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Ni-Mo alloys electrodeposited under direct current from citrate-ammonia plating bath

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Ni-Mo alloys are characterized by high hardness, wear, thermal and corrosion resistance [1, 2]. Due to this reason they could offer an important alternative to hard chromium coatings, which according to EU directives (2000/53/WE, 2011/37/UE) have to be eliminated from manufacturing processes [3]. However, these alloys are difficult to obtain by conventional thermal methods, what is caused by the large difference in metals melting points (Ni . 1455 C, Mo . 2620C) and the limited mutual solubility. Convenient way to produce these type of coatings, which overcomes above mentioned problems, is a low-temperature and relatively simple electrodeposition technique. It enables uniform surface covering with simultaneous control of thickness and microstructure and thus allow to influence the properties of the layer. The mechanism of Ni-Mo alloys electrodeposition is still not clearly understood, although a few hypotheses are presented in the literature [4?€7]. Nevertheless, it is known that molybdenum (as well as another reluctant elements, such as W, Ge) cannot be deposited alone from aqueous solution of their salts. However, it could be readily co-deposited with iron-group metals (such as Ni, which acts as a catalyst) with an alloy formation. This phenomenon was called induced co-deposition by Brenner [8]. However, Ni-Mo coatings deposited from solution containing only molybdenum and nickel ions are of poor quality and contain high amount of molybdenum oxides. This effect is probably related to the formation of multimolecular heteropolymolybdates, which are difficult to electroreduce. Addition of an appropriate complexing agent, such as sodium citrate (characterized also by buffering, leveling and brightening properties), causes decomposition of heteropolymolybdates and the formation of the electroactive molybdenum [MoO4(Cit)H]4. and then nickel [NiCit]. citrate complexes (Cit = C6H5O7 3.) [9]. It results in an im[...]

Mechanical and tribological properties of electrodeposited Ni-Mo coatings

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Hard chromium coatings are widely used in aerospace, automotive, hydraulic and machinery industry to reduce friction, wear and corrosion. Electrodeposition is a frequently used technique to deposit such a coatings because is simple, low cost and low temperature. Main advantage is to possibility of relatively uniform deposition on large, complex shape elements. Nowadays hard chromium that dominates within this group coatings were characterize by good mechanical and corrosion resistance but the deposition process is toxic and carcinogenic. Many environmental friendly materials are proposed to replace electroplated chromium coatings, and one of them are nickel based alloys coatings - Ni-P [1], Ni-Mo [2], Ni-W [3] and nanocomposite coatings like Ni-P-Al2O3 [4], Ni-W-SiC [5]. Pure Ni coatings are not hard and wear resistant, but their properties could be significantly improved by alloying and reducing the grain size to nanocrystalline structure. It is reported that Ni-Mo deposits have a good corrosion resistance but there is a lack of tribological studies. For coating-substrate systems wear is not only determined by hardness as showed classical theories of friction [6]. Increase of hardness is usually associated with rise of stiffness and frequently cause change of deformation mechanisms from plastic to brittle fracture. Many studies indicate than wear depends on quotient of hardness H and modulus of elasticity E [7] which is called plasticity index. This value can be used to quantitatively assess the load that may be carried on by the coating in the elastic stress regime. Wear at the elastic state, when abrasive wear dominates is significantly lower than under plastic deformation. The value of the contact force that caused plastic deformation of the coating can be calculated from Hertz theory [8]: P R R pl E l ind = 0 68 3 ⋅ ⋅ 3 2 2 . π (1) where Eind is the reduced modulus of elasticity of coating and the [...]

Optimization of galvanic bath and operating parameters for amorphous Ni-W electrodeposition

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Functional coatings obtained by electrodeposition are widely applied commercially to provide enhanced surface performance for engineering applications [1]. In recent years much effort has been put into searching for alternatives to electroplated hard chromium, the properties of which are often used as a standard for other functional coatings [2]. Usually, high hardness, wear and corrosion resistance are the critical properties. Despite the excellent properties of the hard chromium coatings, used in automotive and aviation industries, they should be eliminated from the manufacturing process (according to the EU directives), due to high toxicity and carcinogenity of hexavalent chromium. Up to now, only electroless hard Ni and Ni-P coatings have found use in selected applications [3]. Electrodeposited Ni-based alloys containing refractory metal (molybdenum or tungsten) could be an important alternative to hard chromium. Ni-W alloys are known for their excellent mechanical and tribological characteristics, but they are very difficult to obtain by conventional metallothermic methods, due to the large differences in melting points (Ni - 1455°C, W - 3410°C) as well as limited mutual solubility of both metals [4]. Hence, electrochemical deposition can be proposed as an alternative technique. Electrodeposition, as a cheap, low temperature and simple method, is a superior technique for manufacture of Ni-W coatings. However, compact and well adherent to the substrate Ni-W coatings are still electrodeposited with technical difficulties. Although tungsten cannot be separately electrodeposited from aqueous electrolytes, it can be easy co-deposited with iron-group metals such as nickel to form an alloy [5]. However, the presence of appropriate complexing agents is necessary. This phenomena is classified by Brenner as induced co-deposition [6]. Among many galvanic baths proposed for the Ni-W alloys electroplating (such as sulfamate, pyrophos[...]

Electrodeposition of nanocrystalline Ni-Mo coatings from citrate electrolyte solution

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Alloys containing molybdenum, e.g. Ni-Mo, are characterized by high hardness, high wear, thermal and corrosion resistance as well as good catalytic properties for hydrogen evolution in alkaline solutions. They also offer an important alternative to hard chromium coatings as chromates are known to be highly toxic and according to EU directive they should be eliminated from manufacturing process, e.g. in automotive and aviation industries [1]. However, these alloys are difficult to prepare by conventional thermal methods due to the large differences in their melting points (Ni - 1455°C, Mo - 2620°C) and their limited mutual solubility [2]. Several techniques have been proposed for the Ni-Mo alloy preparation: mechanical alloying, plasma sputtering, gas condensation and electrocrystallization. Electroplating is a relatively simple, low cost and low temperature method suitable for mass production, where the substrates of complex shape can be uniformly plated by coatings with high rate. However, it is known that molybdenum in a metallic state cannot be separately electrodeposited from an aqueous solution of its salts, but it can readily be co-deposited with iron-group metals such as nickel, forming an alloy. The phenomenon is called induced co-deposition [3]. The mechanism is still not clearly understood, although a few hypotheses are presented in literature [3÷6]. The electrodeposition of Ni-Mo alloys have been extensively investigated [7÷11]. A great number of galvanic baths were studied, e.g.: sulphate, sulpho- salicylate, acetate, tartrate, pyrophosphate, amonia and citrate. Most deposits obtained from these baths were poor quality, porous or cracked and electrodeposition process was not efficient, however pyrophosphate and citrate electrolytes are the most promising. In the present work the correlation of electrodeposition conditions from citrate solution with the coating properties was determined. The surface morphology as a f[...]

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