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

Abstract

The coating of hard metals by conventional CVD technology causes a deterioration of the mechanical properties and restricts their technical application as cutting tools. By the combination of PVD- and CVD-coatings a significant improvement in strength and lifetime was obtained in comparison to conventional CVD-coatings. The investigated coatings were deposited on a WC–(Ti,Ta,Nb,W)C–Co hard metal. The PVD/CVD-coating systems consisted of a TiAlN-PVD coating on which the functional TiN–TiCN–TiN-coating systems were deposited by a high temperature chemical vapour deposition (HTCVD) or a medium temperature chemical vapour depositon (MTCVD) process. Mechanical investigations were carried out under monotonically increasing and cyclic loads at room temperature. These investigations led to the result that especially the mechanical behaviour of coated hard metals under cyclic loads restricts the technical application of these coated materials as cutting tools. Failure mechanisms in the microstructure were investigated by TEM and SAM. It was shown that the TiAlN–PVD-layer exhibits a strong resistance against the propagation of subcritical cracks. Cutting tests with the mentioned PVD/CVD-layer systems showed increased life times compared to CVD coated hard metals. These results were in agreement with the results of the mechanical and microstructural investigations.