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10ο ΠΑΝΕΛΛΗΝΙΟ ΕΠΙΣΤΗΜΟΝΙΚΟ ΣΥΝΕΔΡΙΟ ΧΗΜΙΚΗΣ ΜΗΧΑΝΙΚΗΣ, ΠΑΤΡΑ, 4-6 ΙΟΥΝΙΟΥ, 2015.

EFFECT OF ORGANIC ADDITIVES IN THE STRUCTURE AND FUNCTIONAL PROPERTIES OFNI–P COMPOSITE COATINGS REINFORCED BY NANO-SIC AND MWCNT

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

Artia NanoEngineering & Consulting

I. DeligkioziCenter for Technology Research and Innovation

ABSTRACTMetal Matrix Composite (MMC’s) protective coatings constitute a class of coatings which are mostly used formechanical and tribological applications. Among these materials, nickel-phosphorous coatings reinforced bynanoparticles have shown excellent mechanical properties due to nickel-phosphorous matrix and especially goodtribological properties. In this work, nickel-phosphorous coatings containing SiC nano-particles (45–55 nm) orMWCNT were electrodeposited from a sulphate bath containing nickel salts and phosphorous acid. Thestructural and functional properties of the composite coatings were compared to pure nickel-phosphorousdeposits prepared under the same conditions. The effect of the additives mixtures, the current density andreinforcing species on the structure, morphology and functional properties has been evaluated. The coatings wereinvestigated 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 improvedmechanical properties in comparison to pure nickel-phosphorous electrodeposits. This improvement is associatedto structural modifications of the Ni–P matrix as well as the morphology of the electrodeposited layers due to thepresence of reinforcing particles.

INTRODUCTIONWear 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 decadeshard chromium plating has been the most used coating method to protect components operating in high wearenvironment. This is due to its superior hardness and corrosion inhibiting qualities. However, hard chromiumcoatings are applied from electrolytic baths containing hexavalent chromium (Cr+6). Public and governmentagencies 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 regulationsagainst hard chromium plating in order to protect public health and workers involved with handling chromiumplating (i.e. chromium platers) [2].

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

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

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purchased from Emfutur. Prior to deposition, the SiC particles with concentrations of 1–10 g/L wereultrasonically dispersed in small volumes of the electrolyte and the resulted SiC dispersion was poured in theplating bath and stirrered for 24 h. This is necessary because SiC nanoparticles are hydrophobic and time isneeded 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 excellentdispersion of the nano-powders as well as to improve the deposition of compact and functional coatings. Amonga 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–Pelectroplating were maintained at 50 ± 2°C and 1 ± 0.2, respectively. Nickel-sulphur balls in titanium basketwere 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 ultrasonicallycleaned in acetone and distilled water, anodized in alkaline electrolyte and then activated in 1:8 HCl, washed indistilled water and then immersed immediately in the plating bath to allow the electrodeposition of the targetcomposite coatings. Prior to the electrodeposition process, the bath was ultrasonicated for 30 min and afterwardcontentiously stirred to keep uniform electrolyte distribution. Samples were prepared at current densities from2.5 to 20 Adm-2 maintaining the composition of the bath constant.

The surface morphology of composite coatings was examined by scanning electron microscope (SEM) usinga FEI Quanta 200model operated at 30 kV. The chemical composition of the deposits was determined using theenergy dispersive X-ray spectroscopy (EDS) system attached to the SEM. X-ray diffraction (XRD) was used todetermine the phase present and the preferred orientation of the coatings. X-ray diffraction analysis was carriedout using a Siemens D-5000 diffractometer with a Cu-Kα radiation. Diffraction diagrams were recorded with astep 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 crystallinephases 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 Vickersmicrohardness (HV or GPa) of composite Ni–P coatings were performed on the surface by using a WilsonInstruments microhardness tester under 100 g load (HV0.1) and a duration of 15 s. The corresponding finalvalues represented in the results of this report were determined as the average of 10 microhardnessmeasurements on each sample. The surface roughness of the substrate samples before and after compositecoating was measured by Hommel T1000 surface roughness tester. The roughness average, Ra, is used todescribe the surface roughness. All the reported data represents the average of at least five surface roughnessmeasurements.

