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Electrolytic and electroless coatings of Ni–PTFE composites
Surface and Coatings Technology 107 (1998) 85–93
Electrolytic and electroless coatings of Ni–PTFE composites Study of some characteristics E. Pena-Munoz a, P. Berc¸ot b,*, A. Grosjean b, M. Rezrazi b, J. Pagetti b a 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 Me´gevand, 25000 Besanc¸on, France Received 14 April 1998; accepted 12 June 1998
Abstract This work consists of the realization and characterization of composite coatings of Ni–PTFE carried out by electroless deposition on the one hand and electrolytic deposition on the other hand, obtained by pulsed current (CPS) and d.c. current (DC ). The evolution of some properties of these coatings generated by various procedures, properties such as morphology, hardness, ductility and wear resistance, are highlighted. © 1998 Elsevier Science S.A. All rights reserved. Keywords: D.c. current; Electrolytic and electroless coatings of composites Ni–PTFE; Physical characteristics; Pulsed current
1. Introduction The deposition of particles finely dispersed in a metal matrix by the process of electrocodeposition led to a new generation of composites, which present particular chemical and physical properties. These properties depend not only on the concentration, size, distribution inside deposited material, nature and morphology of the particles, but also on the type of solution used and physicochemical parameters (pH, temperature, density of current, etc.). One recalls that a composite is a polyphase solid in which two or several components are associated in order to obtain, on a macroscopic scale and at least in certain directions, an original whole of properties which the components taken separately do not make it possible to reach [1]. Among the shapes of current most employed for the implementation of the electrolytic coatings, we used d.c. current and pulsed currents. One of the most important differences between these shapes of current is the maximum value which the density of current can reach. Indeed, the reaction of deposition consumes metal ions and therefore tends to impoverish the solution in the immediate vicinity of the cathode. This impoverishment is naturally compensated by the diffusion of metal ions
* Corresponding author.
of comparable nature which move from the centre of the solution towards the impoverished area near the electrode and which, in this way, feed the reaction. However, the speed with which the ions can diffuse in the electrolyte is limited, which restricts the acceptable intensity of current in DC. The technique of electrodeposition, called ‘‘pulsed currents’’, consists in the use of discontinuous currents, the most used according to the literature have the shape of rectangular pulses, as shown in Fig. 1. The use of pulsed currents enables very high current densities by the application of impulses of current following a rest period. The duration of the impulses must be limited in order not to drastically impoverish the cathode/solution interface in metal cations, which makes it possible to avoid the problems involved in diffusion. The rest period must be long enough to allow a sufficient restocking of this zone. The amplitude of current thus reached involves an important modification of the deposit microstructure and thus properties. A plot of transitory curves V=f(t) makes it possible to follow the evolution of the various electrochemical processes which are established on the surface of the work electrode [2]. From these curves, it is possible to define the lower limits and higher Faradaic ranges to locate the Faradaic reaction where only the transfer of loads corresponding to the deposit occurs. Fig. 2 shows the Faradaic range obtained from the nickel sulphamate bath. This makes
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Fig. 1. Form of the pulsed currents.
it possible to fix the parameters related to the shapes of current. Concerning electroless coatings, they are well mastered and used: commercial processes exist, e.g. NiflorA [3]. Many articles cover the performance of this kind of deposition [4–6 ]. Concerning electrolytic coatings, they are studied but still remain today a curiosity of the laboratory. Helle and Opschoor [7] were pioneers in showing that incorporation of PTFE particles by electroplating deposition depends upon two vital factors: the mode of agitation to keep the particles buoyant and the use of surfactants to keep the particles from agglomeration. The aim of this study is a better knowledge of the influence of PTFE on some characteristics of coatings (morphology, hardness, ductility, coefficient of friction and distribution of thickness). Coatings are obtained in an electroless way and an electrolytic way. In this latter case, the form of current, namely DC and CPS, is allowed to vary.
