Growth and characterization of TiAlN/CrAlN superlattices prepared by reactive direct current magnetron sputtering

TiAlN and CrAlN coatings were prepared using a reactive direct current magnetron sputtering system from TiAl and CrAl targets. Structural characterization of the coatings using x-ray diffraction (XRD) revealed the B1 NaCl structure of TiAlN and CrAlN coatings with a prominent reflection along the (111) plane. The XPS data confirmed the bonding structures of TiAlN and CrAlN single layer coatings. Subsequently, nanolayered multilayer coatings of TiAlN/CrAlN were deposited on silicon and mild steel (MS) substrates at different modulation wavelengths (Lambda) with a total thickness of approximately 1.0 mu m. The modulation wavelengths were calculated from the x-ray reflectivity data using modified Bragg's law. TiAlN/CrAlN multilayer coatings were textured along (111) for Lambda < 200 A and the XRD patterns showed the formation of superlattice structure for coatings deposited at Lambda=102 A. The x-ray reflectivity data showed reflections of fifth and seventh orders for multilayer coatings deposited at Lambda=102 and 138 A, respectively, indicating the formation of sharp interfaces between TiAlN and CrAlN layers. The cross-sectional scanning electron microscopy image of TiAlN/CrAlN multilayer coatings indicated a noncolumnar and dense microstructure. A maximum hardness of 39 GPa was observed for TiAlN/CrAlN multilayer coatings deposited at Lambda=93 A, which was higher than the rule-of-mixture value (30 GPa) for TiAlN and CrAlN. Study of thermal stability of the coatings in air using micro-Raman spectroscopy indicated that the TiAlN/CrAlN multilayer coatings were stable up to 900 degrees C in air. TiAlN/CrAlN multilayer coatings also exhibited improved corrosion resistance when compared to the MS substrate.