Table 1. Composition of the electroplating baths.

Bath Code Basic bath compositionSDS Lactic acid Saccharin CTAB Nanoparticles

g/LW-SL-SC1

NiSO4 •6H2O 150 g/LNiCl2 •6H2O 45 g/LH3PO4 49 g/LH3PO3 41 g/L

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

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

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10ο ΠΑΝΕΛΛΗΝΙΟ ΕΠΙΣΤΗΜΟΝΙΚΟ ΣΥΝΕΔΡΙΟ ΧΗΜΙΚΗΣ ΜΗΧΑΝΙΚΗΣ, ΠΑΤΡΑ, 4-6 ΙΟΥΝΙΟΥ, 2015.

Figure 1. Weight percentage of phosphorous versus current density for (a) Ni–P/SiC and (b) Ni–P/CNTcomposite coatings for different composition of electroplating baths.

The main factor however is conversion of the saccharin on the cathode [9,10]. The process, as well as conversionof the phosphite anion and incorporation in the deposit, proceeds with consumption of active hydrogen on thecathode surface. The result is reduction of the expendable hydrogen, and reduction intensity of phosphorus anionconversion and P incorporation. Thus, phosphorus incorporation in Ni-P deposition is inhibited by addition andincreasing the saccharin concentration in electrolyte and saccharin effect is more pronounced at relatively lowpH and weakens as pH increases [11]. Similar trend of decreasing phosphorous content in composite coatingswas observed by addition of CTAB in the plating baths (Figure 1b).

The surface morphology of Ni–P composites produced at I=10 Adm-2 from baths with various loadings ofSiC nanoparticles is displayed in Fig. 2(a-d). The presence of SDS surfactant in the electrolyte improves theuniform distribution of SiC particles as large scale agglomerates (size >300nm) were not observed. Thedispersibility of SiC particles in electrodeposit depends on the dispersion state of SiC particles in electrolyte.

Figure 2. SEM micrographs of: (a) reference Ni–P coating, I=10 A/dm2, W-SL, P=15.4% Wt; (b) Ni–P/SiC,I=10 A/dm2, W-SL-SiC1, P=15.8% wt; (c) Ni–P/SiC, I=10 A/dm2, W-SL-SiC5, P=15.4% Wt; (d) Ni–P/SiC,I=10 A/dm2, W-SL-SiC10, P=16.5% wt; (e) Ni–P/SiC, I=10 A/dm2, W-SLSac-SiC5, P=5.0% Wt; (f) Ni–P/SiC,I=20 A/dm2, W-SLSac-SiC5, P=4.3% Wt; (g) Ni–P/CNT, I=10 A/dm2, W-SL-CNT1, P=15.9% Wt; and (h) Ni–P/CNT, I=10 A/dm2, W-SLC-CNT1, P=5.9% Wt.

(a) (b) (c) (d)

(e) (f) (g) (h)

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10ο ΠΑΝΕΛΛΗΝΙΟ ΕΠΙΣΤΗΜΟΝΙΚΟ ΣΥΝΕΔΡΙΟ ΧΗΜΙΚΗΣ ΜΗΧΑΝΙΚΗΣ, ΠΑΤΡΑ, 4-6 ΙΟΥΝΙΟΥ, 2015.

Figure 3. XRD (a) Ni–P/SiC and (b) Ni–P/CNT composite coatings.

