Cutting tools are subject to high mechanical and tribological loads during machining, which together with thermal influences can lead to failure. In particular, the cutting edge area is at the center of this load. Completed investigations have already shown that residual stresses in coated carbide tools influence tool life and can be adjusted by the coating process.
However, the necessary X-ray residual stress measurements are only possible in the area of the rake or flank surface, since the necessary diffraction conditions are fulfilled in this area. In the area of the cutting edge, a measurement with classical X-ray methods is therefore not possible. A simplified transfer of the measurement results to the cutting edge area is also not possible due to the curvature.
In order to make residual stress measurements in the cutting edge area nevertheless possible, the use of alternative measurement methods is therefore necessary. Raman spectroscopy, which is based on the analysis of backscattered coherent laser light, offers a possible solution here. The advantages are, on the one hand, the significantly smaller measuring point (diameter < 3 µm) and the short measuring time compared to the established X-ray measuring methods. The method can be applied to Raman-active cutting materials/coatings such as diamond, boron nitride or nitride coatings.
Previous studies have shown that residual stresses can be reliably detected with this method in titanium aluminum nitride coatings when introduced by a mechanical process such as wet blasting. This significant residual stress difference could be detected spatially resolved in the cutting edge region. The measurement of coating-induced residual stresses, which are influenced by a change in the coating parameters, is initially not possible by a pure Raman measurement due to a strong influence on the crystal lattice. However, with a parallel X-ray reference measurement in the uncurved region, the relative distribution of residual stresses in the cutting edge region can be quantified.
The application of Raman spectroscopy to the high hardness cutting materials PCD and PcBN is currently under investigation. X-ray verified load stresses in the bending load case have already been demonstrated.
Currently, the influence of the established preparation methods grinding/brushing for PcBN tools and grinding and eroding for PCD tools on the residual stress state is being determined. Furthermore, machining by nano- and femtosecond lasers is planned for both cutting materials. The high pulse length differences of both processes are also expected to have an influence on the residual stress states.
Finally, the use of the manufactured tools on hardened 100Cr6 (PcBN) as well as hypereutectic aluminum-silicon alloy is planned to quantify the influence of the measured residual stresses on tool life.
With the collected knowledge of the novel application of this measuring method, the area of the cutting edge can be specifically designed in the future by a targeted post-treatment or preparation.