The PTFE particles are added in the form of emulsion. They have an average size lower than 0.5 mm. Dispersion is in pseudo-equilibrium, stabilized using a non-ionic dampening agent. In order to make a comparative study, pure nickel deposits are also carried out under the same operating conditions of work. The experimental device is constituted of the following elements ( Fig. 3): – a cell containing the electrolysis bath with a magnetic stirrer; – electrodes immersed in the bath: an anode made up of nickel ‘‘rounds’’ and a 12 cm2 plate of copper placed as a cathode (substrate); – the electric device made up of conductors supplying the electrodes connected to a generator of current: the whole device is controlled by a computer; – an oscilloscope to visualize the transitory curves V=f(t) corresponding to the current impulses. For a better comparison of the results obtained with DC and CPS, we allow the parameters J and T to c c
2. Experimental conditions 2.1. Electrolytic coating A bath containing some nickel sulphamate [8], without either brightener or antipitting agent, is used in the pH range from 4 to 5 and at a temperature of 55 °C.
Fig. 2. Faradaic range.
Fig. 3. Electrochemical device.
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Fig. 4. Micrography on the surface of a pure nickel coating, carried out in DC with J=3 A dm−2.
Fig. 5. Micrography on the surface of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in DC with J=3 A dm−2.
vary, in order to always keep the same average density of current, namely J =3 A dm−2. This value is equal m to the current density of the DC experiment. 2.2. Electroless coatings The bath used contains nickel sulphate and sodium hypophosphite and works within the pH range from 4 to 5, at a temperature of 85 °C. It enables us to obtain a coating of nickel–phosphorus containing phosphorus at 10% level. The %P was measured by means of quantitative X-ray microanalysis. A cell with agitation induced by fluid circulation with a centrifugal pump is used. A ‘‘circulation overflowing’’ cell with two compartments, one regulating the temper-
ature and the other doing the plating, was chosen. This kind of cell, with upward circulation, has the advantage of keeping the particles suspended in the solution and of ensuring the speed of particles in the solution, providing the agitation.
3. Experimental results 3.1. Morphology Traditional optical microscopy and electronic scanning microscopy are used to realize the various stereotypes.
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Fig. 6. Micrography on the surface of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in electroless plating.
Fig. 7. Micrography out-of-cut of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in DC with J=3 A dm−2.
3.2. Study of the surface morphology Micrographies presented allow comparison between a pure nickel coating (Fig. 4) and nickel–PTFE coatings carried out with 10 g l−1 of PTFE in the bath, one electrolytic (Fig. 5) and the other electroless ( Fig. 6). The pure nickel deposit has a rather regular surface, whereas the electrolytic coating develops in a nodular way as PTFE is introduced into the solution. Moreover, we noticed that the size of the nodules increases with PTFE concentration. The morphology of the electroless coating nickel–PTFE is uniform and the PTFE particles are clearly visible on the surface with a homogeneous distribution. The surface analysis of the Ni–PTFE coatings is not
the most suitable method to determine the presence of PTFE in the nickel matrix, but it is appropriate to observe the morphology and brightness changes. Conversely, one of the most reliable methods to observe the distribution of the PTFE particles in the nickel matrix consists of taking photographs of out-of-cut deposits. 3.3. Study of morphological cuts The observation requires three successive stages: the first consists in coating a part of the covered copper plate in a chemically inert resin; the second in polishing it with various granulometries; and finally a chemical attack of the surface is necessary to reveal the presence of PTFE particles incorporated in the coating. A nitric
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Fig. 8. Micrography out-of-cut of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in pulsed currents with J =30 A dm−2 and c J =3 A dm−2. m
Fig. 9. Inclusion rate according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic ones for DC and CPS with J=J =3 A dm−2. m
Fig. 10. Hardness measurement according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic ones for DC and CPS with J=J =3 A dm−2. m
acid 12 N solution was used during 5 s for our experiments. The micrographies performed show the presence and distribution of the PTFE particles in the metal matrix. They appear as black points, of size at least regular.
The micrographies presented allow comparison between nickel–PTFE coatings carried out with 10 g l−1 PTFE in the bath: two electrolytic coatings, one DC ( Fig. 7) and the other CPS (Fig. 8), and an electroless coating.
