An in-depth investigation of the cutting speed impact on the degraded microstructure of worn PCBN cutting tools

The impact of an increased cutting speed on the degradation of a low content polycrystalline cubic boron nitride (PCBN) tool material is investigated by advanced microscopy techniques. The locally affected microstructure of worn PCBN cutting tools, after dry hard turning, is studied by high precision in situ lift-out cross sections taken from across the crater, formed on the rake face. The cross sections are studied with scanning electron microscopy, transmission electron microscopy (TEM) with electron energy loss spectroscopy and, primarily, energy filtered TEM. Advanced analysis techniques are crucial to illustrate the degradation mechanisms taking place locally at micro- and nano-metre levels during the machining operation. Results show that a higher cutting speed drastically affects the wear surface of the cutting edge. While an adherent layer, consisting of elements from the workpiece material, covers practically the whole wear surface at a lower cutting speed, it is only partially distributed at a higher cutting speed. Results also show significant differences in the local microstructure of the affected worn zone with an increase in cutting speed. The chemical degradation will go from tool-workpiece interface wear with smooth wear surfaces and almost no interaction with material below the wear surface at lower cutting speed to a severe penetration into the tool material by partially oxidised Fe-rich features at higher cutting speed. The more aggressive degradation behaviour at the higher cutting speed is also more localised. Single chemically worn cBN grains are for example shown. The dominating wear mechanism is shown to be chemical degradation, which accelerates with a higher cutting speed. The cBN phase is more affected than the major matrix phase, Ti(C,N). (C) 2011 Elsevier B.V. All rights reserved.