Thermal stability of metal nitride superlattices studied by means of Atom Probe Tomography
Experimental Condensed Matter Physics
Final Report Abstract
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.
Publications
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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
