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EFFECT OF ORGANIC ADDITIVES IN THE STRUCTURE ... ... increasing the saccharin concentration in electrolyte and saccharin effect is more pronounced at relatively low pH and weakens

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    A. Zoikis-Karathanasis, T. Kosanovic Milickovic Centre for Research & Technology Hellas, Institute for Research and Technology of Thessaly

    Artia NanoEngineering & Consulting

    I. Deligkiozi Center for Technology Research and Innovation

    ABSTRACT Metal Matrix Composite (MMC’s) protective coatings constitute a class of coatings which are mostly used for mechanical and tribological applications. Among these materials, nickel-phosphorous coatings reinforced by nanoparticles have shown excellent mechanical properties due to nickel-phosphorous matrix and especially good tribological properties. In this work, nickel-phosphorous coatings containing SiC nano-particles (45–55 nm) or MWCNT were electrodeposited from a sulphate bath containing nickel salts and phosphorous acid. The structural and functional properties of the composite coatings were compared to pure nickel-phosphorous deposits prepared under the same conditions. The effect of the additives mixtures, the current density and reinforcing species on the structure, morphology and functional properties has been evaluated. The coatings were investigated with scanning electron microscopy (SEM–EDS), X-ray diffraction (XRD) and Vickers micro- hardness. The Ni–P/SiC and Ni–P/CNT MMC’s coatings, prepared at optimum conditions, exhibited improved mechanical properties in comparison to pure nickel-phosphorous electrodeposits. This improvement is associated to structural modifications of the Ni–P matrix as well as the morphology of the electrodeposited layers due to the presence of reinforcing particles.

    INTRODUCTION Wear and corrosion of materials cost up to 3-4% of developed countries’ national income (GDP) [1]. Every year, billions of Euros are spent on capital replacement and control methods for wear and corrosion infrastructure, hence, prevention of wear and corrosion is of crucial importance for the European economy. For several decades hard chromium plating has been the most used coating method to protect components operating in high wear environment. This is due to its superior hardness and corrosion inhibiting qualities. However, hard chromium coatings are applied from electrolytic baths containing hexavalent chromium (Cr+6). Public and government agencies having already recognized the extremely harmful impact of Cr+6 in both human health and environment (cancers, respiratory problems, contamination of aquifer etc.) have begun to enact legislations and regulations against hard chromium plating in order to protect public health and workers involved with handling chromium plating (i.e. chromium platers) [2].

    Composite deposits used as protective surface coatings for engineering components to improve their wear resistance and service life are the one of alternatives to hard chromium [3]. The nanocomposites can exhibit enhanced mechanical and chemical (e.g. corrosion resistance) properties compared with their conventional micron-scale (or larger) counterparts [4]. The reduced size of the reinforcement phase down to the nano-scale is such that interaction of particles with dislocations becomes of significant importance and, when added to other strengthening effects typically found in conventional MMC’s, results in a remarkable improvement of mechanical properties [5]. Particularly, electrolytic composite coatings based on a Ni–P alloy matrix containing fine particles of SiC, WC, B4C, MWCNT, or TiO2 have attracted attention due to their good mechanical and chemical properties including high hardness and enhanced wear resistance combined with a good corrosion resistance [4]. However, homogenous dispersion of the reinforcing nano-particles in the metallic matrix is a prerequisite in order to enhance their mechanical/tribological properties [6].

    In the present work, the effect of organic additives at the structure and functional properties of Ni–P based nano-composite coatings has been studied.

    EXPERIMENTAL A standard sulfate based Ni–P bath reinforced with SiC nanoparticles or MWCNTs have been investigated. The composition of the electroplating baths are given in Table 1. Analytical reagents and distilled water were used to prepare the baths. Particle size of SiC (Nanoamor, β-SiC) was 45–55 nm. Multiwalled carbon nanotubes (3–15 walls, MWCNTs) with outer diameter of 5–20 nm, inner diameter 2–6 nm and tube length 1–10 μm were


