electrolytic and electroless coatings of ni.ptfe composites. study of some characteristics

Surface and Coatings Technology 107 (1998) 8593

Electrolytic and electroless coatings of NiPTFE compositesStudy of some characteristics

E. Pena-Munoz a, P. Bercot b,*, A. Grosjean b, M. Rezrazi b, J. Pagetti ba Universidad de Monterry, Morones Prieto 4500 Pte., Garza Garcia, N.L., Mexico

b Laboratoire de Corrosion et Traitements de Surface, Universite de Franche-Comte, 32 rue Megevand, 25000 Besancon, France

Received 14 April 1998; accepted 12 June 1998

Abstract

This work consists of the realization and characterization of composite coatings of NiPTFE carried out by electroless depositionon the one hand and electrolytic deposition on the other hand, obtained by pulsed current (CPS) and d.c. current (DC). Theevolution of some properties of these coatings generated by various procedures, properties such as morphology, hardness, ductilityand wear resistance, are highlighted. 1998 Elsevier Science S.A. All rights reserved.

Keywords: D.c. current; Electrolytic and electroless coatings of composites NiPTFE; Physical characteristics; Pulsed current

1. Introduction of comparable nature which move from the centre ofthe solution towards the impoverished area near theelectrode and which, in this way, feed the reaction.The deposition of particles finely dispersed in a metal

matrix by the process of electrocodeposition led to a However, the speed with which the ions can diVuse inthe electrolyte is limited, which restricts the acceptablenew generation of composites, which present particular

chemical and physical properties. These properties intensity of current in DC.The technique of electrodeposition, called pulseddepend not only on the concentration, size, distribution

inside deposited material, nature and morphology of the currents, consists in the use of discontinuous currents,the most used according to the literature have the shapeparticles, but also on the type of solution used and

physicochemical parameters (pH, temperature, density of rectangular pulses, as shown in Fig. 1.The use of pulsed currents enables very high currentof current, etc.).

One recalls that a composite is a polyphase solid in densities by the application of impulses of currentfollowing a rest period. The duration of the impulseswhich two or several components are associated in order

to obtain, on a macroscopic scale and at least in certain must be limited in order not to drastically impoverishthe cathode/solution interface in metal cations, whichdirections, an original whole of properties which the

components taken separately do not make it possible to makes it possible to avoid the problems involved indiVusion. The rest period must be long enough to allowreach [1].

Among the shapes of current most employed for the a suYcient restocking of this zone.The amplitude of current thus reached involves animplementation of the electrolytic coatings, we used d.c.

current and pulsed currents. One of the most important important modification of the deposit microstructureand thus properties. A plot of transitory curves V=f(t)diVerences between these shapes of current is the maxi-

mum value which the density of current can reach. makes it possible to follow the evolution of the variouselectrochemical processes which are established on theIndeed, the reaction of deposition consumes metal ions

and therefore tends to impoverish the solution in the surface of the work electrode [2].From these curves, it is possible to define the lowerimmediate vicinity of the cathode. This impoverishment

limits and higher Faradaic ranges to locate the Faradaicis naturally compensated by the diVusion of metal ionsreaction where only the transfer of loads correspondingto the deposit occurs. Fig. 2 shows the Faradaic range

* Corresponding author. obtained from the nickel sulphamate bath. This makes

0257-8972/98/$ see front matter 1998 Elsevier Science S.A. All rights reserved.PII S0257-8972 ( 98 ) 00547-7

86 E. Pena-Munoz et al. / Surface and Coatings Technology 107 (1998) 8593

Fig. 1. Form of the pulsed currents.

it possible to fix the parameters related to the shapes The PTFE particles are added in the form of emulsion.of current. They have an average size lower than 0.5 mm. Dispersion

Concerning electroless coatings, they are well mas- is in pseudo-equilibrium, stabilized using a non-ionictered and used: commercial processes exist, e.g. dampening agent. In order to make a comparative study,NiflorA [3]. Many articles cover the performance of this pure nickel deposits are also carried out under the samekind of deposition [46 ]. operating conditions of work.

