Surface and Coatings Technology 155 (2002) 279284

0257-8972/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972 02 .00116-0

Graded NiPPTFE coatings and their potential applicationsQ. Zhao *, Y. Liu , H. Muller-Steinhagen , G. Liua, a b c

Department of Mechanical Engineering, University of Dundee, Dundee DD1 4HN, UKa

Institute for Thermodynamics and Thermal Engineering, University of Stuttgart, Pfaffenwaldring 6, 70550 Stuttgart, Germanyb

School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR Chinac

Received 10 January 2002; accepted in revised form 4 March 2002

Abstract

The effect of the ratio of a cationic surfactant concentration to PTFE concentration in the plating solution on the electrolessNiPPTFE deposition rate and on the PTFE content in the coatings has been investigated. The adhesion of the NiPPTFElayer is significantly improved by gradually increasing the PTFE content from the substrate to the top surface. It has beendemonstrated that these graded electroless NiPPTFE coatings can reduce the formation of deposits on heat exchanger surfacessignificantly. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electroless deposition; NiPPTFE; Graded coating; Adhesion

1. Introduction

The first composite coatings of electroless (or auto-catalytic) nickelphosphorus and polytetrafluoroethyl-ene (PTFE) were introduced approximately 18 yearsago w1,2x. Electroless nickelphosphorous is widelyused in the chemical, mechanical and electronic indus-tries because of its corrosion and wear resistance andits inherently uniform coating thickness. The incorpo-ration of PTFE particles into the NiP matrix can takeadvantage of the different properties of NiP alloy andPTFE w3x. PTFE is chemically very inert and has arelatively high melting point (325 8C). Its coefficient offriction is lower than that of almost any other polymers.Because of its extremely low surface energy (18.6 mNym), PTFE has excellent non-stick properties w4x. Theresulting properties of electroless NiPPTFE coatings,such as non-stick, non-galling, anti-adhesive, higher drylubricity, lower friction, good wear and good corrosionresistance, have been used successfully in many indus-tries w3,5x. A concise review of the field of compositeelectrodeposition and highlights of the importance ofprocess control in obtaining critical deposit characteris-tics for a variety of demanding industrial applications

*Corresponding author. Tel.: q44-1382-345651; fax: q44-1382-345508.

E-mail address: q.zhao@dundee.ac.uk (Q. Zhao).

were given by Kerr and Barker et al. w6x and Helle andWalsh w7x. Celis et al. gave a detailed survey on theunderstanding of the mechanism of electrolytic codepos-ition and presented some trends and expected futuredevelopments of composite plating w8x.PTFE particles readily coagulate and precipitate in

the plating solution since PTFE is a water-repellentmaterial. Due to this agglomerate formation it is difficultto obtain a uniform dispersion of PTFE particles in aplating bath. This will not only reduce the PTFE contentin the coatings, but also increase the surface roughnessas larger PTFE particles are incorporated w9,10x. Toachieve optimum properties of electroless NiPPTFEcoatings, the PTFE particles must be uniformly distrib-uted throughout the NiP matrix. Areas where less PTFEis incorporated do not have the same physical andmechanical properties as areas where the PTFE contentis uniform and higher. This influences the mechanicaland tribological (friction, lubrication, bearing and hydro-dynamic) properties of the coating, and is a mainproblem in many electroless NiPPTFE coatings w5x.Matsuda et al. w9,10x investigated the effect of thepresence of a variety of surfactants on the suspensionof PTFE particles in electroless nickel plating solutions.They found that the suspension of the PTFE particleswas improved by the addition of non-ionic surfactants.Nishira et al. w11x studied the effects of agitation

280 Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279284

Table 1The pretreatment and coating procedures for electroless NiPPTFE

Procedures Conditions

Alkaline cleaning 6080 8C, 510 minRinsing Room temperatureCathodic electrocleaning Room temperature, 23 minRinsing Room temperaturePickling Room temperature, 0.51 minActivation Room temperature, 13 minElectroless plating NiP 8590 8C, pH: 4.85.0Rinsing Room temperatureElectroless plating graded NiPPTFE 8590 8C, pH: 4.85.0Rinsing Room temperature

Table 2Bath composition and operating conditions for electroless NiPPTFE

NiSO 6H O: 25 gyl4 2 NaH PO : 30 gyl2 2H C H O 6H O: 18 gyl3 6 5 7 2 NaCH COO: 18 gyl3PTFE(60 wt.%): 450 mlyl (NH ) CS: 1 ppm2 2C H F N O I (FC-4): 01.0 gyl20 20 23 2 4 pH: 4.8; Temperature: 8893 8C

methods on the particle size distribution of PTFE aggre-gates in a plating solution. One of their findings wasthat an ultrasonic homogeniser was more effective thanmechanical agitation.An important potential application of electroless Ni

