Microstructure and corrosion behavior of TiN coatings deposited on nitrided stainless steel by PBII&D

L. VACA; S. BRUHL; A. MARQUEZ

Congreso; Plasma Based Ion Implantation and Deposition 2015; 2015

Austenitic stainless steels are widely used infood and chemistry industry as well as biomaterials because they have anexcellent corrosion resistance but the low hardness limit their potentialapplications. Plasma nitriding is usually used to improve the tribologicalproperties of these steels but not always corrosion resistance is maintained. TiNcoatings are commonly used for hardening cutting tools and they are alsocorrosion resistant.In this work, Ti-TiN bilayer coatings obtainedby PBII&D on nitrided austenitic stainless steel in order to improve notonly hardness but also corrosion resistance compared with the samples whichwere only nitrided. AISI316L samples were DC plasma nitrided in anindustrial equipment (IONAR S.A., Argentina) during 20 h at 420 °C with 20%nitrogen partial pressure and hydrogen. After that, they were coated by meansof a DC cathodic arc running 130 A current discharge between a Ti cathode andthe vacuum chamber employed as anode. The samples were placed 30 cm away fromthe cathode surface and was connected to 6kV negative pulsed bias voltage. TiNfilms were obtained employing a continuous nitrogen flux of about 25 sccm and aworking pressure in the range of 1-3.10-2 Pa. The morphology and structuralproperties have been characterized by means of an optical microscope, scanningelectron microscopy (SEM) and X rays diffraction (XRD). The coating adhesionwas assessed by means of Scratch Test and Rockwell C indentation. The corrosionbehavior was evaluated etching the samples with Marble reagent and by means ofanodic polarization tests in NaCl solution. Ti and TiN peaks could be identified in the XRDpatterns of the coated samples, meanwhile, the XRD patterns of the nitridedsamples showed the expanded austenite phase. SEM observation showed thepresence of macroparticles in the layer structure and the film thickness wasabout 1 µm. The Scratch Test and Rockwell C indentation allowed determined thatthe film had a good adhesion. The PBII&D samples resisted the marblereagent attack and in the polarization experiment, presenting the best corrosionbehavior.