Concerning electrolytic coatings, they are studied but The experimental device is constituted of the followingstill remain today a curiosity of the laboratory. Helle elements (Fig. 3):and Opschoor [7] were pioneers in showing that incorpo- a cell containing the electrolysis bath with a mag-ration of PTFE particles by electroplating deposition netic stirrer;depends upon two vital factors: the mode of agitation electrodes immersed in the bath: an anode made upto keep the particles buoyant and the use of surfactants of nickel rounds and a 12 cm2 plate of copperto keep the particles from agglomeration. placed as a cathode (substrate);

The aim of this study is a better knowledge of the the electric device made up of conductors supplyinginfluence of PTFE on some characteristics of coatings the electrodes connected to a generator of current:(morphology, hardness, ductility, coeYcient of friction the whole device is controlled by a computer;and distribution of thickness). Coatings are obtained in an oscilloscope to visualize the transitory curvesan electroless way and an electrolytic way. In this latter V=f(t) corresponding to the current impulses.case, the form of current, namely DC and CPS, is

For a better comparison of the results obtained withallowed to vary.DC and CPS, we allow the parameters Jc and Tc to

2. Experimental conditions

2.1. Electrolytic coating

A bath containing some nickel sulphamate [8], with-out either brightener or antipitting agent, is used in thepH range from 4 to 5 and at a temperature of 55 C.

Fig. 3. Electrochemical device.Fig. 2. Faradaic range.

87E. Pena-Munoz et al. / Surface and Coatings Technology 107 (1998) 8593

Fig. 4. Micrography on the surface of a pure nickel coating, carried out in DC with J=3 A dm2.

Fig. 5. Micrography on the surface of a composite coating NiPTFE with 10 g l1 PTFE, carried out in DC with J=3 A dm2.

vary, in order to always keep the same average density ature and the other doing the plating, was chosen. Thiskind of cell, with upward circulation, has the advantageof current, namely Jm=3 A dm2. This value is equal

to the current density of the DC experiment. of keeping the particles suspended in the solution andof ensuring the speed of particles in the solution, provid-ing the agitation.2.2. Electroless coatings

The bath used contains nickel sulphate and sodiumhypophosphite and works within the pH range from 4to 5, at a temperature of 85 C. It enables us to obtain 3. Experimental resultsa coating of nickelphosphorus containing phosphorusat 10% level. The %P was measured by means of 3.1. Morphologyquantitative X-ray microanalysis.

A cell with agitation induced by fluid circulation with Traditional optical microscopy and electronic scan-ning microscopy are used to realize the variousa centrifugal pump is used. A circulation overflowing

cell with two compartments, one regulating the temper- stereotypes.


Fig. 6. Micrography on the surface of a composite coating NiPTFE with 10 g l1 PTFE, carried out in electroless plating.

Fig. 7. Micrography out-of-cut of a composite coating NiPTFE with 10 g l1 PTFE, carried out in DC with J=3 A dm2.

3.2. Study of the surface morphology the most suitable method to determine the presence ofPTFE in the nickel matrix, but it is appropriate toobserve the morphology and brightness changes.Micrographies presented allow comparison between a

pure nickel coating (Fig. 4) and nickelPTFE coatings Conversely, one of the most reliable methods to observethe distribution of the PTFE particles in the nickelcarried out with 10 g l1 of PTFE in the bath, one

electrolytic (Fig. 5) and the other electroless (Fig. 6). matrix consists of taking photographs of out-of-cutdeposits.The pure nickel deposit has a rather regular surface,

whereas the electrolytic coating develops in a nodularway as PTFE is introduced into the solution. Moreover, 3.3. Study of morphological cutswe noticed that the size of the nodules increases withPTFE concentration. The observation requires three successive stages: the

first consists in coating a part of the covered copperThe morphology of the electroless coatingnickelPTFE is uniform and the PTFE particles are plate in a chemically inert resin; the second in polishing

it with various granulometries; and finally a chemicalclearly visible on the surface with a homogeneousdistribution. attack of the surface is necessary to reveal the presence

of PTFE particles incorporated in the coating. A nitricThe surface analysis of the NiPTFE coatings is not


Fig. 8. Micrography out-of-cut of a composite coating NiPTFE with 10 g l1 PTFE, carried out in pulsed currents with Jc=30 A dm2 andJm=3 A dm2.

