Graded Ni–P–PTFE coatings and their potential applications

Surface and Coatings Technology 155 (2002) 279–284

Graded Ni–P–PTFE coatings and their potential applications b ¨ Q. Zhaoa,*, Y. Liua, H. Muller-Steinhagen , G. Liuc a Department of Mechanical Engineering, University of Dundee, Dundee DD1 4HN, UK Institute for Thermodynamics and Thermal Engineering, University of Stuttgart, Pfaffenwaldring 6, 70550 Stuttgart, Germany c School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China

b

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 electroless Ni–P–PTFE deposition rate and on the PTFE content in the coatings has been investigated. The adhesion of the Ni–P–PTFE layer is significantly improved by gradually increasing the PTFE content from the substrate to the top surface. It has been demonstrated that these graded electroless Ni–P–PTFE coatings can reduce the formation of deposits on heat exchanger surfaces significantly. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electroless deposition; Ni–P–PTFE; Graded coating; Adhesion

1. Introduction The first composite coatings of electroless (or autocatalytic) nickel–phosphorus and polytetrafluoroethylene (PTFE) were introduced approximately 18 years ago w1,2x. Electroless nickel–phosphorous is widely used in the chemical, mechanical and electronic industries because of its corrosion and wear resistance and its inherently uniform coating thickness. The incorporation of PTFE particles into the Ni–P matrix can take advantage of the different properties of Ni–P alloy and PTFE w3x. PTFE is chemically very inert and has a relatively high melting point (325 8C). Its coefficient of friction is lower than that of almost any other polymers. Because of its extremely low surface energy (18.6 mNy m), PTFE has excellent non-stick properties w4x. The resulting properties of electroless Ni–P–PTFE coatings, such as non-stick, non-galling, anti-adhesive, higher dry lubricity, lower friction, good wear and good corrosion resistance, have been used successfully in many industries w3,5x. A concise review of the field of composite electrodeposition and highlights of the importance of process control in obtaining critical deposit characteristics for a variety of demanding industrial applications *Corresponding author. Tel.: q44-1382-345651; fax: q44-1382345508. E-mail address: [email protected] (Q. Zhao).

were given by Kerr and Barker et al. w6x and Helle and Walsh w7x. Celis et al. gave a detailed survey on the understanding of the mechanism of electrolytic codeposition and presented some trends and expected future developments of composite plating w8x. PTFE particles readily coagulate and precipitate in the plating solution since PTFE is a water-repellent material. Due to this agglomerate formation it is difficult to obtain a uniform dispersion of PTFE particles in a plating bath. This will not only reduce the PTFE content in the coatings, but also increase the surface roughness as larger PTFE particles are incorporated w9,10x. To achieve optimum properties of electroless Ni–P–PTFE coatings, the PTFE particles must be uniformly distributed throughout the Ni–P matrix. Areas where less PTFE is incorporated do not have the same physical and mechanical properties as areas where the PTFE content is uniform and higher. This influences the mechanical and tribological (friction, lubrication, bearing and hydrodynamic) properties of the coating, and is a main problem in many electroless Ni–P–PTFE coatings w5x. Matsuda et al. w9,10x investigated the effect of the presence of a variety of surfactants on the suspension of PTFE particles in electroless nickel plating solutions. They found that the suspension of the PTFE particles was improved by the addition of non-ionic surfactants. Nishira et al. w11x studied the effects of agitation

0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 1 1 6 - 0

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Table 1 The pretreatment and coating procedures for electroless Ni–P–PTFE Procedures

Conditions

Alkaline cleaning Rinsing Cathodic electrocleaning Rinsing Pickling Activation Electroless plating Ni–P Rinsing Electroless plating graded Ni–P–PTFE Rinsing

60–80 8C, 5–10 min Room temperature Room temperature, 2–3 min Room temperature Room temperature, 0.5–1 min Room temperature, 1–3 min 85–90 8C, pH: 4.8–5.0 Room temperature 85–90 8C, pH: 4.8–5.0 Room temperature

methods on the particle size distribution of PTFE aggregates in a plating solution. One of their findings was that an ultrasonic homogeniser was more effective than mechanical agitation. An important potential application of electroless Ni– P–PTFE coatings is to reduce fouling, which is generally defined as the accumulation of unwanted material on the surfaces of processing equipment w12x. A typical example is the formation of limescale on heat exchanger surfaces or on household heating elements. Fouling has been recognised as a nearly universal problem in design and operation of processing equipment. Any method for preventing fouling or lengthening processing time through minimising fouling will give substantial cost savings. It has long been known that poorest scale adhesion occurs on materials with low surface energies. Many attempts have been made to achieve this by coating surfaces with PTFE or other polymer layers w13x. However, the poor thermal conductivity, poor abrasion resistance and poor adhesion to metal substrate of these low surface energy coatings currently inhibit their commercial use w14x. Because electroless Ni–P– PTFE coatings are metal-based, their thermal conductivity, anti-abrasive property and mechanical strength are superior to standard PTFE coatings. However, the adhesion of standard Ni–P–PTFE coatings to the substrate needs to be improved, since the coatings were generally found 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 Ni– P–PTFE coating rate and PTFE content in the coatings. Furthermore, a method of improving the adhesion of the Ni–P–PTFE coating is described.