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From the coatings obtained, the PTFE particles seem to be irregularly distributed in the layer of nickel. We noticed that for a level of 30 g l−1, the presence of PTFE in the nickel matrix is most important, while it is relatively weaker at 50 g l−1. The particles then have a tendency to agglomerate during their incorporation. 3.4. Inclusion rate The processing of preceding images allows us to carry out measurements of the rate of PTFE inclusion in the layer of nickel. The technique used proceeds in three quite distinct stages, described as follows. (1) Acquisition and digitalization of the image using a miniature camera of type C.C.V. MICAM provided with an objective of 16 mm. (2) Storage of the image in a 256×64 matrix by acquisition on computer. The image is then memorized under 64 nuances of grey. (3) A thresholding operation which consists in bringing back the image from 64 to two nuances of grey: one corresponding to the ‘‘black’’ points, and the other at the ‘‘white’’ bottom. Consequently, a counting of the inclusion forms is carried out right before calculations of surface for various levels of pixels; the pixel being a measurement representing an elementary point of the image. A percentage of black points is then estimated; this corresponds in fact to the rate of built-in particles. Statistical processing of several images is carried out. The results presented depend on the conditions of development of the deposit, like the density of current, PTFE concentration in the bath, shape of current employed, etc. Fig. 9 reproduces the rate of incorporation (according to the PTFE concentration in the bath) and allows comparison of the three techniques of development (DC, CPS and electroless). The results, in agreement with the literature [9–11], indicate that the rate of inclusion has a maximum in the vicinity of 30 g l−1 PTFE in the bath. Concerning the electrolytic coatings, the use of pulsed currents makes it possible to reach rates of inclusion up to 1.7 times higher than those obtained in DC, with a maximum close to 12% under the best conditions. Nevertheless, electroless coatings have the most important rates, able to reach 30% for a concentration of 30 g l−1 PTFE in the solution.
Fig. 11. Ductility test in accordance with the standard ISO 4524/5.
of hardness is always influenced by the substrate, we used the Jo¨nsson and Hogmark method [12], able to dissociate the contributions of the substrate and coating on measured hardness. In addition, we noted that the hardness measurement is not influenced by the thickness, which shows that the result of measurement is not influenced by the substrate and that the selected method is thus valid. Fig. 10 presents hardness measurements corrected with this method for coatings carried out with the three techniques of deposition. In the case of electrolytic coatings, the values show that the hardness of the coating is slightly influenced by
3.5. Measurements of Vickers’ hardness These measurements are taken on samples from approximately 10 mm thickness by using a microhardness instrument of Vickers’ type, calibrated with a load of 300 g to obtain a print of sufficient size in order to improve the measuring accuracy. Since the measurement
Fig. 12. Formation of the cracks in the composite coatings.
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Fig. 13. Thickness distribution of the composite coatings.
the presence of PTFE, increasing when PTFE is added to the bath and then incorporated in the coating. For the samples carried out in CPS, the same behaviour as in DC can be observed. Hardness is generally larger for a given PTFE concentration in the bath. Hardness varies with the rate of incorporation of particles in the metal matrix which, as we showed previously, varies with the PTFE concentration in the bath (Fig. 10). In the case of electroless plating, the presence of PTFE involves a significant reduction in hardness, indeed the nickel–phosphorus coating without PTFE has a hardness higher than that obtained with electrolytic coatings, and decreases strongly to become very much lower than those in the presence of PTFE. 3.6. Measurements of ductility In order to study the ductility of coatings, we carried out a test in accordance with the standard ISO 4524/5, which consists in folding the samples as indicated in Fig. 11. This test does not allow us to give a quantitative measurement, but only a qualitative estimate of the coating behaviour with respect to this property. The bar on which the samples are folded has a diameter of 4 mm, and the folding angle h is 30°. The operation is carried out three consecutive times. Then, an observation under microscope (50×) enables us to detect the formation of some cracks on the sample surface. The number and size of created cracks depend on the operating conditions, like the density of current, concentration of PTFE particles in the bath, shape of current employed, etc. In all cases, ductility is improved by the presence of PTFE in the coating. Moreover, tests carried out in CPS show that the coatings generally have a good ductility. In conclusion, usually the application of CPS appears to improve ductility. However, as the difficulty in quantifying this characteristic is concerned, this test remains
