Structure and properties of reactive direct current magnetron sputtered niobium aluminum nitride coatings
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Kanwal Preet Bhatti and Sujeet Chaudhary Department of Physics, Indian Institute of Technology, New Delhi 110 016, India (Received 24 September 2007; accepted 20 December 2007)
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.
I. INTRODUCTION
Binary nitrides of transition metals such as TiN, CrN, or NbN have been explored to a great extent due to their excellent mechanical properties and resistance to chemical attack.1 NbN coatings exhibit a variety of interesting properties like high hardness, chemical inertness, and high electrical conductivity. At low temperatures, NbN becomes a superconductor and therefore finds applications in infrared imaging photodetectors, superconducting magnets, and other microelectronic devices.2,3 NbN coatings have also been used as cathode material for field emission in vacuum microelectronic devices.4 Nanolayered multilayer coatings based on NbN such as TiN– NbN5,6 or CrN–NbN7 have been found to exhibit improved mechanical and tribological properties compared to single-layer coatings. Recently, Nb–NbN multilayer films have been developed for solar-selective applications.8 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0168 1258 J. Mater. Res., Vol. 23, No. 5, May 2008 http://journals.cambridge.org Downloaded: 01 Jul 2014
Incorporation of additional elements like aluminum, silicon, or chromium into transition-metal nitride matrices has led to the formation of ternary nitrides with improved properties. TiAlN and CrAlN coatings, which have been widely studied, exhibit higher hardness and thermal stability when compared to TiN and CrN.9–1
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