The microstructure of the affected zone of a worn PCBN cutting tool characterised with SEM and TEM

Abstract

The local effect of wear on the microstructure of a commercial low content Polycrystalline Cubic Boron Nitride (PCBN) tool material, with ∼55 vol% cBN in a Ti(C,N)-matrix, has been investigated. The PCBN tool was used in dry hard turning of a case hardened steel. A crater developed on the rake face of the tool edge and the microstructure of this area has been studied in detail using Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray analysis and Transmission Electron Microscopy (TEM) with Electron Energy-Loss Spectroscopy and Energy Filtered TEM. A lift-out technique in a Focused Ion Beam/SEM instrument was used to produce thin foil specimens, <80 nm in thickness. This technique provided the possibility to produce specimens from a specific location with sub-micron resolution. In this case, specimens were taken from across the rake face of the worn PCBN tool. The spatial resolution was substantially improved by using thin foils and it was possible to investigate the degraded microstructure down to the nanometre scale. The results show an adherent layer smearing the crater on the rake face of the PCBN tool cutting edge. Closer investigation of this layer reveals an inner layered structure with some characteristic features. This layer consists of elements from the workpiece material. The tool–adherent layer interface is smooth, showing rounded, concave wear surfaces of micron-sized cBN instead of the original flat grain surfaces. Larger Fe-rich areas are found which are oxidised on the surface, sometimes containing smaller amounts of Cr and Ni. Mn and S are found as larger, often elongated areas primarily close to the tool–adherent layer interface. The dominating degrading mechanism appears to be chemical degradation. The detailed microstructure of the tool–adherent layer interface and the adherent layer is presented.