Thermal stability of metal nitride superlattices studied by means of Atom Probe Tomography
Experimentelle Physik der kondensierten Materie
Zusammenfassung der Projektergebnisse
The thermal stability of AlN/CrN multilayered hard coatings was investigated. The coatings were produced in a modified in-house RF-magnetron sputter deposition chamber. Coatings with different bilayer periods were examined with respect to the as-deposited crystal structures and hardness values. The main findings can be summarized as follows: The AlN/CrN superlattices with a bilayer period of 4 nm show the best hardness (~32 GPa) which exceed the rule of mixture value for single coating AlN and CrN by 86%. This effect is caused by the AlN layer stabilization to the cubic crystal structure resulting in special microstructure with columnar grains and coherent multilayers. Good thermal stabilities for AlN/CrN were found for temperatures up to 700°C/60 min. After exposure to temperatures ≥ 800°C the AlN/CrN coatings undergo microstructural changes and layer dissolution. For instance, the AlN layers are found to pinch-off at the grain boundaries (GBs) caused by the higher GB energy as compared to the CrN layers. The pinch-off mechanism at GB junctions leads to the formation of nanocrystalline hcp AlN. A mechanism for this transformation was proposed. The nanocrystalline hcp AlN coarsens with progressive annealing time. Cr atoms are rejected from the CrN layers by the coarsen AlN and accumulated in a separate phase. The advanced microstructural changes result in drop of the hardness. A high chemical stability of AlN with stoichiometric compositions in all analyzed samples was revealed by APT. On the other hand, CrN layers were found to lose N at temperatures ≥ 800°C until a chemically stable composition of Cr2N was reached. Despite the chemical transformation of CrN to Cr2N, no transformation of the crystal structure for the residual multilayer regions could be detected.
Projektbezogene Publikationen (Auswahl)
-
2013, Interface-directed spinodal decomposition in TiAlN/CrN multilayer hard coatings studied by atom probe tomography: Acta Materialia, v. 61, p. 7534-7542
Povstugar, I., P.-P. Choi, D. Tytko, J.-P. Ahn, and D. Raabe
