Vacuum 81 (2006) 479–488 Deposition and characterization of TiAlN/Si 3 N 4 superhard nanocomposite coatings prepared by reactive direct current unbalanced magnetron sputtering Harish C. Barshilia à , B. Deepthi, K.S. Rajam Surface Engineering Division, National Aerospace Laboratories, Post Bag No. 1779, Bangalore–560 017, India Received 19 April 2006; received in revised form 3 July 2006; accepted 9 July 2006 Abstract Superhard nanocomposite coatings of TiAlN/Si 3 N 4 with varying silicon contents were synthesized using reactive direct current (DC) unbalanced magnetron sputtering. The Si and TiAl targets were sputtered using an asymmetric bipolar-pulsed DC power supply and a DC power supply, respectively, in Ar+N 2 plasma. The structural and mechanical properties of the coatings were characterized using X-ray diffraction (XRD) and nanoindentation techniques, respectively. The elemental composition of the TiAlN/Si 3 N 4 nanocomposite coatings was determined using energy-dispersive X-ray analysis and the bonding structure was characterized by X-ray photoelectron spectroscopy. The surface morphology of the coatings was studied using atomic force microscopy. The XRD data showed that the nanocomposite coatings exhibited (1 1 1) and (2 0 0) reflections of cubic TiAlN phase. The broadening of the diffraction peaks with an increase in the silicon content in the nanocomposite coatings, suggested a decrease in the average crystallite size. The TiAlN/Si 3 N 4 nanocomposite coatings exhibited a maximum hardness of 43 GPa and an elastic modulus of 350 GPa at a silicon concentration of approximately 11 at%. The hardness and the elastic modulus of the nanocomposite coatings decreased significantly at higher silicon contents. Micro-Raman spectroscopy was used to characterize the structural changes as a result of heating of the nanocomposite coatings in air (400–850 1C) and in vacuum (900 1C). The Raman data of the nanocomposite coatings annealed in air and vacuum showed better thermal stability as compared to that of the TiAlN coatings. Similarly, the nanocomposite coatings deposited on mild steel substrates exhibited improved corrosion resistance. r 2006 Elsevier Ltd. All rights reserved. Keywords: TiAlN/Si 3 N 4 superhard nanocomposite coatings; Unbalanced magnetron sputtering; Structural and mechanical properties; Thermal stability; Corrosion behavior 1. Introduction Thin films of transition metal nitrides have been widely used in many engineering applications especially due to their high hardness, chemical inertness and excellent wear resistance. Among them, the properties and the applica- tions of TiN coatings have been studied extensively. The main disadvantage of TiN is its limited oxidation resistance (approximately 450–500 1C). The addition of other ele- ments such as Al, Cr, Si, etc. increases the oxidation resistance of TiN [1–3]. Initially, TiAlZrN-based thin films were developed for an electronic application in 1972 [4]. Subsequently, TiAlN coatings have been developed for the engineering applications as an alternative to TiN since 1986 [5]. It has been reported that, the addition of aluminum to TiN, thus forming TiAlN, improves the oxidation behavior and the thermal stability of the coating, by forming a stable oxide layer on the surface of the film during oxidation [6]. The properties of TiAlN coatings are mainly controlled by the aluminum content. TiAlN coatings with hardness values varying in the range 20–35 GPa have been reported in the literature [7,8]. Even hardness as high as 47 GPa has been achieved for TiAlN coatings prepared using magne- tron sputtering [9]. These superhard films were reported to be nanocomposite films of TiAlN/AlN, having relatively ARTICLE IN PRESS www.elsevier.com/locate/vacuum 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.07.003 à Corresponding author. Tel.: +91 80 2508 6494; fax: +91 80 2521 0113. E-mail address: harish@css.nal.res.in (H.C. Barshilia).