PPTFE coatings is to reduce fouling, which is generallydefined as the accumulation of unwanted material onthe surfaces of processing equipment w12x. A typicalexample is the formation of limescale on heat exchangersurfaces or on household heating elements. Fouling hasbeen recognised as a nearly universal problem in designand operation of processing equipment. Any method forpreventing fouling or lengthening processing timethrough minimising fouling will give substantial costsavings. It has long been known that poorest scaleadhesion occurs on materials with low surface energies.Many attempts have been made to achieve this bycoating surfaces with PTFE or other polymer layersw13x. However, the poor thermal conductivity, poorabrasion resistance and poor adhesion to metal substrateof these low surface energy coatings currently inhibittheir commercial use w14x. Because electroless NiPPTFE coatings are metal-based, their thermal conductiv-ity, anti-abrasive property and mechanical strength aresuperior to standard PTFE coatings. However, the adhe-sion of standard NiPPTFE coatings to the substrateneeds to be improved, since the coatings were generallyfound to peel-off during fouling tests with water.The purpose of this paper is to describe the effect of

cationic surfactant and PTFE emulsion addition on NiPPTFE coating rate and PTFE content in the coatings.Furthermore, a method of improving the adhesion of theNiPPTFE coating is described.

2. Experimental procedure

2.1. Operating parameters

Electroless NiPPTFE composite coating is alsocalled a dispersion coating. In the bath, the reducingagent NaH PO reduces the metallic ion Ni on the2q2 2surface of a catalytic solid (substrate), so that the NiPalloy coating is formed. The PTFE particles which aresuspended in the bath diffuse to the surface of thesubstrate under the action of stirring and Coulombsforce. They are then absorbed physically and chemicallyon the surface. The PTFE particles are buried in thegrowing NiP coating to form the NiPPTFE com-posite coating. Operating parameters and procedure ofelectroless NiPPTFE are very similar to normalelectroless NiP coating. The pretreatment and coatingprocedures are listed in Table 1. The composition andthe plating conditions for the electroless NiPPTFEsolution used in the present investigation are listed inTable 2. The electroless NiPPTFE bath solution hasa good stability and can be used continuously for over4 weeks. A 60% PTFE emulsion from Aldrich withparticle size in the range of 0.050.5 mm and a cationicsurfactant were used. Both the PTFE emulsion and thesurfactant were diluted with demineralised water andstirred for 1 h before use.

2.2. Deposition rate

To determine coating rates under various conditions,NiPPTFE was coated onto copper plates of 0.35 mm

281Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279284

Fig. 1. Effect of cationic surfactant concentration on NiPPTFEcoating rate.

Fig. 2. Effect of temperature on NiPPTFE coating rate.

thickness. Then, the NiPPTFE coating rate g(mmyh) is given by:

hgs (1)

t

where h is the coating thickness and t is the coatingtime. In this investigation the coating thickness isdetermined by analysing a cross-section of the coatedcopper plate by SEM.

2.3. PTFE content in the coating

PTFE particles are distributed throughout the nickelphosphorus matrix. By altering bath composition andoperating conditions, the PTFE content in the compositedeposit can be varied. The PTFE content is usuallydetermined either by titration (determining the contentsof Ni and P) or by filtration (weighting the PTFE cakeleft on a filter). For these two methods, the NiPPTFE coating needs to be dissolved in 50% nitric acid.Obviously these two methods are time-consuming. Inthis investigation, the PTFE content is therefore deter-mined by measuring the coating thickness h. Assumingthat porosity is negligible in the calculation of the totalcoating volume, the following two equations areobtained:

r V qr V sVr (2)NiyP NiyP PTFE PTFEV qV sV (3)NiyP PTFEwhere r , r and r are the densities of electrolessNiP PTFENiP coating, PTFE and NiPPTFE coating, respec-tively. V , V and V are the corresponding volumesNiP PTFEof NiP, PTFE and overall NiPPTFE coating, respec-tively. r varies slightly with the content of phospho-NiPrus in the electroless NiP coating. In general, thephosphorus content is approximately 810 wt.%, andthe corresponding density of the NiP coating is approx-imately 7.9 gycm . The density of PTFE is approxi-3mately 2.2 gycm . By combining Eqs. (2) and (3), the3PTFE content, V yV is then determined by:PTFE

