Structure and composition of built-up layers on coated tools during turning of Ca-treated steel

Built-up layers formed on CVD coated cutting tools during turning of Ca-treated steel were investigated. The built-up layers were analysed using scanning electron microscopy and transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy, electron diffraction, energy-filtered TEM and high resolution TEM. This is the first paper where the inclusion layer is studied and analysed using TEM. The structure and composition of the inclusion layer is reported in detail and compared with the inclusion population in the workpiece material. The part of the inclusion layer that was subjected to the highest temperature during cutting was crystalline, and is composed of round sulphide grains (Ca0.85Mn0.15S) and deformed elongated aluminate grains (CaO·6Al2O3). These are the high-melting point phases of the inclusions present in the steel and it appeared that only these compounds can contribute to the formation of the solid built-up layer.