2. Experimental procedure 2.1. Operating parameters Electroless Ni–P–PTFE composite coating is also called a dispersion coating. In the bath, the reducing agent NaH2PO2 reduces the metallic ion Ni2q on the surface of a catalytic solid (substrate), so that the Ni–P alloy coating is formed. The PTFE particles which are suspended in the bath diffuse to the surface of the substrate under the action of stirring and Coulomb’s force. They are then absorbed physically and chemically on the surface. The PTFE particles are buried in the growing Ni–P coating to form the Ni–P–PTFE composite coating. Operating parameters and procedure of electroless Ni–P–PTFE are very similar to normal electroless Ni–P coating. The pretreatment and coating procedures are listed in Table 1. The composition and the plating conditions for the electroless Ni–P–PTFE solution used in the present investigation are listed in Table 2. The electroless Ni–P–PTFE bath solution has a good stability and can be used continuously for over 4 weeks. A 60% PTFE emulsion from Aldrich with particle size in the range of 0.05–0.5 mm and a cationic surfactant were used. Both the PTFE emulsion and the surfactant were diluted with demineralised water and stirred for 1 h before use. 2.2. Deposition rate To determine coating rates under various conditions, Ni–P–PTFE was coated onto copper plates of 0.35 mm

Table 2 Bath composition and operating conditions for electroless Ni–P–PTFE NiSO4Ø6H2O: 25 gyl H3C6H5O7Ø6H2O: 18 gyl PTFE(60 wt.%): 4–50 mlyl C20H20F23N2O4I (FC-4): 0–1.0 gyl

NaH2PO2: 30 gyl NaCH3COO: 18 gyl (NH2)2CS: 1 ppm pH: 4.8; Temperature: 88–93 8C

Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279–284

VPTFE rNiyPyr s V rNiyPyrPTFE

281

(4)

To determine the coating density r, Ni–P–PTFE was coated onto copper plates of 0.35 mm thickness. Each copper plate (approx. 20=20 mm2) was weighted before and after coating, using a Sartorius electronic scale with 10y5 g precision. The relation between Ni–P–PTFE coating thickness, h (mm) and coating density, r (gy cm3) is given by the following equation: Fig. 1. Effect of cationic surfactant concentration on Ni–P–PTFE coating rate.

thickness. Then, the Ni–P–PTFE coating rate g (mmyh) is given by: gs

h t

(1)

where h is the coating thickness and t is the coating time. In this investigation the coating thickness is determined by analysing a cross-section of the coated copper plate by SEM.

hs

hCu B waywb E B rCu E FØC F ØC 2 D wb G D r G

where hCu is the thickness of the copper plate before coating (350 mm in this investigation); wb and wa are the weights of the copper plate before and after coating, respectively, and rCu is the copper density (8.9 gycm3). As both sides of the copper plates are coated, the above equation is divided by 2. By combining Eqs. (4) and (5), the PTFE content, VPTFE yV is then determined by: VPTFE 1 s V rNiyPyrPTFE B

ØCrNiyPyrCuØ

2.3. PTFE content in the coating

D

PTFE particles are distributed throughout the nickel– phosphorus matrix. By altering bath composition and operating conditions, the PTFE content in the composite deposit can be varied. The PTFE content is usually determined either by titration (determining the contents of Ni and P) or by filtration (weighting the PTFE cake left on a filter). For these two methods, the Ni–P– PTFE coating needs to be dissolved in 50% nitric acid. Obviously these two methods are time-consuming. In this investigation, the PTFE content is therefore determined by measuring the coating thickness h. Assuming that porosity is negligible in the calculation of the total coating volume, the following two equations are obtained: rNiyPØVNiyPqrPTFEØVPTFEsVØr

(2)

VNiyPqVPTFEsV

(3)

where rNi–P, rPTFE and r are the densities of electroless Ni–P coating, PTFE and Ni–P–PTFE coating, respectively. VNi–P, VPTFE and V are the corresponding volumes of Ni–P, PTFE and overall Ni–P–PTFE coating, respectively. rNi–P varies slightly with the content of phosphorus in the electroless Ni–P coating. In general, the phosphorus content is approximately 8–10 wt.%, and the corresponding density of the Ni–P coating is approximately 7.9 gycm3. The density of PTFE is approximately 2.2 gycm3. By combining Eqs. (2) and (3), the PTFE content, VPTFE yV is then determined by:

(5)

hCu waywb E F Ø 2h wb G

(6)

The 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 concentration has a significant effect on the Ni–P–PTFE deposition rate. For a given PTFE concentration, the coating rate decreases sharply with increasing surfactant concentration, and then levels-off at a low value. For a given surfactant concentration, the coating rate increases with increasing PTFE concentration in a solution.