V r yrPTFE NiyPs (4)V r yrNiyP PTFETo determine the coating density r, NiPPTFE was

coated onto copper plates of 0.35 mm thickness. Eachcopper plate (approx. 20=20 mm ) was weighted before2and after coating, using a Sartorius electronic scale with10 g precision. The relation between NiPPTFEy5coating thickness, h (mm) and coating density, r (gycm ) is given by the following equation:3

B E B Eh w yw rCu a b CuC F C Fhs (5)2 w rD G D Gb

where h is the thickness of the copper plate beforeCucoating (350 mm in this investigation); w and w areb athe weights of the copper plate before and after coating,respectively, and r is the copper density (8.9 gycm ).3CuAs both sides of the copper plates are coated, the aboveequation is divided by 2.By combining Eqs. (4) and (5), the PTFE content,

V yV is then determined by:PTFEV 1PTFEsV r yrNiyP PTFE

B Eh w ywCu a bC F r yr (6)NiyP Cu 2h wD GbThe coating thickness is determined by analysing a

cross-section of the coated copper plate by SEM.

3. Results and discussion

3.1. Effect of the cationic surfactant concentration

Fig. 1 shows that the cationic surfactant concentrationhas a significant effect on the NiPPTFE depositionrate. For a given PTFE concentration, the coating ratedecreases sharply with increasing surfactant concentra-tion, and then levels-off at a low value. For a givensurfactant concentration, the coating rate increases withincreasing PTFE concentration in a solution.

282 Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279284

Fig. 3. Effect of PTFE concentration on NiPPTFE coating rate.Fig. 5. Optimum ratio between the concentration of cationic surfactantand the concentration of PTFE particles.

Fig. 4. PTFE content in coating vs. PTFE concentration in solution. Fig. 6. Uniform distribution of PTFE particles in coating.

3.2. Effect of temperature

Fig. 2 shows that, for constant cationic surfactantconcentration and PTFE concentration, the coating rateincreases with increasing temperature of the platingsolution. For both investigated temperatures, the coatingrate decreases sharply with increasing surfactantconcentration.

3.3. Effect of suspended PTFE concentration

Fig. 3 demonstrates that, for a given cationic surfac-tant concentration, the coating rate increases withincreasing PTFE concentration in solution until reachinga maximum value at approximately 15 mlyl.

3.4. PTFE content in the coating

As shown in Fig. 4, the volumetric PTFE content inthe coating increases with increasing PTFE concentra-tion in the solution up to a maximum value which isapproximately 15 mlyl. For higher PTFE concentrations,a significant reduction is observed.It was found that the cationic surfactant is effective

in suspending the PTFE particles in the plating solution.However, for any constant surfactant concentration, thePTFE particles were found to agglomerate and settle-out during the plating process, once the PTFE concen-tration in the solution exceeded a certain value. This

indicates that the given amount of cationic surfactantwas not sufficient to disperse the PTFE particles andhence maintain them in suspension. For example, for acationic surfactant concentration of 0.5 gyl agglomera-tion and settling occurs when the PTFE concentrationin the solution exceeds 24 mlyl. This is the reason whythe PTFE content in coatings decreased so sharply. Ifthe solution is saturated with the cationic surfactant, allPTFE particles will be surrounded by surfactant. How-ever, if too much surfactant is added and the solutionbecomes super-saturated, then the rest of the surfactantwill accumulate near the depositing surface, and hencesuppress the diffusion of PTFE particles and Ni ionstowards the interface. In this case, the deposition rate isagain quite slow (see Figs. 1 and 2) and only a fewPTFE particles are incorporated into the NiP matrix. Itis, therefore, critical to determine the optimum ratiobetween the concentration of cationic surfactant and theconcentration of PTFE particles.In order to find the optimum ratio and the maximum

PTFE content in the coatings, over 70 copper plates

283Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279284

Fig. 7. Cross-section of gradient NiPPTFE coating.

Fig. 8. Comparison of fouling behaviour of a NiPPTFE-coated sur-face with an untreated surface.

have been coated with NiPPTFE using various con-centrations of cationic surfactant and PTFE particles.Fig. 5 shows the optimum ratio between the concentra-tion of cationic surfactant and the concentration of PTFEparticles.The SEM picture displayed in Fig. 6 shows that the

(black) PTFE particles are uniformly distributedthroughout the NiP matrix without using any mechan-ical agitation or ultrasonic homogeniser.

