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Progress in Organic Coatings 74 (2012) 788– 793

Contents lists available at SciVerse ScienceDirect

Progress in Organic Coatings

j ourna l ho me p ag e: www.elsev ier .com/ locate /porgcoat

he performance of surfactant on the surface characteristics of electrolessickel coating on magnesium alloy

. Sudagara, J.S. Liana,∗, Q. Jianga, Z.H. Jianga, G.Y. Lia, R. Elansezhianb

College of Materials Science and Engineering, Nanling Campus, Jilin University, Changchun, ChinaDepartment of Mechanical Engineering, Pondicherry Engineering College, Puducherry State, India

r t i c l e i n f o

rticle history:eceived 17 May 2011eceived in revised form 25 August 2011ccepted 13 October 2011vailable online 18 December 2011

eywords:

a b s t r a c t

Magnesium and its alloys corrode rapidly in the electrolyte bath. Surfactants while used extensively assurface active agents in the electrolyte bath, have been little studied on magnesium surfaces. The influenceof surfactants cetyltrimethyl ammonium bromide (CTAB) and sodium lauryl sulfate (SLS) on the surfaceproperties such as roughness, morphology and topography of electroless Ni–P deposits on magnesiumalloy was researched. The research reveals that the surfactant solutions has significant influence on thecomposition of coating, surface roughness and surface morphology. In addition, it has marginal effect on

lectroless nickelurfactantsurface morphologyurface topographyicrohardness

the microhardness. Electroless coatings with addition of surfactants produce a smooth surface and aver-age roughness value of 1.412 �m for CTAB and 1.789 �m for SLS, which are less than the value (2.98 �m)without surfactant addition. There was a significant improvement in the rate of deposition. However, thesurfactants influence reached maximum at critical micelle concentration and above this value it gets sta-bilized. The initial structure appears to be dependent upon the percent occluded surfactants. The surface

ssed

agnesium alloy microstructures are discu

. Introduction

Electroless nickel coating has received widespread acceptances it provides a uniform deposit on irregular surfaces, direct deposi-ion on surface activated non-conductors, formation of less porouseposits, high hardness and excellent resistance to wear, abra-ion and corrosion [1]. All smooth surfaces possess some degreef roughness, even if only at the atomic level. Correct functionf the fabricated component often is critically dependent on itsegree of roughness. Every machining operation bequeaths someharacteristic on the machined surface. This characteristic micro-rregularities left by the cutting tool which are termed as surfacerregularity or surface roughness [2]. Roughness is sometimes anndesirable property, as it may cause friction, wear, drag andatigue, but it is sometimes beneficial, as it allows surfaces to trapubricants and prevents them from welding together. Magnesiumlloys have promising properties for several industrial applica-ions, because of their low density [3]. Magnesium alloy with

etallic (electroless/electroplating) deposit are being used, in newight-weight engines which are less in weight and hence consumeess energy. However, metallic coatings in magnesium are having

ultitudinous problems caused by surface roughness. Example of

∗ Corresponding author. Tel.: +86 431 85095875; fax: +86 431 85095876.E-mail address: lianjs@jlu.edu.cn (J.S. Lian).

300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2011.10.022

in line with the experimental observations.© 2011 Elsevier B.V. All rights reserved.

mechanical malfunction can be found in high-performance enginemachine parts which are required to move or rotate at high speedwithout wear. Excess surface roughness can lead to unacceptablyhigh levels of frictional heating, causing damage and even failure[4].

Surfactants are specifically added into the electrolyte bath toreduce the vertical component of surface tension forces, whichbinds the nickel particles to the hydrogen gas bubbles generatedduring the plating reaction. Due to this, uniform and pit-free coat-ing can be obtained. Smooth and pit-free electroless Ni–P depositswere obtained by adding 150 ppm of sodium dodecyl sulfate (SDS)to the electroless nickel bath [4]. Similarly, a very brief conclusionwas derived by Hagiwara et al. [5] as well, who studied the effectof three different surfactants added in the Ni–P electroless bath onthe morphology of the resulting Ni–P particles. Many attempts havebeen made to find out the effect of surfactants on the roughness ofelectrodeposited Ni–P coatings. Tripathy et al. [6] and Wheeler et al.[7] studied a numerical model to explain the influence of catalyticsurfactant on roughness evaluation. Alsari et al. [8] research stud-ied on SDS effect on the electroplating deposion. Medina-Valtierraet al. [9] studied the influence of CTAB on the roughness of titaniasol–gel films. Elansezhian et al. had investigated SDS and CTAB [10]and showed that there is a possibility of significant improvement

in the average surface finish of electroless Ni–P deposit on mildsteel. However, there was no such investigation on magnesiumalloy and moreover it is complicated because of corrosive naturein the electrolyte bath.

