Microstructure, mechanical properties and oxidation behaviors of magnetron sputtered NbNx coatings

• Chemical and phase compositions and microstructure of NbNx coatings were investigated.
• Maximum hardness is obtained for nano-composite coating with mixed Nb2N and NbN phases.
• Activation energies are 219.3 and 192.3 kJ/mol for oxidation of Nb2N and NbN coatings.
• Non-protective Nb2O5 scales with cracks and pores lower oxidation resistance of NbN coating.

Mechanical properties and oxidation resistance are of importance for the NbNx coatings as used in cutting and forming tools. In this study, the NbNx coatings were deposited by magnetron sputtering at nitrogen partial pressure ranging from 0 to 40%. The chemical and phase compositions, morphologies, mechanical properties and oxidation behaviors of the NbNx coatings were investigated by electron probe microanalysis, X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, scanning and transmission electron microscopy, and nanoindentation measurements. The results reveal the composition evolution of the NbNx coatings as α-Nb (0%), β-Nb2N (5%), a mixture of β-Nb2N and δ-NbN (10%), and δ-NbN (20–40%). The single phase coatings exhibit columnar structure while the mixed phases coating shows nano-composite structure. Compared with the single phase δ-NbN coatings (21.6 ± 0.8–28.0 ± 1.2 GPa), higher hardness of the single phase β-Nb2N coating (30.9 ± 1.0 GPa) is due to the higher covalent character and much finer grains. The maximum hardness reaches 33.3 ± 1.5 GPa for the nano-composite coating with mixed phases of β-Nb2N and δ-NbN. The oxidation results demonstrate that the activation energies are 219.3 and 192.3 kJ/mol for the Nb2N and NbN coatings respectively. Non-protective Nb2O5 scales with cracks and pores result in poorer oxidation resistance of the NbN coating in comparison to the Nb2N coating.

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