TEM and DFT investigation of CVD TiN/κ–Al 2O 3 multilayer coatings

Abstract

This paper investigates the interfacial structure in hot-wall CVD TiN/κ–Al 2O 3 multilayer coatings using both HREM and DFT modeling. Two multilayers with different thicknesses of the TiN layers (50 and 600 nm) separating the κ–Al 2O 3 layers are analyzed. The general microstructure of the two multilayers is relatively similar. The TiN layer in the thicker TiN/κ–Al 2O 3 coating is thick enough to be several TiN grains high. This means that epitaxial columns, which are often found in the thinner TiN/κ–Al 2O 3 coatings, are not present. However, the orientation relationships at the TiN/κ–Al 2O 3 interfaces are the same in both multilayers. The HREM investigations show that κ–Al 2O 3 (001) planes can grow directly on flat (111) TiN faces, without any other phases or detectable amounts of impurities, such as sulphur, present. Where the TiN layers are more curved, γ–Al 2O 3 can be grown, at least partly stabilized by the cube-on-cube orientation relationship between γ–Al 2O 3 and the underlying TiN. The DFT calculations show very similar adsorption strengths for an O monolayer positioned on Ti-terminated TiC(111) and TiN(111) surfaces, with preferred adsorption in the fcc site. O adsorption on N-terminated TiN(111) is much weaker, with preferred adsorption in the top site. Calculated elastic-energy contributions yield a higher stability for κ–Al 2O 3 on TiN(111) than on TiC(111) and a higher stability for κ–Al 2O 3 than for α–Al 2O 3 on both TiC and TiN. This indicates that the observed higher stability of κ–Al 2O 3 on TiC(111) than on TiN(111) is not due to the lattice mismatch, while the preferred epitaxial growth of κ–Al 2O 3 over α–Al 2O 3 can be partly attributed to the mismatch.