Mechanical behaviour of PVD- and CVD-coated hard metals under cyclic loads

Abstract

The aim of this work is to investigate the influence of different coating processes and layer systems on the mechanical properties of hard metals. A WC–(Ti,Ta,Nb,W)C–Co hard metal was coated by PVD (physical vapour deposition) and HTCVD (high temperature chemical vapour deposition). The PVD coating consisted of 3 μm TiN and the CVD coatings of 9 μm TiN-Ti(C,N)-TiN. The mechanical investigations focus on the fatigue behaviour of the coated hard metals under cyclic loads. The mechanical properties of the coated hard metals were tested under monotonically increasing and cyclic loads at room temperature. After these investigations, microstructural characterizations by TEM, SAM, SFM, SEM with EDS and measurements of the internal stresses by X-ray analysis were carried out. It turned out that the inert strengths and the fatigue life times of the CVD coated hard metals were strongly decreased in comparison to the uncoated materials. The reason for this deterioration are the layer thickness, an embrittlement of the interface between coating and substrate, and tensile stresses in the coating resulting from different thermal expansion coefficients of the substrate and the coating. Due to the HTCVD coating and the damage by the coating procedure subcritical crack growth into the substrate is made easier. This leads to a strong decrease of the life times under cyclic loads. It could be shown, that the mechanisms degrading the mechanical properties are found in the coating and the adjacent region of the substrate with a thickness of 20 μm. Due to its brittle behaviour hard coatings always cause a decrease of strength. The fatigue behaviour of the PVD-coated hard metal is determined by the compressive stresses in the coating. The sensitivity of the PVD-coated hard metal to fatigue is reduced, because the compressive stresses in the coating provide a hindrance to subcritical crack propagation. In summary this work demonstrates that the knowledge about the mechanical behaviour of coated hard metals under cyclic loads is important with regard of their application as cutting tools.