J. Sudagar et al. / Progress in Organic

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ig. 1. Effect of added surfactant at its CMC value on the rate of the electroless Ni–Poating on magnesium alloy.

Hence, in this investigation, two types of surfactants namelyodium lauryl sulfate (SLS) and cetyl tri-methyl ammonium bro-ide (CTAB) were used in the electrolyte for electroless Ni–P

eposit on magnesium alloy, which affects the surface tension

ig. 2. (a) XRD patterns of electroless Ni–P coating with-out/with added surfactant; theirlloy.

Coatings 74 (2012) 788– 793 789

forces between solid/liquid interfaces [11] and influencing thecoating process through this mechanism. The deposition rates ofthe electroless Ni–P deposit depends on many factors. For exam-ple, they might be the temperature, pH of bath, bath loading,concentrations of nickel and the reducing agent, and the sur-face properties of the substrates. Wetting agents, such as ionicand nonionic surfactants, are often added to increase the wet-tability of surfaces to be deposited. Despite the complicatedbehavior of the deposition reactions and adsorption of surfac-tants to the as-deposited substrates, qualitative discussions onthe effects of added surfactants (SLS and CTAB) to the plat-ing bath solutions on the deposition rates, surface morphology,surface topography and microhardness are investigated in thisstudy.

2. Experimental

2.1. Sample and bath preparations

The substrate mainly contained about 9.1 wt.% Al, 0.64 wt.% Zn,0.17 wt.% Mn, 0.001 wt.% Fe and Mg balance (AZ91D die cast magne-sium alloy with a size of 20 mm × 15 mm × 5 mm). All pre-process

including, alkaline cleaning and chrome plus HF pre-treatmentprior to electroless bath for magnesium alloy can be found in ourearlier studies [12]. The treatment processing of magnesium alloysis always difficult, if the substrate covers with an oxide film. Hence,

parameters (b) CTAB added; (c) SLS added, electroless Ni–P deposit on magnesium

790 J. Sudagar et al. / Progress in Organic Coatings 74 (2012) 788– 793

F t-freec

satr

TC

ig. 3. SEM morphology (200× and 1000×) of electroless Ni–P coated: (a) surfactanoncentration, on magnesium alloy substrate.

amples should be transferred as quickly as possible between the

ny two steps of the treatments. The bath composition and allhe operation parameters for the electroless Ni–P deposition areeported in Table 1.

able 1ompositions of plating bath for electroless Ni–P deposits used.

Electroless Ni–Pbath composition

Bath-1(/L)for CTAB

Bath-2(/L)for SLS

Conditions

NiSO4·6H2O 0.09 mol 0.09 molNaH2PO2·H2O 0.3 mol 0.3 mol pH 6 ± 0.2HF (40%, v/v) 0.6 mol 12 ml TemperatureNaC2H3O2 0.15 mol 0.15 mol 84 ± 2 ◦CNH4HF2 0.14 mol 0.14 molStabilizer 1 ppm 1 ppmNaC12H25SO4 (0–2.0) –C19H42BrN – (0–1.8)

, (b) with added CTAB at their CMC concentration, (c) with added SLS at their CMC

2.2. Added surfactants

The surfactants (surface active agent) are usually organic com-pounds that are amphiphile. Surfactants lower surface tensionby absorbing onto surfaces such as liquid/air or solid/liquid. Inlow concentration, surfactants exist as isolated molecules. Whenconcentration increases, it forms aggregates called micelles. Thecritical micelle concentration (CMC) (for SLS) in pure water at25 ◦C is 0.0082 M [13]. However, this micelle formation dependson many factors. According to previous report [14], the CTABand SLS get optimum at 0.00411 and 0.00416 M respectively, inelectroless Ni–P deposit on mild steel. Due to turbulence createdby the surfactant the coating bath get de-stabilized (0.6 g/l andabove). In mild steel substrate, the coating was delayed by as

much 40 min due to the formation of surfactant monolayer onthe substrate surface preventing direct contact of electrolyte withsubstrate. Hence, a nickel strike was given prior to dip in the elec-trolyte having surfactant [11]. In magnesium alloy substrate, the

J. Sudagar et al. / Progress in Organic

Fig. 4. The average surface roughness vs. added concentration of surfactant. Thevariation of the surface roughness with added surfactant of the electroless Ni–Pc

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face morphology from a non-smooth state to smooth state and the

oatings was determined by AFM.

oth rate of corrosion and rate of nickel coating was initiated atame time. Hence, a thick nickel strike is needed before proceednto the bath having surfactant and moreover, good skill is alsoeeded.

