Abstract
Plasma nitriding is a widely used technique for increasing the surface hardness of stainless steels, and consequently, for improving their tribological properties. It is also used to create an interface between soft stainless steel substrates and hard coatings to improve adhesion. This paper reports on the mechanical and corrosion properties of AlSI301 stainless steel (SS) after a duplex treatment consisting of plasma nitriding followed by deposition of Cr bond coat and CrSiN top layer by magnetron sputtering. Mechanical properties of the deposited films, such as hardness (H) and reduced Young's modulus (Er), were measured using depth-sensing indentation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to evaluate resistance to localized and to general corrosion, respectively. The corrosion behavior has been correlated with the microstructure and composition of the surface layers, determined by complementary characterization techniques, including XRD, SEM, and EDS. The CrSiN layers exhibited an H value of 24 GPa, whereas the nitrided layer was shown to present a gradual increase of H from 5 GPa (in the nitrogen-free SS matrix) to almost 14 GPa at the surface. The electrochemical measurements showed that the nitriding temperature is a critical parameter for defining the corrosion properties of the duplex-treated SS. At a relatively high temperature (723 K), the nitrided layer exhibited poor corrosion resistance due to the precipitation of chromium nitride compounds and the depletion of Cr in the iron matrix. This, in turn, leads to poor corrosion performance of the duplex-treated SS since pores and defects in the CrSiN film were potential sites for pitting. At relatively low nitriding temperature (573 K), the nitrided interface exhibited excellent corrosion resistance due to the formation of a compound-free diffusion layer. This is found to favor passivation of the material at the electrode/electrolyte interface of the duplex-treated SS. (C) 2010 Elsevier B.V. All rights reserved.