Surface and Coatings Technology 167 (2003) 207211

0257-8972/03/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(02)00899-X

Development of electroless NiPPTFESiC composite coatingY.S. Huang, X.T. Zeng*, I. Annergren, F.M. Liu

Singapore Institute of Manufacturing Technology, Singapore 638075, Singapore

Abstract

Electroless nickel (EN) and EN composite coatings with polytetrafluoroethylene (PTFE) andyor SiC were deposited bychemical deposition. The microstructure analysis was conducted with scanning electron microscopy and X-ray diffraction.Differential scanning calorimetry was used to study the phase transition of the coating during the heat treatment. The mechanicaland tribological properties were measured using hardness indentation, scratch test and pin-on-disc wear test. The surface energywas analysed using water droplet surface contact angle measurement. The synergistic effects of SiC and PTFE on the wear andanti-sticking properties of the coatings are discussed. 2002 Elsevier Science B.V. All rights reserved.

Keywords: EN-composite; Wear; Hardness; PTFE; SiC; Anti-sticking

1. Introduction

Electroless nickel (EN) coatings are widely used inthe chemical, mechanical and electronic industriesbecause of their wear and corrosion resistance properties.The EN deposition process is based on a redox reactionin which the reducing agent is oxidized and Ni ions2qare reduced on the substrate surface. An advance inelectroless NiP deposition is the co-deposition of solidparticles into coatings to further improve certain prop-erties w1x. Electroless NiPPTFE films present excel-lent self-lubricating and anti-sticking properties w24x,however, the coating hardness is correspondinglydecreased depending on the volume fraction of polytet-rafluoroethylene (PTFE) particles in the coating.On the other hand, electroless NiPSiC coatings are

recognized for their hardness and wear resistance andare considered as a replacement for hard chromium inthe aerospace industry w5,6x. One of the issues with theco-deposited NiPSiC coatings is the higher frictioncoefficient compared with electroless NiP coatings.Nowadays, complex composite coatings containing

both hard and lubricating particles are receiving moreand more interest. Losiewicz et al. w7x studied the phasecomposition and surface morphology of an electrolytic

*Corresponding author. Tel.: q65-67938521; fax: q65-67922779.E-mail address: xzeng@simtech.a-star.edu.sg (X.T. Zeng).

NiPTiO PTFE composites for an electrochemical2reaction electrode; Deng et al. w8x studied electrodepos-ited ReNiWSiCPTFE composite and their prop-erties; Straffelini et al. w9x studied the dry slidingbehavior of a double NiPySiCPTFE co-deposit com-posite; Wang et al. w10x reported the tribological behav-ior of compression molding polyetheretherketonecomposite filled with nanometer sized SiC and PTFEpaticles. Up to now, few studies were reported onelectroless plating of complex composite coatings.This work aimed to develop electroless plating of

ENPTFESiC composite coatings to achieve highhardness, low friction coefficient and low surface energy.A comparison study of the properties of EN, ENSiC,ENPTFE and ENSiCPTFE composite coatings wasconducted. The effects of SiC and PTFE particles onthe coating properties will be discussed.

2. Experiments

EN and EN composite coatings containing PTFE andyor SiC particles were deposited on 25=6 mild steelsubstrate. ENLUBE dispersion of PTFE was used as thePTFE additive. Micro-sized (average size 4.5 mm) b-silicon carbide particles were used for SiC co-deposition.Surfactants were used for particle dispersion and surfacecharge adjustment. Magnetic stirring was used to keepparticles from sediment. The composition of the solutionand the plating parameters for electroless ENPTFESiC co-deposition are listed in Table 1.

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Table 1Composition of EN and EN-composite bath and plating conditions

EN ENPTFE ENSiC ENPTFESiC

NiSO (g l )y14 2630 2630 2630 2630NaH PO H O (g l )y12 2 2 2830 2830 2830 2830CH COONa3H O (g l )y13 2 3545 3545 3545 3545Latic acid (85%) (ml l )y1 2025 2025 2025 2025Thiourea (mg l )y1 35 35 35 35PTFE dispersion (ml l )y1 68 68SiC powder (g l )y1 810 810Surfactants (mg l )y1 100200 100200pH 4.8 4.8 4.8 4.8Plating temperature 8890 8C 8890 8C 8890 8C 8890 8C