.3. Deposition rate and characterization of electroless Ni–Peposits

The deposition rate R of the electroless Ni–P alloy is expressedn terms of the weight gain during the deposition process. Thes-deposited sample was dried and weighed repeatedly until nourther change in the reading to ensure complete drying of the Ni–Peposit. Accordingly, the deposition rate can be expressed as theollowing equation:

Deposition-rate) R = w × 104

dAt

here R is the deposition rate (�/h), w is the weight gain (g), d is theensity of the deposit (g/cm3), A is the surface area of depositioncm2), and t is the deposition time (h). However, the nickel is asso-iated with phosphorous content in this electroless NiP deposit. Inhis work, the phosphorus content in the electroless Ni–P depositsas found to be nearly 6–9 wt.%. The experimental results reported

n this work are the average values of several runs, which are repro-ucible within ±5 �g.

The structures were studied by the X-ray diffractometerXRD) (D/max 2500PC, Rigaku, Japan) with a Cu K� radiation� = 0.154178 nm) and a monochromator at 50 kV and 300 mAith the scanning rate and step being 4◦/min and 0.02◦, respec-

ively. The surface morphology of the electroless nickel coatingsere observed by using scanning electron microscope (SEM) (JEOL

SM-5310, Japan) and an EDX attachment was used for qualita-ive elemental chemical analysis. Atomic force microscope (AFM)Nanoscope IIIa, Digital Instruments, USA) was employed by usinghe silicon cantilever for the tapping mode. The microhardness wasvaluated by using a HXD-1000 microhardness tester with Vickers

ndenters, employing a constant load of 300 g for 15 s. The five dif-erent places on the deposit were measured, and then the values getveraged.

Coatings 74 (2012) 788– 793 791

3. Results and discussion

3.1. Deposition rate of electroless Ni–P deposit with surfactantson magnesium

Fig. 1 shows the effect of added surfactant on the rate of elec-troless Ni–P deposition. The concentration of the surfactant wasmaintained at CMC levels as discussed in the experimental part. TheCMC condition was established through the concentration variationof a particular surfactant so as to achieve the maximum depositionrate. It clearly shows that the addition of a surfactant at its opti-mum level increases the deposition rate by about a factor of 15%and 26% for SLS and CTAB, respectively; the bath with addition ofCTAB gives a higher deposition rate than that added with SLS. Theseare in good agreement with the previously reported effect of sur-factants in electroless Ni–P deposited on brass substrate [15]. Theeffect of the added surfactant concentration on the deposition ratehas also been studied in wide range from lower concentration rangeof a micro grams to 1.8–2.0 g/l and the surfactants of CTAB or SLS atconcentration less than to 300 ppm yield almost no variations thansurfactant-free bath. When the concentration is higher than 0.6 g/l,this surfactants effect start forming, as the concerned micelles arestart forming above this level [11,14].

3.2. Surface characteristics of electroless Ni–P deposits withsurfactants on magnesium

The XRD pattern of the electroless Ni–P deposit with-out andwith surfactants is shown in Fig. 2(a). The deposits had only asingle very broad peak and the reflections corresponding to the(1 1 1) plane of a face-centered cubic nickel phase is observed (Grainsizes calculated using Debye–Scherrer method). The intensity ofthe (1 1 1) peak was much stronger than that of (2 0 0) and (2 2 0)peaks in samples. The variations of the full-width at half-maximum(FWHM), height of (1 1 1) peak and the nano-crystal size as a func-tion of the added surfactant concentration are shown in Fig. 2(b)and (c). It can be seen that the crystal size decreased with theincrease of the added concentration of CTAB (Fig. 2(b)). The FWHMand height of (1 1 1) peak increased with the increase of the addedCTAB concentration, except some fluctuation in the height of peak.The comprehensive analysis showed that the crystallization qual-ity of the sample was deteriorated gradually with the increaseof added concentration. It reveals that after addition of CTAB, itincreases coating thickness and mixture of amorphous and nano-crystalline phase. The XRD parameters of electroless Ni–P depositswith SLS showing similar behavior (Fig. 2(c)). This suggested thatthe behavior for SLS was similar to that of CTAB.

