Design and Selection of Chemically Deposited Ni-P-W Coatings for Optimum Tribological Behavior

Design and Selection of Chemically Deposited Ni-P-W Coatings for Optimum Tribological Behavior

Prasanta Sahoo (Jadavpur University, India), Supriyo Roy (Jadavpur University, India) and J. Paulo Davim (University of Aveiro, Portugal)
DOI: 10.4018/978-1-4666-5141-8.ch002
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Chemically deposited nickel coatings possess superior tribological properties such as high hardness, good wear, and corrosion resistance. The quest for improved tribological performance has led to the design and selection of newer variants of these coatings. The present chapter deals with the development of Ni-P-W coating on mild steel substrate and the improvement of tribological characteristics through modification of the coating process parameters. Three coating process parameters, concentration of nickel source, concentration of reducing agent, and concentration of tungsten source along with the annealing temperature, are optimized for minimum friction and wear of the coating. Friction and wear tests are carried out in a multi-tribotester using block on roller configuration under dry conditions. Taguchi-based grey relational analysis is employed for the optimization of this multiple response problem using L27 orthogonal array. Analysis of variance shows that the concentration of nickel source, the interaction between nickel source concentration, and reducing agent concentration, and also the interaction between nickel source concentration and tungsten source concentration have significant influence in controlling the friction and wear behavior of chemically deposited Ni-P-W coating. It is observed that wear mechanism is a mild adhesive in nature. The structural morphology, composition, and phase structure of the coating are studied with the help of Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray analysis (EDX), and X-Ray Diffraction analysis (XRD), respectively.
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The performance and life of engineering components can be enhanced by applying hard coatings over the surface of the components. The thickness of coatings range between 10 and 500 μm and their rates of deposition can provide the required product quality at relatively low capital and operating costs. The coating processes are broadly classified into two groups: electroplating and electroless or chemical coating. Since the study of electroless/chemical coating process by Brenner and Riddell (1946), a series of research studies have been performed and the process is accepted by various industries like electrical, aerospace, automotive, chemical, electronics, etc (Mallory & Hadju, 1991; Riedel, 1991). In chemically deposited coating process, the substrate is submerged in to a chemical solution called bath which consists of solution of metal ions, reducing agent, complexing agents, stabilizers, buffer solution etc. Few metal coatings applied by electroplating can match the thickness uniformity of an electroless coating finish. Because these protective coatings are chemically applied, they create deposits of highly consistent depth across all surfaces, including edges and complex interior geometries. Electroless coating has several advantages over conventional electroplating technique, except the life of the bath. The advantages are that the quality of the deposit, namely the physical, tribological and mechanical properties improve. Moreover the non-conductive substrate can be coated by this electroless method. In this process, a sharp edge receives the same thickness of deposit as a blind hole does and it offers extremely bright deposits, which are comparable with electroplated bright nickel. The desirable properties can be varied by choosing different pH, temperature and composition of the bath. Electroless nickel coatings has assumed the greatest commercial importance among the electroless coating.

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