Chemical vapor deposited alumina hard coatings: Residual stress states decisively determined by the physical material properties

Abstract

Machining tools made from cemented carbides coated with protective hard layers are indispensable in machining technology and are examined down to the smallest detail for reasons of patent law. As established in thin film analysis practice, a biaxial stress model is in general used for residual stress evaluation. But important information gets lost as a result of this simplification and the potential for product improvement is wasted. If someone goes into more detail the appearance changes and enables a deeper and more fundamental understanding. The X-ray diffraction study presented demonstrates that the residual stress state in the top α-Al2O3-layer is more complex than the usual model assumption of a biaxial stress nature suggests. It can again be shown that the internal stress state arising during the chemical vapor deposition process, is strongly influenced by the thermal expansion behavior of the material under investigation. But the presented results reveal that the impact is much higher than discussed in previous studies. The internal stress field is triaxial, anisotropic and strongly direction-dependent, presumably due to the crystal anisotropy in combination with growth textures of [0 0 1] type. Nevertheless, with respect to direction-dependent grain interaction the sample elasticity is not affected, and the sample state proves to be quasi-isotropic.