The particles with large negative or positive zeta potential will repel each other and so the agglomeration isinhibited [12,13]. Adversely, the particles with low zeta potential will approach each other and so the aggregatecan be formed [14]. The effect of SDS on increasing the negative surface charge of SiC particles leads to higherrepulsive force between them and consequently lowers their tendency for agglomeration. Depending uponsurfactants variation, surface topographies of samples exhibited considerable variations. The surface morphologyof Ni–P and Ni–P/SiC (Figure 2a-b) appears to be composed of larger nodules of “cauliflower” type structurestypical for amorphous materials. Increase of SiC loading in the plating bath leads to formation of polyhedralcrystals (Fig. 2c-d). Addition of saccharin in electroplating bath changes the coating's morphology to sphericallyshaped crystallites as deposited at I=10 A/dm2 (Fig. 2e). Ni–P/SiC and Ni–P/CNT composites obtained at ahigher current densities (I=15–20 A/dm2) in the presence of saccharin comprise compact and smooth refinedsurfaces (Fig. 2f-g). Refined grains with flat surfaces with small pittings were observed at the surface of Ni-P/CNT coating deposited from baths contining CTAB (Fig. 2g). Cationic surfactant, such as CTAB, plays animportant role in particles co-deposition, since it can modify the state of particle charge (zeta potential) intopositive value by particles-surfactant adsorption phenomenon. Thus, it would increase the adhesion forcetowards cathode, resulting in higher rate of particles incorporation into the growing nickel matrix [15].

The structure of the electrodeposited Ni–P composite coatings obtained at different current densities,containing various concentrations of phosphorous was studied using XRD. The XRD findings are in agreementwith the data available in literature for electrodeposited Ni–P composite coatings. It is generally accepted that thecrystallographic structure of Ni–P alloys is influenced mainly by the amount of P present in the alloy and itevolves from a crystalline to an amorphous one with increasing P content [16,17,18,19]. However, thecomposition at which this transition occurs is not well defined. The X-ray diffraction pattern of the as-depositedNi–P samples at 20 Adm-2 in the absence of saccharin in plating bath, presented in Fig. 3a, shows fcc reflectionscharacteristic of nickel and no peaks from NixPy compounds are noticed. The broadening of peak in XRDsignifies that the deposit is of amorphous nature. Decreasing of current densities lead to the formation along ofamorphous phase and crystalline NixPy compounds such as Ni12P5 and Ni8P3. Addition of saccharin in electrolyteleads to amorphous structures at lower current densities (I=2.5 Adm-2) while by increasing current density Ni–Pcomposite coatings exhibit a reinforcement of the (111) line accompanied with an attenuation of the (200).Electrodeposits obtained at current density ≥ 15 Adm-2 are characterized by an intense (200) diffraction linecorresponding to a (100) texture (Fig. 3(b-c)). Hence, it is obvious that the addition of saccharin in the

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electroplating bath modifies the (111) preferred orientation to soft-mode (100) texture. Therefore, these as-platedNi–P composites can be best described as a supersaturated solid solution of phosphorous dissolved in crystallinenickel. Ni– P/CNT electrodeposits from baths with CTAB shows fcc(111) reflections characteristic of nickel andno peaks from other Ni–P compounds are noticed. The average grain size of the Ni–P deposit was calculatedfrom the (111) or (200) X-ray diffraction peak broadening using the Debye-Scherrer equation. It has beenobserved that with the incorporation of phosphorous in the coating, the average grain size of the crystallitesdecreases from a few micrometers (pure Ni) to few nanometers (7–20 nm). In some cases, when the phosphorouscontent was 15 wt% and above, grain size measurements were not possible as the samples were X-rayamorphous. It should be noticed that neither SiC phases nor CNT have been detected in the XRD patterns ofelectroplated Ni–P/SiC and CNT coatings. Absence of such peaks in samples might be due to low content ofparticles beyond the detection limit by XRD. The energy dispersive X-ray (EDX) analysis of the NiP/SiCcoatings reveal 0.2 to 0.8 % Wt SiC incorporation with increasing loading of SiC in bath from 1 to 10 g/L.Overall, XRD analysis and SEM observation demonstrate that the effect of SiC particles on the microstructure ofcoatings mostly depends on the grain structure of the Ni-P matrix.