The surface morphology of the deposits was analyzed by SEM.The SEM micrographs of electroless deposit with different con-centration of CTAB and SLS are presented in Fig. 3 (200× and1000×). Without addition (Fig. 3(a)), the matrix of the coatingconsists of relatively bigger grains and particles than after addi-tion. Evidence is provided in Fig. 3(b) for CTAB and Fig. 3(c) forSLS that the surface morphology has changed from non-smoothnodular appearance to a smooth surface. The effect of the addedsurfactant on the surface morphology has also been studied at abeginning stage of electroless Ni–P deposits as well (without thicknickel strike), the surfactants including CTAB as well as SLS yieldsseveral pin-holes. This enhances the substrate to corrode more,instead of nickel deposition. The influence of surfactant on the sur-face roughness of electroless Ni–P coating is an interesting aspect.In the presence of surfactants the Ni–P deposits changes the sur-

reason is the amount of nickel particles deposited on the depositsurface is increased. This is due to the fact that at CMC concen-tration of surfactant the contact angle is reduced and this leads to

792 J. Sudagar et al. / Progress in Organic Coatings 74 (2012) 788– 793

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Fig. 5. The average microhardness of electroless Ni–P coa

he better wettability of Ni–P deposit. It is apparent that surfac-ant addition during Ni–P deposit can improve the roughness atower concentration. The surface of electroplated Ni–P deposited

ith SDS addition was smoother at higher (CMC) concentrations7]. The addition of SDS during electro-deposition of Ni–P modifiedhe surface roughness and surface morphology of deposit for underump metallization in microelectronic industry [16] and it is goodgreement with our present observations. In addition, the phos-horus content (not shown) in the coating improved significantlyith addition of CTAB from 5.9 wt.% to 9 wt.% resulted in better cor-

osion resistance and with addition of SLS the improvement wasnly to 6.9 wt.%. In addition, crystalline phase to amorphous plusano crystalline phase, this results smoother and flatter surface.heir structures are reported as rather amorphous and comparablymoother than low phosphorous [17].

The topography of the coatings was analyzed by using AFM.he AFM images of electroless Ni–P coatings without and withurfactant were presented in Fig. 4. They show that the surfaceopography of without surfactant have more roughness; and afterddition of surfactant, the surfaces improved to be more smooth-ess. The average surface roughness value Ra is 1.412 �m for CTABnd 1.789 �m for SLS with the CMC quantity of surfactant, whichre much smaller than the value of 2.98 �m measured from theoating surface topography without surfactants.

.3. Microhardness of electroless Ni–P deposits with surfactantsn magnesium

The microhardness of electroless Ni–P deposits increases withncreasing the surfactant concentration up to its CMC value andhen decreased. Fig. 5(a) shows the variation of microhardness oflectroless Ni–P deposit with respect to CTAB surfactant concen-rations. In as-plated condition, hardness is 400 VHN300. In theresence of CTAB the hardness increased up to 565 VHN300, which

s attributed to the change in crystalline structure. In the absencef surfactant, the deposits are purely crystalline and with addi-ion of CTAB it is changed to a mixture of nano crystalline andmorphous structure. This can be testified by the widening of the-ray diffraction peak of electroless Ni–P deposits (Fig. 2(b)) withddition of CTAB at various concentrations. When CTAB is added

nto the electrolyte bath the phosphorus content is increased from% to 9%. At higher concentrations above the CMC value of CTABhe phosphorus content is high (∼9%). Fig. 5(b) shows the vari-tion of microhardness of electroless Ni–P deposit with respect

s. added surfactant concentrations: (a) CTAB and (b) SLS.

to SLS surfactant concentrations. In the presence of SLS the hard-ness increased up to 485 VHN300 only. The reason is due to thechange in crystalline structure. The EN deposits are changed to amixture of nano crystalline and amorphous structure and the corre-sponding X-ray diffractograms of EN deposits with addition of CTAB(Fig. 2(c)). Microhardness value increases significantly for CTAB andmarginal for SLS.

4. Conclusions

Electroless nickel coatings on Mg alloy was successfully car-ried out with addition of surfactants CTAB and SLS. The followingconclusions have been drawn:

• The addition of small concentration of surfactants to the coatingbaths could increase the deposition rates by 26% compared to thatfrom the surfactant-free bath. XRD diffractograms shows additionof CTAB and SLS promotes formation of amorphous plus nanocrystalline phases in the deposits than as-deposited Ni–P.

• Surface morphology and surface topography reveals that thesmoothness increased by 52% compared to that from thesurfactant-free bath which resulting in good surface finish on theelectroless Ni–P deposited magnesium alloy. Microhardness wasimproved from 400 VHN to 565 VHN for CTAB and to 490 VHNfor SLS addition which is good for better wear resistance.

On the overall, surfactants enhanced surface properties of theelectroless Ni–P deposit on magnesium alloy and further heat treat-ment will improve greater.

Acknowledgements

This work was supported by National Nature Science Founda-tion (Grant No. 50871046), the Foundation of National Key BasicResearch and Development Program (No. 2010CB631001) and theProgram for Changjiang Scholars and Innovative Research Team inUniversity. One of our authors (J. Sudagar) acknowledges to Indo-China cultural exchange scholarship program by the Ministry ofHuman Resource Department (MHRD, India) and Ministry of Edu-cation (MOE, China).

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