Fig. 2. Effect of temperature on Ni–P–PTFE coating rate.

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Fig. 3. Effect of PTFE concentration on Ni–P–PTFE coating rate.

3.2. Effect of temperature Fig. 2 shows that, for constant cationic surfactant concentration and PTFE concentration, the coating rate increases with increasing temperature of the plating solution. For both investigated temperatures, the coating rate decreases sharply with increasing surfactant concentration. 3.3. Effect of suspended PTFE concentration Fig. 3 demonstrates that, for a given cationic surfactant concentration, the coating rate increases with increasing PTFE concentration in solution until reaching a maximum value at approximately 15 mlyl. 3.4. PTFE content in the coating As shown in Fig. 4, the volumetric PTFE content in the coating increases with increasing PTFE concentration in the solution up to a maximum value which is approximately 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, the PTFE particles were found to agglomerate and settleout during the plating process, once the PTFE concentration in the solution exceeded a certain value. This

Fig. 4. PTFE content in coating vs. PTFE concentration in solution.

Fig. 5. Optimum ratio between the concentration of cationic surfactant and the concentration of PTFE particles.

indicates that the given amount of cationic surfactant was not sufficient to disperse the PTFE particles and hence maintain them in suspension. For example, for a cationic surfactant concentration of 0.5 gyl agglomeration and settling occurs when the PTFE concentration in the solution exceeds 24 mlyl. This is the reason why the PTFE content in coatings decreased so sharply. If the solution is saturated with the cationic surfactant, all PTFE particles will be surrounded by surfactant. However, if too much surfactant is added and the solution becomes super-saturated, then the rest of the surfactant will accumulate near the depositing surface, and hence suppress the diffusion of PTFE particles and Ni ions towards the interface. In this case, the deposition rate is again quite slow (see Figs. 1 and 2) and only a few PTFE particles are incorporated into the Ni–P matrix. It is, therefore, critical to determine the optimum ratio between the concentration of cationic surfactant and the concentration of PTFE particles. In order to find the optimum ratio and the maximum PTFE content in the coatings, over 70 copper plates

Fig. 6. Uniform distribution of PTFE particles in coating.

Q. Zhao et al. / Surface and Coatings Technology 155 (2002) 279–284

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above, say 30%, again produces an insufficient adhesion of the Ni–P–PTFE layer to the Ni–P sub-layer, and hence a fragile coating. The results of several heat transfer fouling experiments showed that the Ni–P– PTFE coating was likely to peel-off after being exposed to hot water for several days. To improve Ni–P–PTFE coating adhesion, a gradient composite coating method is developed by gradually increasing the PTFE content from the substrate to the top surface. Since there is no obvious interface between coatings, the coating adhesion is improved significantly. These ‘multi-layer’ coatings can in practice be achieved by installing several Ni–P– PTFE baths with different ratios of cationic surfactant to PTFE particles. Fig. 7 shows the cross-section of such a gradient coating. 3.6. Surface energy measurements

Fig. 7. Cross-section of gradient Ni–P–PTFE coating.

have been coated with Ni–P–PTFE using various concentrations of cationic surfactant and PTFE particles. Fig. 5 shows the optimum ratio between the concentration of cationic surfactant and the concentration of PTFE particles. The SEM picture displayed in Fig. 6 shows that the (black) PTFE particles are uniformly distributed throughout the Ni–P matrix without using any mechanical agitation or ultrasonic homogeniser. 3.5. Improved adhesion of Ni–P–PTFE coating Since the PTFE particles are chemically inert and do not adhere to the substrate, an increase in PTFE content in the coating is usually accompanied by a decrease in the adherence of the coating to the substrate. Ni–P– PTFE is, therefore, usually coated onto a Ni–P sub-layer to enhance the adhesion of the coating. However, it was found that a sudden transition to a PTFE concentration

