Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond Corrosion resistance of low-carbon steel modified by plasma nitriding and diamond-like carbon Cleber Pereira Fenili a, ⁎ , Fernando Sílvio de Souza b , Guilherme Marin a , Sônia Maria Hickel Probst a , Cristiano Binder a , Aloísio Nelmo Klein a a LabMat – Laboratório de Materiais, GRRC – Grupo de Revestimentos Resistentes a Corrosão, Departamento de Engenharia Mecânica, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil b UniSociesc, Sociedade Educacional de Santa Catarina, 89206001 Joinville, SC, Brazil ARTICLE INFO Keywords: Diamond-like carbon Plasma CVD Polarization Electrochemical impedance spectroscopy Passive film Corrosion ABSTRACT In this paper, the corrosion resistance of nitrided and diamond-like carbon (DLC)-coated SAE 1020 steel samples was investigated. The DLC coating was deposited after plasma-nitriding. The structure and composition of dif- ferent phases of the samples were characterized by Vickers micro-hardness, scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The corrosion behavior of the samples was investigated by electrochemical methods using a 0.1 mol L -1 H 2 SO 4 solution. The results obtained from sample characterization agreed well with those obtained from electrochemical corrosion methods. The 500-W DLC coating exhibited the best cor- rosion-resistance. The DLC-coated samples showed better corrosion-resistance than uncoated samples. 1. Introduction Protective surface coatings are widely employed to improve elec- trochemical and mechanical properties such as corrosion resistance, wear, and hardness. Techniques using plasma atmosphere are employed in material coating processes to improve the surface energy, friction, and hardness, along with the adhesion to other materials or coatings. During plasma nitriding, nitrogen atoms are introduced into the surface of a sample, thus producing a compound layer and a diffusion layer. The characteristics of a nitrided steel surface depend on the che- mical composition of the substrate and process parameters such as time, temperature, and especially the nitriding potential of the treatment medium [1]. Therefore, according to Colijin et al. [2] and Lampe et al. [3] the microstructure of nitrided layers can be subdivided into two distinct regions: a compound layer and a diffusion zone. Compound layer is the outermost layer and may be either single- phase (γ′-Fe 4 N or ε-Fe 2–3 N) or polyphase (γ′-Fe 4 N+ ε-Fe 2–3 N nitrides of alloying elements). It is very hard, and hence contributes to the wear and corrosion resistance of nitride layers. Diffusion zone lies below the compound layer and is formed by the saturation of ferrite by nitrogen, precipitation of very thin and coales- cence-resistant nitrides (α″-Fe 16 N 2 and γ′-Fe 4 N), and generation of re- sidual stresses. It contributes significantly to the improvement of the wear resistance and fatigue properties [4] of nitride layers. Low-nitrogen and carbon-free atmospheres favor the formation of the γ′-Fe 4 N phase. This phase has a face-centered cubic (FCC) crystal structure and exhibits a narrow solubility range for nitrogen (5.7–6.1%) [5,6]. The ε-Fe 2–3 N phase starts precipitating when the nitrogen content is ~6.1% by weight. Hence, high nitrogen content and the presence of carbon favor the precipitation of ε-Fe 2–3 N. This phase can dissolve up to 11.5% (by weight) of nitrogen at temperatures below 500 °C and has a packed hexagonal crystal structure [1,6]. However, a brittle and porous compound layer affects the char- acteristics of nitride layers adversely, thus limiting their applications [7,8]. To eliminate the effect of the brittleness and porosity of compound layers, nitride layers are coated with diamond-like carbon (DLC) films, which are amorphous carbon composite films that have carbon-carbon and carbon-hydrogen bonds, the carbon atoms being sp 2 and sp 3 hy- bridized. These bonds impart diamond-like characteristics such as hardness and chemical inertness to these films [9]. These films can be classified into various types depending upon the amount of hydrogen and the ratio between the sp 2 and sp 3 bonds [10]. DLC coatings have good chemical stability in acidic and alkaline solutions at room temperature, which makes them corrosion-resistant [11–13]. However, DLC films cannot be deposited directly onto SAE 1020 steel, because it is a ductile substrate, and their modulus of elasticity difference is not adequate for mechanical stresses. Thereby, a https://doi.org/10.1016/j.diamond.2017.11.001 Received 9 August 2017; Received in revised form 20 October 2017; Accepted 6 November 2017 ⁎ Corresponding author. E-mail address: cleber.fenili@labmat.ufsc.br (C.P. Fenili). Diamond & Related Materials 80 (2017) 153–161 Available online 08 November 2017 0925-9635/ © 2017 Elsevier B.V. All rights reserved. MARK