filtered TEM.
The microstructure of the tool material contained micron-sized cBN grains, with flat grain boundaries, surrounded by a matrix mainly consisting of Ti(C,N). A fraction of smaller cBN grains, down to nanometre size, were found scattered in the matrix. The matrix also displayed a varying grain size. Both XRD and electron microscopy observations suggested that only limited reactions occur during the sintering process. Al2O3 was the major reaction product identified and some occurrence of TiB2 and AlN phases was also observed. Differences in C and N content in Ti(C,N) were also noted which indicated a material far from equilibrium.
The worn PCBN tool exhibited a crater on the rake face and flank wear with an adherent layer covering the affected cutting edge area. Cross section specimens were taken from across the crater, on the rake face of the worn tool, using a lift-out technique in the focused ion beam/SEM. These cross-sections contained both the area close to the wear surface of the PCBN tool and the adherent layer covering it. This allowed a detailed investigation of the interface between them. The adherent layer consisted of elements from the workpiece material. Large Fe-rich areas were found, which were oxidised on the surface and sometimes containing small amounts of Cr and Ni. Mn and S were detected in larger areas, often with rounded shapes. Traces of Al, probably from the PCBN tool material, was found close to the interface in the adherent layer. No B, N or Ti were found in the layer and must have been transported away by the moving chip. cBN grains on the tool wear surface showed concave shapes instead of the common flat shapes. The major degradation mechanism appears to be of chemical nature.
Keywords: cBN, PCBN, microstructure, wear, adherent layer, TEM, SEM, EDX, EFTEM, EELS