Fig. 9. Inclusion rate according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic ones for DC andCPS with J=Jm=3 A dm2.

Fig. 10. Hardness measurement according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic onesfor DC and CPS with J=Jm=3 A dm2.

acid 12 N solution was used during 5 s for our The micrographies presented allow comparisonbetween nickelPTFE coatings carried out withexperiments.

The micrographies performed show the presence and 10 g l1 PTFE in the bath: two electrolytic coatings,one DC (Fig. 7) and the other CPS (Fig. 8), and andistribution of the PTFE particles in the metal matrix.

They appear as black points, of size at least regular. electroless coating.


From the coatings obtained, the PTFE particles seemto be irregularly distributed in the layer of nickel. Wenoticed that for a level of 30 g l1, the presence ofPTFE in the nickel matrix is most important, while it isrelatively weaker at 50 g l1. The particles then have atendency to agglomerate during their incorporation.

3.4. Inclusion rate

The processing of preceding images allows us to carryout measurements of the rate of PTFE inclusion in thelayer of nickel. The technique used proceeds in threequite distinct stages, described as follows.

Fig. 11. Ductility test in accordance with the standard ISO 4524/5.(1) Acquisition and digitalization of the image using a

miniature camera of type C.C.V. MICAM providedwith an objective of 16 mm. of hardness is always influenced by the substrate, we

(2) Storage of the image in a 25664 matrix by acquisi- used the Jonsson and Hogmark method [12], able totion on computer. The image is then memorized dissociate the contributions of the substrate and coatingunder 64 nuances of grey. on measured hardness.

(3) A thresholding operation which consists in bringing In addition, we noted that the hardness measurementback the image from 64 to two nuances of grey: is not influenced by the thickness, which shows that theone corresponding to the black points, and the result of measurement is not influenced by the substrateother at the white bottom. Consequently, a count- and that the selected method is thus valid.ing of the inclusion forms is carried out right before Fig. 10 presents hardness measurements correctedcalculations of surface for various levels of pixels; with this method for coatings carried out with the threethe pixel being a measurement representing an ele- techniques of deposition.mentary point of the image. A percentage of black In the case of electrolytic coatings, the values showpoints is then estimated; this corresponds in fact to that the hardness of the coating is slightly influenced bythe rate of built-in particles. Statistical processingof several images is carried out.

The results presented depend on the conditions ofdevelopment of the deposit, like the density of current,PTFE concentration in the bath, shape of currentemployed, etc.

Fig. 9 reproduces the rate of incorporation (accordingto the PTFE concentration in the bath) and allowscomparison of the three techniques of development (DC,CPS and electroless).

The results, in agreement with the literature [911],indicate that the rate of inclusion has a maximum inthe vicinity of 30 g l1 PTFE in the bath.

Concerning the electrolytic coatings, the use of pulsedcurrents makes it possible to reach rates of inclusion upto 1.7 times higher than those obtained in DC, with amaximum close to 12% under the best conditions.Nevertheless, electroless coatings have the most impor-tant rates, able to reach 30% for a concentration of30 g l1 PTFE in the solution.3.5. Measurements of Vickers hardness

These measurements are taken on samples fromapproximately 10 mm thickness by using a microhardnessinstrument of Vickers type, calibrated with a load of300 g to obtain a print of suYcient size in order to

Fig. 12. Formation of the cracks in the composite coatings.improve the measuring accuracy. Since the measurement


Fig. 13. Thickness distribution of the composite coatings.