The morphology and microstructure of the coatingswere analysed using scanning electron microscopy(SEM) and X-ray diffraction (XRD) using Cu Ka X-rays. Post-deposition annealing was conducted, and dif-ferential scanning calorimetry (DSC) was used to studythe phase transition of the coating during the heattreatment. In this experiment, the DSC samples wereprepared by stripping off the coatings from aluminumsubstrates using 65% NaOH solution. The surface energyof the coating was analysed using water droplet surfacecontact angle measurement.The coating hardness was measured using microhard-

ness indentation. The adhesion strength was measuredusing a Teer 2200 scratch tester at a constant speed of10 mm min under a normal load, which linearlyy1increased from 10 to 128 N at a constant rate of 100N min . Optical microscopy was used to confirm they1starting point of the adhesive failure along the scratchtrack. To evaluate the tribological properties of thecoatings, wear tests were carried out at an air humidityof 45"10 RH% and a temperature of 24"1 8C using apin-on-disc tribometer with the sample placed horizon-tally on a turntable. The tests were performed usingalumina pins with a radius of 4.5 mm and at a linearspeed of 20 cm s for a total sliding distance of 4000y1m. A normal load of 10 N was used during the weartests. The wear loss and normalized wear rate werecalculated from the wear scar cross-sectional area, asmeasured by a Talysurf profilometer, and the wear trackdiameter.

3. Results and discussion

Fig. 1 shows the SEM pictures of EN and ENcomposite coatings. All the samples studied had athickness of approximately 15 mm. ENPTFE and ENSiC were deposited with PTFE or SiC particles uniform-ly distributed in coatings. However, when SiC and PTFEparticles were deposited together, some interferenceeffects occurred, resulting in the particles being non-uniformly distributed in the coatings. The SiC particleswere much heavier than PTFE, which required strongagitation to keep them from flocculation. This affected

the co-deposition and distribution of the PTFE particlesin the coating.Fig. 2 shows a typical DSC thermogram of EN

PTFESiC composite coating. The heating temperaturewas controlled at a rate of 10 8C min . There werey1two exothermic peaks observed in the DSC curve. Thepeak at approximately 340 8C was associated with thephase transition from the amorphous NiP matrix to amixed structure of polycrystalline Ni and Ni P alloy.3This was confirmed by XRD analysis. The small peakat 280 8C may be due to the stress relaxation of theparticlesmatrix interfaces. No phase transition wasobserved by XRD for the samples heat-treated at 3008C.Fig. 3a and b show XRD patterns of EN and EN

composite coatings before and after annealing. It can beseen that the as-plated coatings had an amorphous NiPstructure. The overlapped sharp peaks at approximately2us458 were from the high speed steel substrate. Afterannealing at 400 8C, the crystallized Ni P alloy phase3was observed. The relative intensity of Ni P to NiP3peaks was much weaker in the SiC or PTFE co-deposited samples, with the ENPTFESiC the weak-est, compared with pure EN samples, indicating that theembedded particles hindered the Ni P grain growth. The3DSC results confirmed that both SiC and PTFE particleswere stable in EN composite coating during the anneal-ing at 400 8C. These particles played a very importantrole in the wear performance of the composite coating.The hardness, friction coefficient and the normalized

wear rates of the EN and EN composite coatings arelisted in Table 2. The hardness of all four type coatingswas significantly increased after heat treatment, due tothe formation of Ni P alloy phase (Fig. 3). The intro-3duction of PTFE or SiC particles into the EN resultedin a substantial decrease or increase in hardness. Ahardness of 1365 HV was measured for the heat-treatedENSiC, which is approximately 2.5 times higher thanthat of the EN without SiC particles. The ENPTFESiC composite coating showed a moderate hardnessbetween ENPTFE and ENSiC, depending on thecorresponding concentration of the PTFE and SiC par-ticles in the coatings.

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Fig. 2. DSC thermogram of the ENPTFESiC coating obtained at aheating rate of 10 8C min .y1

Fig. 1. SEM images of (a) EN, (b) ENPTFE, (c) ENSiC and (d) ENPTFESiC composite coatings.

The adhesion of the coatings to substrates was eval-uated by the critical load in scratch test. The results arealso shown in Table 2. Apparently, co-deposition of bothPTFE and SiC particles into the EN coating reduced theadhesion. This may be explained by Guglielmis twostep adsorption theory w11x. According to this theory,the dispersed particles were first physically adsorbed onthe substrate, producing a layer of loosely adsorbedparticles with a relatively high surface coverage, thensome relative strongly adsorbed (both physical and staticelectrical) particles were built in to the depositing metalmatrix. Almost no other adhesive force may have existedbetween the co-deposited particles and the substrate.Thus the adhesion strengths of the EN composite coat-ings were negatively affected.To estimate the surface energy of the EN composite

coatings, the contact angle of de-ionised water droplet

210 Y.S. Huang et al. / Surface and Coatings Technology 167 (2003) 207211

Fig. 3. XRD patterns of (a) as-plated and (b) heat-treated at 400 8Cfor 1 h of EN and EN composite coatings.

Fig. 4. Friction coefficient vs. sliding distance for EN and EN com-posite coatings.

