Microstructure and performance of CVD kappa-Al2O3 multilayers

In this paper, the microstructure and mechanical properties of different multilayer coatings of CVD kappa-Al2O3 have been investigated. The individual kappa-Al2O3 layers were separated by thin intermediate layers called kappa-modification layers. All the multilayers examined had the same total thickness, 8 mu m, but different numbers of layers, 8, 15 and 32. The aim of this work was to describe the microstructure of the coatings and relate this to cutting test results. The detailed microstructures, such as surface morphologies, twinning, dislocation density, porosity, orientation relationships and grain size were examined. The presence of alpha-Al2O3 in the layers was described and related to the nucleation surface. Columns of epitaxially grown grains were found, which often extended to the top surface of the coating. The best cutting performance was exhibited by 8 kappa followed by 15 kappa and 32 kappa. The failure mechanism of all the coatings studies was crater wear (diffusion wear) and the 8 kappa multicoating clearly outperformed the other multicoatings. The TRS value was highest for 8 kappa, lower for 15 kappa and lowest for 32 kappa.