the presence of PTFE, increasing when PTFE is added qualitative and does not allow us to know the influenceof the other influential parameters. Only the PTFEto the bath and then incorporated in the coating.influence is strong enough to be highlighted. Fig. 12For the samples carried out in CPS, the same behavi-shows some micrographies of the surface quality of theour as in DC can be observed. Hardness is generallysamples after the standardized tests have beenlarger for a given PTFE concentration in the bath.performed.Hardness varies with the rate of incorporation of par-

ticles in the metal matrix which, as we showed pre-3.7. Distribution thicknessviously, varies with the PTFE concentration in the bath

(Fig. 10).Here only electrolytic coatings (DC and CPS) areIn the case of electroless plating, the presence of

compared. The distribution thickness is directly relatedPTFE involves a significant reduction in hardness,to the problems of edge eVects. These eVects do notindeed the nickelphosphorus coating without PTFEexist in the case of electroless coatings.has a hardness higher than that obtained with electro-

The thicknesses of the coatings are measured with anlytic coatings, and decreases strongly to become veryX-ray fluorescence apparatus Fisher 1600. A series ofmuch lower than those in the presence of PTFE.100 measurements (1010) is carried out on eachsample to allow a more complete statistical analysis.3.6. Measurements of ductility

The results obtained with this technique allow us tocarry out a 3D representation of the measured thick-In order to study the ductility of coatings, we carriednesses, and hence the general form of the surface. Inout a test in accordance with the standard ISO 4524/5,this form the edge eVects are clearly revealed accordingwhich consists in folding the samples as indicated into the various parameters of electrolysis.Fig. 11. This test does not allow us to give a quantitative

The use of pulsed currents greatly reduces edge eVects,measurement, but only a qualitative estimate of the conversely to the use of DC. This is checked by the factcoating behaviour with respect to this property. that the standard deviation calculated on the whole of

The bar on which the samples are folded has a the measurement thicknesses reaches a minimal value ofdiameter of 4 mm, and the folding angle h is 30. The 1.11 in CPS, whereas it reaches 3.84 in DC.operation is carried out three consecutive times. Then, The 3D representation of thickness distribution showsan observation under microscope (50) enables us to a significant diVerence when one compares pulsed cur-detect the formation of some cracks on the sample rent and DC. Indeed, the observations carried outsurface. (Fig. 13) reveal that, on DC, the surfaces present valleys,

The number and size of created cracks depend on the which cause rather irregular thicknesses. On the otheroperating conditions, like the density of current, concen- hand, with the use of CPS, a more planar surface tendstration of PTFE particles in the bath, shape of current to be obtained, thus leading to more regular thicknessesemployed, etc. and close to the awaited thickness. These results are

In all cases, ductility is improved by the presence of obtained in CPS when cathodic currents Tc are imposedPTFE in the coating. Moreover, tests carried out in for short times.CPS show that the coatings generally have a goodductility. 3.8. CoeYcients of friction

In conclusion, usually the application of CPS appearsto improve ductility. However, as the diYculty in quanti- A standard tribometer ball/plan is used to test the

tribological properties of the composite coatingsfying this characteristic is concerned, this test remains


system, relatively important with respect to the tradi-tional tests of pawn-plan, makes it possible to increasethe phenomenon of stickslip, which indicates the modi-fications of tribological conditions. The signal deliveredby the gauges is digitized and stored in the form of afile by microcomputer. The evolution of the averagefriction coeYcient is then measured, according to thenumber of cycles. The measurement is always done atthe same place with each passage of the wiper (detectedby a position encoder).

Fig. 14 shows the evolution of the coeYcient of fric-tion according to the number of cycles of wear forvarious samples.

The first test realized on the coatings without PTFEshows an important diVerence between the electrolesscoatings and the electrolytic coatings. This is probablydue to the presence of phosphorus in the electrolesscoating. The diVerence between DC and CPS for theconditions of pulsation used is not significant.

