Structure and properties of reactive direct current magnetron sputtered niobium aluminum nitride coatings

A reactive direct current magnetron sputtering system was used to prepare NbAlN coatings at different nitrogen flow rates and substrate bias voltages. Various properties of NbAlN coatings were studied using x-ray diffraction, scanning electron microscopy, atomic force microscopy, x-ray photoelectron spectroscopy, nanoindentation, the four-probe method, a solar spectrum reflectometer and emissometer, spectroscopic ellipsometry, micro-Raman spectroscopy, and potentiodynamic polarization techniques. Single-phase NbAlN with B1 NaCl structure was obtained for the coatings prepared at a nitrogen flow rate in the range of 1.5–3 sccm, a substrate bias voltage of −50 to −210 V, and a substrate temperature of 300 °C. Nanoindentation data showed that the optimized NbAlN coating exhibited a maximum hardness of 2856 kg/mm². An approximately 100-nm-thick NbAlN–NbAlON tandem on copper substrate exhibited a high absorptance (0.93) and a low emittance (0.06), suitable for solar-selective applications. The spectroscopic ellipsometry and resistivity data established the metallic nature of NbAlN and the semitransparent behavior of NbAlON coatings. The corrosion resistance of NbAlN coatings was superior to that of the mild steel substrate. The addition of aluminum in NbN coatings increased the onset of oxidation in air from 350 to 700 °C. Vacuum-annealed NbAlN coatings were structurally stable up to 700 °C and retained their high hardness up to a temperature of 650 °C.