Surface free energy is one of the most important physico-chemical properties of a solid surface, since it is a direct measure of intermolecular or inter-atomic attractive forces. When a surface is immersed in an aqueous solution, molecules or atoms at the surface tend to interact with molecules or atoms in the solution, and the types of forces or interactions depend on the chemistry of both solid and liquid. In general, the lower the surface free energy of a solid, the weaker is the adhesion of deposit on it w14x. Rankin and Adamson w15x reported that PTFE-coated surfaces prevented scale formation from seawater during 256 h of evaporator operation. Any scale which formed sloughed-off because of low adhesion. As surface energy or contact angle depend only on the properties of the top layer of the coating, gradual Ni–P–PTFE coatings will maintain low surface energy properties due to the higher PTFE content in the top layer. In this investigation, surface energy was measured with a CAHN DCA-315 instrument. It operates on the Wilhelmy plate principle, i.e. the accurate measurement of the force ("10y3 dyne) acting on a sample of constant cross-section, during its immersion in a test liquid at a low and constant speed. The results show that the surface energy of the Ni–P–PTFE coatings was in the range of 27–34 mNym, which is much lower than that of pure Ni–P coating (75 mNym) and copper (87 mNym). 3.7. Fouling experiments

Fig. 8. Comparison of fouling behaviour of a Ni–P–PTFE-coated surface with an untreated surface.

Fouling tests were carried out in a convective heat transfer and flow boiling test rig. Aqueous CaSO4 solution was pumped from the supply tank through two parallel test sections, each consisting of an electrically heated cylindrical stainless steel heater rod which is mounted concentrically within the surrounding vertical pipe. More details about the experimental set-up are

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given in w16x. One test heater was coated with the gradual Ni–P–PTFE method, whereas the other was untreated as a control. Fig. 8 shows the measured heat transfer coefficients as a function of time during a fouling run with a CaSO4 concentration of 2.0 gyl and a flow velocity of 80 cmys. For the untreated heater, the heat transfer coefficients reduced gradually towards a low asymptotic value as a CaSO4 scale was formed on the surface. Contrariwise, for the Ni–P–PTFE-coated heater, the heat transfer coefficients remained almost constant during the run which means that almost no CaSO4 scale was formed on the treated surface. Obviously, this is a most promising result which indicates substantial benefits if this technology can be applied in practice. 4. Conclusions FC-4 cationic surfactant is effective in suspending PTFE particles in the plating solution. The rate of the cationic surfactant concentration to PTFE concentration in the plating solution has a significant effect on the Ni–P–PTFE deposition rate and on the PTFE content in the coatings. When the optimum ratio between the concentration of cationic surfactant and the concentration of PTFE particles is used, the PTFE content in the coating has a maximum and the PTFE particles are uniformly distributed throughout the Ni–P matrix. The adhesion of electroless Ni–P–PTFE is significantly improved by gradually increasing the PTFE content from the substrate to the top surface. Graded electroless

Ni–P–PTFE coatings have a tremendous potential for reducing heat exchanger fouling. References w1x S.S. Tulsi, P. Ebdon, Electroless Nickel-PTFE Composite Coatings, UK National Corrosion Conference, London, 1982. w2x S.S. Tulsi, The Institute of Metal Finishing, Proceedings of Annual Technical Conference, Bournmouth, UK, 10–14 May, 1983. w3x S.S. Tulsi, Finishing 7 (11) (1983) 14. w4x J.S. Hadley, L.E. Harland, Metal Finishing 85 (12) (1987) 51. w5x K.-H. Pietsch, Prod. Finishing (Cincinnatti) 63 (5) (1999) 34. w6x C. Kerr, D. Barker, F. Walsh, J. Archer, Trans. Inst. Metal Finishing 78 (5) (2000) 171, Sep. w7x K. Helle, F. Walsh, Trans. Inst. Metal Finishing 75 (2) (1997) 53, Mar. w8x J.P. Celis, J.R. Roos, C. Buelens, J. Fransaer, Trans. Inst. Metal Finishing 69 (4) (1991) 133–139. w9x H. Matsuda, M. Nishira, Y. Kiyono, O. Takano, Trans. Inst. Metal Finishing 73 (1) (1995) 16, Feb. w10x H. Matsuda, Y. Kiyono, M. Nishira, O. Takano, Trans. Inst. Metal Finishing 72 (2) (1994) 55, May. w11x M. Nishira, K. Yamagishi, H. Matsuda, M. Suzuki, O. Takano, Trans. Inst. Metal Finishing 74 (2) (1996) 62, Mar. w12x Q. Zhao, H. Muller-Steinhagen, ¨ 1999 United Engineering Foundation Conference: Mitigation of Heat Exchanger Fouling and Its Economic and Environmental Implications, Banff, Canada, 11th–16th July, 1999. w13x Y.G. Mussalli, J. Tsou, Proc. of 51st American Power Conference 51 (1989) 1094. w14x H. Muller-Steinhagen, ¨ Q. Zhao, Chem. Eng. Sci. 52 (19) (1997) 3321. w15x B.H. Rankin, W.L. Adamson, Desalination 13 (1973) 63. w16x H. Muller-Steinhagen, ¨ Q. Zhao, A. Helali-Zadeh, X. Ren, Can. J. Chem. Engn. 78 (2000) 12.