The friction coeYcient m decreases considerably withPTFE in the bath, and the number of cycles N is higherwhen the PTFE concentration increases. This behaviouris explained by the fact that the inclusion rate in self-lubricating particles in the coating is higher for strongPTFE concentrations in the bath, at least until a limitof saturation which is in the vicinity of 30 g l1. Indeed,for the coatings carried out in DC, the rates of inclusionare respectively 4.1, 6.2 and 7.2% for concentrations of10, 20 and 30 g l1, and N then varies from 320 to550 cycles.

In the case of pure nickel, wear is much faster andappears even for the first cycles. It is also noted that, inall cases, the friction coeYcient becomes stable in thevicinity of 0.5 for a number of cycles higher than N.

4. Conclusion

The type of coating (electroless or electrolytic), theshape of the current employed as well as the eVect ofthe incorporation of PTFE particles appears to have aninfluence on certain characteristics of the deposits.

Fig. 14. CoeYcients of friction. Deposits in DC, CPS and electroless plating werecompared.

The pure nickel deposit has a regular surface. On theNiPTFE. The studied surface is moved with a rotary other hand, with PTFE introduction into the solution,movement at a controlled angular velocity lower than the surface of the electrolytic coatings presents some300 tr min1. The wiper resting against the surface is a nodules whose size increases with PTFE concentration.steel ball 100 C6 having a diameter of 10 mm. The Besides, the morphology of the electroless nickelPTFEnormal load applied to the ball is obtained by fixing coatings is uniform and the PTFE particles are definitelyweights above the wiper. This system has the advantage visible on the surface. They are distributed in a homo-of guaranteeing a constant load during the test. The geneous way.force sensor is a stainless steel beam embedded in a The rate of inclusion increases with concentration ofbracket and carrying the wiper on the other end. Gauges particles in the bath, and has a maximum in the vicinityof deformation stuck on the blade measure the inflection of 30 g l1 PTFE in the bath. The most important rateof the blade, which is proportional to the eVort of is reached with the electroless coatings and the pulsed

currents allow an increase compared to the DC.friction generated in the contact. The elasticity of the


[2] P. Bercot, Etude des revetements dor obtenus par courants pulses:With regard to the hardness of the coatings, it iscorrelation entre plage faradique et certaines caracteristiques dehigher in CPS. In the case of the electroless coatings,tels revetements, Thesis, Universite de Franche-Comte, Besan-

the hardness decreases with concentration of particles. con, 1988.For ductility measurements, the samples carried out [3] P.R. Ebdon, J. Mater. Product Technol. 1 (2) (1986) 290.

[4] P.R. Ebdon, Plat. Surf. Finish. September (1988) 65.in DC show poor characteristics. But these properties[5] R.N. Duncan, Metal Finish. September (1989) 33.improve with the application of pulsed currents. In all[6 ] S.S. Tulsi, Trans. Inst. Metal Finish. 61 (1983) 147.cases, ductility is improved by the presence of PTFE.[7] K. Helle, A. Opschoor, Proc. Interfinish 80 (1980) 234.

The same holds for wear resistance, with a much [8] R. Tournier, Bain au nickel sulfamate, Les Fiches Techniquesgreater eVect in the case of electroless coatings. G.O.T.S.

[9] M. Guglielmi, J. Electrochem. Soc. 119 (8) (1972) 1009.Finally, the use of pulsed currents allows us to obtain[10] F. Eba, Contribution a` letude du mecanisme dincorporation decoatings with more homogeneous thickness distribution.

particules de PTFE dans un depot electrolytique de nickel: influ-ence dune agitation ultrasonore, Thesis, Universite de Franche-Comte, Besancon, 1998.

References [11] W. Metzger, T.H. Florian, Metalloberflache 34 (7) (1980) 67.[12] D. Chicot, J. Lesage, Durete de materiaux deposes en couches

minces, Materiaux et Techniques 1011 (1994) 19.[1] M. Ruimi, K.V. Quang, Revetements composites electrodeposes,Techniques de lIngenieur M1626 (10) (1990) 2.

electrolytic and electroless coatings of ni.ptfe composites. study of some characteristics

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