Table 2Main properties of EN and EN-composites

Microhardness (HV50) Friction Wear rate Critical Contact

As-plated Heat-treatedcoefficient (10# mm N# m# )6 3 1 1 load (N) angle(average)

EN 453 956 2.52 70 95.0ENPTFE 340 421 0.48 0.36 62 120.2ENSiC 530 1365 0.59 0.45 60 79.2ENPTFESiC 450 701 0.52 0.36 60 101.7

to the coating surface was measured using a goniometer.The results are also shown in Table 2. The inclusion ofPTFE particles effectively increased the surface contactangle, indicating a decrease in surface energy. The ENPTFE coating showed the highest surface contact angle(upto 1208), representing the lowest surface energyamong the four tested coatings. This is desired in manyapplications for anti-sticking performance.Fig. 4 shows the friction coefficients of the four tested

coatings as a function of sliding distance. The unevencurve with the EN coating was caused by the coatingspalling along the wear track. The three ENcompositecoatings showed significantly improved wear resistance.

Among them, the ENPTFE coating showed the lowestfriction coefficient (;0.48) and, correspondingly thelowest wear rate despite the lowest hardness. This couldbe explained by the formation of a PTFE transfer filmbetween coating and pin ball during the wear sliding,which reduced the friction coefficient and wear w12x.The ENSiC coating demonstrated better load-supportdue to the higher hardness. However, the highest frictioncoefficient (;0.59) resulted in a relatively higher wearrate. The ENFTFESiC coating demonstrated a com-bination of the advantages of the ENSiC in high load-bearing capacity and ENPTFE in low surface energyand high wear resistance.

4. Conclusions

EN and EN composite coatings with PTFE andyorSiC particles were deposited by chemical deposition. Acomparison of the properties of EN, ENSiC, ENPTFE and ENSiCPTFE composite coatings was con-ducted. The results showed that:

(1) The as-deposited coatings had an amorphous NiPstructure incorporated with uniformly distributed PTFEor SiC particles. Phase transition from the amorphousNiP to a mixed structure of crystalline Ni and Ni P3alloy was observed at 340 8C.

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(2) Annealing at 400 8C resulted in a substantialincrease in coating hardness. The ENSiC coatingshowed the highest hardness (;1365 HV) while theENPTFE showed the lowest friction coefficient(;0.48). The ENPTFESiC showed a moderate hard-ness and friction coefficient in between, depending onthe percentage of the particles included in the coating.

(3) The ENPTFESiC composite coating demon-strated a combination of the advantages of the ENSiCin high hardness and load-bearing capacity and of theENPTFE coating in low friction coefficient, low sur-face energy and high wear resistance. Under the exper-imental conditions, a sliding wear rate against analumina pin of approximately 3.6=10y7mm N m was measured for ENPTFE and EN3 y1 y1PTFESiC, which was about a seventh of that recordedfor the EN coating.The significantly improved wear resistance plus low

surface energy of the ENPTFESiC coatings will findapplications for moulds, automobile parts and generalwear components.

References

w1x I. Apachitei, J. Duszczyk, L. Katgerman, P.J.B. Overkamp,Scripta Mater. 38 (1998) 13831389.

w2x Lai-Gui Yu, Xu-Shou Zhang, Thin Solid Films 245 (1994)98103.

w3x P.R. Ebdon, Plat. Surf. Finish. 9 (1988) 6568.w4x Z. Serhal, J. Morvan, P. Berc, M. Rezrazi, J. Pagetti, Surf.

Coat. Technol. 145 (2001) 233241.w5x I. Apachitei, J. Duszczyk, L. Katgerman, P.J.B. Overkamp,

Scripta Mater. 38 (1998) 13471353.w6x A. Grosjean, M. Rezrazi, J. Takadoum, P. Bercot, Surf. Coat.

Technol. 137 (2001) 9296.w7x B. Losiewicz, A. Stepien, D. Gierlotka, A. Budnik, Thin Solid

Films 349 (1999) 4350.w8x Lun-Hao Deng, Zhong-Cheng Guo, Xian-Wan Yang, Electro-

plat. Finish. 18 (1999) 911.w9x G. Straffelini, D. Colombo, A. Molinari, Wear 236 (1999)

179188.w10x Qi-Hua Wang, Qun-Ji Xue, Wei-Min Liu, Jian-Min Chen, Wear

243 (2000) 140146.w11x N. Guglielmi, J. Electrochem. Soc. 119 (1972) 10091015.w12x Lai-Gui Yu, Xu-Shou Zhang, Thin Solid Films 245 (1994)

98103.

Development of electroless NiP-PTFE-SiC composite coatingIntroductionExperimentsResults and discussionConclusionsReferences