3.5. Improved adhesion of NiPPTFE coating

Since the PTFE particles are chemically inert and donot adhere to the substrate, an increase in PTFE contentin the coating is usually accompanied by a decrease inthe adherence of the coating to the substrate. NiPPTFE is, therefore, usually coated onto a NiP sub-layerto enhance the adhesion of the coating. However, it wasfound that a sudden transition to a PTFE concentration

above, say 30%, again produces an insufficient adhesionof the NiPPTFE layer to the NiP sub-layer, andhence a fragile coating. The results of several heattransfer fouling experiments showed that the NiPPTFE coating was likely to peel-off after being exposedto hot water for several days. To improve NiPPTFEcoating adhesion, a gradient composite coating methodis developed by gradually increasing the PTFE contentfrom the substrate to the top surface. Since there is noobvious interface between coatings, the coating adhesionis improved significantly. These multi-layer coatingscan in practice be achieved by installing several NiPPTFE baths with different ratios of cationic surfactantto PTFE particles. Fig. 7 shows the cross-section ofsuch a gradient coating.

3.6. Surface energy measurements

Surface free energy is one of the most importantphysico-chemical properties of a solid surface, since itis a direct measure of intermolecular or inter-atomicattractive forces. When a surface is immersed in anaqueous solution, molecules or atoms at the surface tendto interact with molecules or atoms in the solution, andthe types of forces or interactions depend on the chem-istry of both solid and liquid. In general, the lower thesurface free energy of a solid, the weaker is the adhesionof deposit on it w14x. Rankin and Adamson w15x reportedthat PTFE-coated surfaces prevented scale formationfrom seawater during 256 h of evaporator operation.Any scale which formed sloughed-off because of lowadhesion.As surface energy or contact angle depend only on

the properties of the top layer of the coating, gradualNiPPTFE coatings will maintain low surface energyproperties due to the higher PTFE content in the toplayer. In this investigation, surface energy was measuredwith a CAHN DCA-315 instrument. It operates on theWilhelmy plate principle, i.e. the accurate measurementof the force ("10 dyne) acting on a sample ofy3constant cross-section, during its immersion in a testliquid at a low and constant speed. The results showthat the surface energy of the NiPPTFE coatings wasin the range of 2734 mNym, which is much lowerthan that of pure NiP coating (75 mNym) and copper(87 mNym).

3.7. Fouling experiments

Fouling tests were carried out in a convective heattransfer and flow boiling test rig. Aqueous CaSO4solution was pumped from the supply tank through twoparallel test sections, each consisting of an electricallyheated cylindrical stainless steel heater rod which ismounted concentrically within the surrounding verticalpipe. More details about the experimental set-up are

284 Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279284

given in w16x. One test heater was coated with thegradual NiPPTFE method, whereas the other wasuntreated as a control. Fig. 8 shows the measured heattransfer coefficients as a function of time during afouling run with a CaSO concentration of 2.0 gyl and4a flow velocity of 80 cmys. For the untreated heater,the heat transfer coefficients reduced gradually towardsa low asymptotic value as a CaSO scale was formed4on the surface. Contrariwise, for the NiPPTFE-coatedheater, the heat transfer coefficients remained almostconstant during the run which means that almost noCaSO scale was formed on the treated surface. Obvi-4ously, this is a most promising result which indicatessubstantial benefits if this technology can be applied inpractice.

4. Conclusions

FC-4 cationic surfactant is effective in suspendingPTFE particles in the plating solution. The rate of thecationic surfactant concentration to PTFE concentrationin the plating solution has a significant effect on theNiPPTFE deposition rate and on the PTFE contentin the coatings. When the optimum ratio between theconcentration of cationic surfactant and the concentra-tion of PTFE particles is used, the PTFE content in thecoating has a maximum and the PTFE particles areuniformly distributed throughout the NiP matrix. Theadhesion of electroless NiPPTFE is significantlyimproved by gradually increasing the PTFE contentfrom the substrate to the top surface. Graded electroless

NiPPTFE coatings have a tremendous potential forreducing heat exchanger fouling.

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J. Chem. Engn. 78 (2000) 12.

Graded Ni-nPP-nPPTFE coatings and their potential applicationsIntroductionExperimental procedureOperating parametersDeposition ratePTFE content in the coating

Results and discussionEffect of the cationic surfactant concentrationEffect of temperatureEffect of suspended PTFE concentrationPTFE content in the coatingImproved adhesion of Ni-nPP-nPPTFE coatingSurface energy measurementsFouling experiments

ConclusionsReferences