Hard DLC coating deposited over nitrided martensitic stainless steel: analysis of adhesion and corrosion resistance
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ladimir J. Trava-Airoldi Instituto Nacional de Pesquisas Espaciáis (INPE), 12.227-010 São José dos Campos–SP, Brazil
Lisandro Escalada and Silvia N. Simison Surface and Bioprocess Engineering Group, INTEMA, Faculty of Engineering, University of Mar del Plata, Mar del Plata, Argentina (Received 10 June 2016; accepted 23 September 2016)
Surface modification treatments, such as the plasma nitriding improve the tribological properties of AISI 420 stainless steel; however, the corrosion resistance is deteriorated. The DLC (Diamond-Like Carbon) coatings were not only having a low friction coefficient but also good wear and corrosion resistance. In this work, both the corrosion behavior and the adhesion of the DLC hard coating, deposited on nitrided and non-nitrided AISI 420 stainless steel substrates, were studied. The coatings were characterized by means of EDS and Raman. In addition, nitrided layer microstructure and the coatings were analyzed by SEM-FIB and XRD. Corrosion behavior was evaluated by the salt spray fog test and cyclic potentiodynamic polarization tests in NaCl solution. The adhesion was assessed using Rockwell indentation and scratch tests. The a-C:H film and nitrided layer thicknesses were about 2.5 lm and 11 lm respectively. The nitrided layer improved adhesion in both tests. The coated AISI 420 stainless steel proved to have excellent atmospheric corrosion resistance and a passive behavior over 1 V (versus SCE) in the electrochemical tests. The adhesion and the corrosion performance were improved when the coating was deposited after the plasma nitriding treatment.
I. INTRODUCTION
Martensitic stainless steels are used for cutlery; cutting, and surgical instruments in which good wear behavior and corrosion resistance are required. Surface modification treatments such as plasma-assisted nitriding can be used to improve their mechanical properties. This treatment allows surface-hardening of the steel and thus enhanced wear resistance. However, depending on the process conditions, the nitriding process can have adverse consequences on the corrosion resistance due to chromium nitride precipitation.1–3 For this reason, the combination of plasma nitriding with deposition of adherent and hard coatings such as diamond-like carbon (DLC) in a duplex process may provide the solution. DLC coatings are known to have low friction, outstanding wear resistance, and chemical inertness.4–6 They have been applied to mechanical components such as in valves, bearings, piston accessories, injection-molding dies, among others, to mitigate wear. The duplex process (plasma-assisted nitriding followed by DLC coating deposition) combines
the good tribological and passivation properties of DLC coating with the fatigue resistance of the nitrided layer, improving the load-bearing capacity, wear and deformation resistance, and corrosion behavior of the system.7–9 Regarding the corrosion resistance of these duplex systems, the electrochemical behavior usually depends on the coating adhesion and the presence of defects. Such defects cou
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