    purchased from Emfutur. Prior to deposition, the SiC particles with concentrations of 1–10 g/L were ultrasonically dispersed in small volumes of the electrolyte and the resulted SiC dispersion was poured in the plating bath and stirrered for 24 h. This is necessary because SiC nanoparticles are hydrophobic and time is needed to allow the surface of the SiC nanoparticles to hydrate. Simmilar procedure was followed for MWCNTs. Various combinations additives have been introduced into the electrolytic bath in order to achieve excellent dispersion of the nano-powders as well as to improve the deposition of compact and functional coatings. Among a plethora of organic additives the ones used in this studgy were sodium dodecyl sulphate (SDS), lactic acid, saccharin and cetyl-trimethyl-ammonium bromide (CTAB). The operating temperature and pH chosen for Ni–P electroplating were maintained at 50 ± 2°C and 1 ± 0.2, respectively. Nickel-sulphur balls in titanium basket were used as anode. Cathodes, made of mild carbon steel, were positioned in vertical alignment with the anode. The distance between anode and cathode was 4 cm. Before each experiment, the cathode was ultrasonically cleaned in acetone and distilled water, anodized in alkaline electrolyte and then activated in 1:8 HCl, washed in distilled water and then immersed immediately in the plating bath to allow the electrodeposition of the target composite coatings. Prior to the electrodeposition process, the bath was ultrasonicated for 30 min and afterward contentiously stirred to keep uniform electrolyte distribution. Samples were prepared at current densities from 2.5 to 20 Adm-2 maintaining the composition of the bath constant.

    The surface morphology of composite coatings was examined by scanning electron microscope (SEM) using a FEI Quanta 200model operated at 30 kV. The chemical composition of the deposits was determined using the energy dispersive X-ray spectroscopy (EDS) system attached to the SEM. X-ray diffraction (XRD) was used to determine the phase present and the preferred orientation of the coatings. X-ray diffraction analysis was carried out using a Siemens D-5000 diffractometer with a Cu-Kα radiation. Diffraction diagrams were recorded with a step of 0.05° for 2θ ranging from 20° to 100° and measuring time 0.6 s per step. The Difrac® plus Eva software, version 2.0 (Siemens Energy and Automatization, Inc.) was used to identify the crystalline peaks. The crystalline phases of all samples were identified by comparison with the ICDD (International Centre for Diffraction Data) database and Joint Committee on Powder Diffraction Standards (JCPDS) files. Measurements of the Vickers microhardness (HV or GPa) of composite Ni–P coatings were performed on the surface by using a Wilson Instruments microhardness tester under 100 g load (HV0.1) and a duration of 15 s. The corresponding final values represented in the results of this report were determined as the average of 10 microhardness measurements on each sample. The surface roughness of the substrate samples before and after composite coating was measured by Hommel T1000 surface roughness tester. The roughness average, Ra, is used to describe the surface roughness. All the reported data represents the average of at least five surface roughness measurements.

    Table 1. Composition of the electroplating baths.

    Bath Code Basic bath composition SDS Lactic acid Saccharin CTAB Nanoparticles

    g/L W-SL-SC1

    NiSO4 •6H2O 150 g/L NiCl2 •6H2O 45 g/L H3PO4 49 g/L H3PO3 41 g/L

    1 10 0 0 SiC: 1 W-SL-SC5 1 10 0 0 SiC: 5 W-SLSac-SC5 1 10 3 0 SiC: 5 W-SL-SC10 1 10 0 0 SiC: 10 W-SLSac-CNT1 1 10 3 0 MWCNT: 1 W-SLC-CNT1 1 10 0 0.1 MWCNT: 1 W-SL-CNT2 1 10 0 0 MWCNT: 2

    RESULTS AND DISSCUSION Nickel–phosphorous silicon carbide and nickel–phosphorous MWCNT composite coatings were deposited galvanostatically from baths described in Table 1. Composition of the composite depends on the bath chemistry and electroplating parameters. By controlling current density, phosphorous acid concentration, additives and temperature, bright and adherent Ni–P coatings with a phosphorous content from 3% up to maximum of 17% Wt were obtained. Dependence of phosphorous content on the electrolysis conditions and bath chemistry is graphically presented in Figure 1. There is a general trend of a decreasing phosphorous content with increasing current density which is also reported in literature by many authors, although a large scatter between the data points of the different authors is noticed [7]. Introduction of the saccharin in the electroplating bath resulted in the decrease of phosphorous content in the composite coatings. The low phosphorus content in the composites in the presence of saccharin is, among others, the consequence of the occupation of a considerable part of the active centers at the cathode surface by saccharin [8].


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