Microstructure control in TiAlN/SiNx multilayers with appropriate thickness ratios for improvement of hardness and anti-corrosion characteristics

TiAlN/SiNx multilayers were fabricated by a reactive magnetron sputtering system combining r.f. and d.c. power sources. The SiNx layer thickness (l(siNx)) was 0.4 and 1 nm, while the layer thickness ratios (l(TiAlN)/l(SiNx)) of TiAIN to SiNx were adjusted to be 4/0.4 and 4/1, respectively. Characterizations by XRD, TEM, SEM and nano-indentation revealed the dependence of l(SiNx) on the preferred orientation, crystalline behavior, microstructure and hardness. The crystalline SiNx grew epitaxially and formed the coherent interfaces with the TiAIN, exhibiting the maximum hardness of 42 GPa. However, SiNx evidently transformed from crystalline to amorphous when the l(SiNx) increased to 1 nm, while microstructure of films changed from columnar feature to more densified one. The corrosion resistance of coatings in 3.5 wt % NaCl aqueous solution was investigated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS). The denser microstructure exhibited the lower corrosion rate and higher polarization impedance. It was revealed that the amorphous SiNx altered the coherent interfaces and the superlattice structure, leading to the improved anti-corrosion performances. (C) 2012 Elsevier Ltd. All rights reserved.