Figure 4 shows the microhardness of deposits as a function of P concentration obtained at different currentdensities. In general, it has been observed that the microhardness of these alloys is highly dependent upon thephosphorous content and consequently of their structure [20,21]. Also, it has been reported that (200) reflectionpeak for Ni represents to the preferred (100) texture, which is associated with deposits that possess maximumductility and minimum hardness and internal stress [22]. The enhancement of (311) and (111) nickel peaks isattributed to the dispersion of [211] orientation. Therefore, by evaluating main peaks intensity ratios, it can alsobe estimated that the change in mechanical properties by textural modifications of the electrodeposited nickelcomposite coatings. This result suggests that the proper ratio of surfactants is also an essential part to obtain themixed orientations of the nickel crystallite planes for increasing harder and compact deposit. On the other hand,smaller grains are also responsible for the increase in microhardness, which is connected to the Hall-Petchhardening effect [23,24] induced by ultrafine grains. The original dislocation model for this relationship wasbased on the concept that grain boundaries act as barriers to the propagation of dislocations by formingdislocation pile-ups at grain boundaries, resulting in hard deposits.

The obtained composite coatings have showed microhardness about 600 to 800 Vickers. The hardness ofthese alloys further increases up to 1350 HV after heat treatment at 400°C for 1 h due to the precipitation of hardNi3P phase. Specifically, Figure 4 indicates that the microhardness of all Ni–P/nano-SiC composite coatings isconsiderably higher than the Ni–P alloy coatings. Hardness of composite coatings containing SiC has beenattributed to the hindrance of dislocation movement by SiC particles [25,26]. Ni–P/CNT coatings in the asdeposited form reach values of micro hardness around 800 HV. The observed trend in hardness indicates that themechanical properties of Ni–P deposits depend not only on the phosphorous content but also on themicrostructure. In general, Ni–P electrodeposit with P content in the range of 3–7 wt% exhibited higherhardness. Heat treatment of the samples resulted in the precipitation of Ni3P grains and the amount of Ni3Pgrains precipitated increased with increase in the concentration of P in the coating. The average grain size of thedeposits increased from 5–10 nm for as-plated samples) to 20–50 nm for heat-treated samples. The averagesurface roughness versus current density for the deposits obtained from various electrolytes containing SiC orCNT particles are shown in Fig. 4 (on right). As seen, increasing the current density from 2.5 to 20 A/dm2 causesa decrease in the average surface roughness of the deposits.

Figure 4. Microhardness and roughness of Ni–P composite coatings as a function of current density.

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CONCLUSIONSDepending on the applied conditions and proper combinations of organic additives it has been achieved toproduce a wide range of Ni–P MMC coatings with P content in the range of 3–17 wt%. The structure of thecoatings has been proven to be strongly affected by the organic additives used in the electrolyte and the currentdensity. Thus, by altering the bath composition it was able to transform the structure of the coatings from thecompletely amorphous phase to nano-crystalline with the presence either Ni or Ni12P5 and Ni8P3 phases. Themorphology of the coatings is also correlated to the electrolysis conditions. Thus, at low current density sphericalformations and pyramidal or polyhedron crystallites have been presented, while in higher current densitysmoother surfaces with refined grain boundaries have been produced.

Finally, functional properties of coatings such as micro hardness and surface roughness have been measuredin the as plated form. Pure Ni–P coatings exhibited hardness in the range of 550 to 650 HV depending on theirstructure. In general amorphous structure has as a result lower micro-hardness values. Addition of SiCnanoparticles or MWCNT has as a result the increase of micro-hardness up to 800 HV. This increment can beattributed to both the presence of the particles (even in low concentration) in the matrix of the coatings as well asin the modification of the Ni–P matrix due to the presence of the nano-particles.

Overall, it has been proved that functional Ni–P matrix composite coatings can be produced from modifiednickel sulfate baths with the addition of proper combination of organic additives.

ACKNOWLEDGEMENTSThis work has partially received funding from the European Union’s Seventh Framework Programme forresearch, technological development and demonstration under grant agreement No. 606110 (HardAlt – "Newgeneration of protective coatings alternative to hard chrome").

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