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NbN x layers were deposited by reactive magnetron sputtering on MgO(001) substrates in 0.67 Pa pure N 2 at T s = 600-1000 °C. T s ≥ 800 °C leads to epitaxial layers with a cube-on-cube relationship to the substrate: (001) NbN ||(001) MgO and [100] NbN ||[100] MgO. The layers are nearly stoichiometric with x = 0.95-0.98 for T s ≤ 800 °C, but become nitrogen deficient with x = 0.81 and 0.91 for T s = 900 and 1000 °C. Xray diffraction reciprocal space maps indicate a small in-plane compressive strain of-0.0008±0.0004 for epitaxial layers, and a relaxed lattice constant that decreases from 4.372 Å for x = 0.81 to 4.363 Å for x = 0.98. This unexpected trend is attributed to increasing Nb and decreasing N vacancy concentrations, as quantified by first-principles calculations of the lattice parameter vs point defect concentration, and consistent with the relatively small calculated formation energies for N and Nb vacancies of 1.00 and-0.67 eV at 0 K and-0.53 and 0.86 eV at 1073 K, respectively. The N-deficient NbN 0.81 (001) layer exhibits the highest crystalline quality with in-plane and out-of-plane x-ray coherence lengths of 4.5 and 13.8 nm, attributed to a high Nb-adatom diffusion on a N-deficient growth front. However, it also contains inclusions of hexagonal NbN grains which lead to a relatively high measured hardness H = 28.0±5.1 GPa and elastic modulus E = 406±70 GPa. In contrast, the nearly stoichiometric phase-pure epitaxial cubic NbN 0.98 (001) layer has a H = 17.8±0.7 GPa and E = 315±13 GPa. The latter value is slightly smaller than 335 and 361 GPa, the isotropic elastic modulus and the [100]-indentation modulus, respectively, predicted for NbN from the calculated c 11 = 641 GPa, c 12 = 140 GPa, and c 44 = 78 GPa. The electrical resistivity ranges from 171-437 μΩ-cm at room temperature and 155-646 μΩ-cm at 77 K, suggesting carrier localization due to disorder from vacancies and crystalline defects.

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Transition metal nitride hard coatings like CrN or TiN are widely used in several applications due to their excellent wear, corrosion and oxidation resistance. However, the friction coefficient of most transition metal nitride coatings is fairly high (0.6-0.8) and the tribological effectiveness, especially at elevated temperatures, is insufficient. [1,2] Therefore, a lot of effort has been made in recent years to decrease the friction coefficient at room as well as at elevated temperatures. [3][4][5] Adding layered solid lubricants like DLC, MoS 2 or WS 2 to hard coatings by forming nanocomposite structures has been established as a suitable approach to reduce friction. [6][7][8][9] Nevertheless, these lubricating phases fail due to degradation and oxidation at temperatures > 350°C. [5] The incorporation of V as a high temperature lubricant into transition metal nitride coatings forming a solid solution (Ti-Al-V-N) [10] or superlattice structures of hard and lubricating layers (TiAlN/ VN, TiAlN/TiAlVN) [11,12] led to a significant enhancement of the tribological performance in high-temperature applications. At temperatures between 500 and 700°C, so-called Magneli phase oxides (V-O) are formed on the coating surface. Some of them, in particular V 2 O 5 , show low shear strength in the solid state as well as low melting temperatures, thus providing lubrication by solid and/or liquid oxide phases. [13,14] However, these oxides are only formed under certain environmental conditions (high temperatures and oxygen-containing atmosphere). Apart from metal oxide lubricants, soft noble metal phases (Ag, Au) have been reported as promising candidates for lubrication over a wide temperature range (e.g. synthesized as nanocomposites TiC/Ag, [15] yttrium-stabilized zirconia YSZ/Au, [16,17] CrN/Ag [18] ). The benefit of these nanocomposite systems is that noble metals may act as lubricants at room temperature (RT) as well as high temperatures due to their low shear strength and stable thermochemistry. Due to their low sensitivity to the environment, such coatings are applicable in ambient air as well as under vacuum conditions in a wide temperature range.We have recently shown that the incorporation of Ag as a solid lubricant in hard wear-resistant CrN coatings decreases friction and wear at RT. [18] In addition, we found that vacuum annealing at 600°C leads to the formation of Ag agglomerates on the coating surface. In this study, special emphasis is laid on the influence of surface reactions, like the formation of these agglomerates and/or surface oxidation, on the hightemperature tribological behavior at 600°C. Characterization of the friction curves and the wear tracks provides an understanding of the relation between the out-diffusion of Ag (forming agglomerates) and its lubricating effect. ExperimentalThe substrates were ground and polished 304 stainless steel samples that were cleaned in ultrasonic baths of trichloroethane, acetone, methanol, and deionized water, respectively and blown dry with N 2 . The ∼ 3 lm thick coa...

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