Wyniki 1-4 spośród 4 dla zapytania: authorDesc:"EWA BEŁTOWSKA-LEHMAN"


Czytaj za darmo! »

Zaawansowane zastosowania konstrukcyjne, elektryczne, chemiczne, optyczne, nuklearne czy biologiczne wymagają wysoko wydajnych, wielofunkcyjnych elementów roboczych, posiadających więcej niż jedną właściwość użytkową na wysokim poziomie. Materiały takie trudno jest otrzymać w układach monolitycznych objętościowych, wielowarstwowych lub nawet kompozytowych. Spowodowało to rozwój nowego rodzaju [...]

XRD investigations of electrodeposited Ni and Ni/Al2O3 coatings

Czytaj za darmo! »

Recently, metal matrix composite (MMC) coatings containing ceramic particles have been widely investigated due to their enhanced material properties (i.e. higher hardness, wear and corrosion resistance) compared to the pure metal or alloy [1÷4]. The MMC properties depend mainly on the type, structure, shape, size, morphology and content of the inert ceramic particles as well as on their distribution in the metal matrix. Several metals, e.g. nickel, copper, gold, chromium have been mainly used as a metal matrix, whereas metal oxides, carbides, borides and polymers were the co-depositing particles [5]. Electrolytic nickel coatings exhibit specific properties as hardness, durability, good corrosion resistance and catalytic activity in many electrochemical processes [6]. The addition of hard ceramic particles into Ni matrix can improve its hardness and wear resistance. These phenomena are mainly attributed to the hardening of the metal matrix by finely dispersed ceramic particles [7]. Al2O3 particle has many superior properties, such as low price, good chemical stability, high microhardness wear resistance at high temperature [8]. Insoluble particles are suspended in a conventional electrolytic bath and embedded in the growing metal during co-deposition process. The Ni/Al2O3 nanocomposites are one of the most promising materials, that can find wide engineering application as coatings of engine cylinders, high-pressure valves, car accessories, aircraft microelectronics etc. [9÷10]. Functional properties of electrodeposited composites are mainly controlled by their composition and structure. Among others, they depend strongly on their microstructure, residual stresses and due to the anisotropic properties on the distribution of crystallographic orientation. It is widely accepted that the state of residual stress on the coating surface and in the near surface area is one of the most important parameters of surface deposit quality. Mac[...]

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

Czytaj za darmo! »

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

Czytaj za darmo! »

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 [...]

 Strona 1