qualitative and does not allow us to know the influence of the other influential parameters. Only the PTFE influence is strong enough to be highlighted. Fig. 12 shows some micrographies of the surface quality of the samples after the standardized tests have been performed. 3.7. Distribution thickness Here only electrolytic coatings (DC and CPS) are compared. The distribution thickness is directly related to the problems of edge effects. These effects do not exist in the case of electroless coatings. The thicknesses of the coatings are measured with an X-ray fluorescence apparatus Fisher 1600. A series of 100 measurements (10×10) is carried out on each sample to allow a more complete statistical analysis. The results obtained with this technique allow us to carry out a 3D representation of the measured thicknesses, and hence the general form of the surface. In this form the edge effects are clearly revealed according to the various parameters of electrolysis. The use of pulsed currents greatly reduces edge effects, conversely to the use of DC. This is checked by the fact that the standard deviation calculated on the whole of the measurement thicknesses reaches a minimal value of 1.11 in CPS, whereas it reaches 3.84 in DC. The 3D representation of thickness distribution shows a significant difference when one compares pulsed current and DC. Indeed, the observations carried out ( Fig. 13) reveal that, on DC, the surfaces present valleys, which cause rather irregular thicknesses. On the other hand, with the use of CPS, a more planar surface tends to be obtained, thus leading to more regular thicknesses and close to the awaited thickness. These results are obtained in CPS when cathodic currents T are imposed c for short times. 3.8. Coefficients of friction A standard tribometer ball/plan is used to test the tribological properties of the composite coatings
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system, relatively important with respect to the traditional tests of pawn-plan, makes it possible to increase the phenomenon of stick–slip, which indicates the modifications of tribological conditions. The signal delivered by the gauges is digitized and stored in the form of a file by microcomputer. The evolution of the average friction coefficient is then measured, according to the number of cycles. The measurement is always done at the same place with each passage of the wiper (detected by a position encoder). Fig. 14 shows the evolution of the coefficient of friction according to the number of cycles of wear for various samples. The first test realized on the coatings without PTFE shows an important difference between the electroless coatings and the electrolytic coatings. This is probably due to the presence of phosphorus in the electroless coating. The difference between DC and CPS for the conditions of pulsation used is not significant. The friction coefficient m decreases considerably with PTFE in the bath, and the number of cycles N is higher when the PTFE concentration increases. This behaviour is explained by the fact that the inclusion rate in selflubricating particles in the coating is higher for strong PTFE concentrations in the bath, at least until a limit of saturation which is in the vicinity of 30 g l−1. Indeed, for the coatings carried out in DC, the rates of inclusion are respectively 4.1, 6.2 and 7.2% for concentrations of 10, 20 and 30 g l−1, and N then varies from 320 to 550 cycles. In the case of pure nickel, wear is much faster and appears even for the first cycles. It is also noted that, in all cases, the friction coefficient becomes stable in the vicinity of 0.5 for a number of cycles higher than N.
4. Conclusion
Fig. 14. Coefficients of friction.
Ni–PTFE. The studied surface is moved with a rotary movement at a controlled angular velocity lower than 300 tr min−1. The wiper resting against the surface is a steel ball 100 C6 having a diameter of 10 mm. The normal load applied to the ball is obtained by fixing weights above the wiper. This system has the advantage of guaranteeing a constant load during the test. The force sensor is a stainless steel beam embedded in a bracket and carrying the wiper on the other end. Gauges of deformation stuck on the blade measure the inflection of the blade, which is proportional to the effort of friction generated in the contact. The elasticity of the
The type of coating (electroless or electrolytic), the shape of the current employed as well as the effect of the incorporation of PTFE particles appears to have an influence on certain characteristics of the deposits. Deposits in DC, CPS and electroless plating were compared. The pure nickel deposit has a regular surface. On the other hand, with PTFE introduction into the solution, the surface of the electrolytic coatings presents some nodules whose size increases with PTFE concentration. Besides, the morphology of the electroless nickel–PTFE coatings is uniform and the PTFE particles are definitely visible on the surface. They are distributed in a homogeneous way. The rate of inclusion increases with concentration of particles in the bath, and has a maximum in the vicinity of 30 g l−1 PTFE in the bath. The most important rate is reached with the electroless coatings and the pulsed currents allow an increase compared to the DC.
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With regard to the hardness of the coatings, it is higher in CPS. In the case of the electroless coatings, the hardness decreases with concentration of particles. For ductility measurements, the samples carried out in DC show poor characteristics. But these properties improve with the application of pulsed currents. In all cases, ductility is improved by the presence of PTFE. The same holds for wear resistance, with a much greater effect in the case of electroless coatings. Finally, the use of pulsed currents allows us to obtain coatings with more homogeneous thickness distribution.
References [1] M. Ruimi, K.V. Quang, Reveˆtements composites e´lectrode´pose´s, Techniques de l’Inge´nieur M1626 (10) (1990) 2.
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[2] P. Bercot, E´tude des reveˆtements d’or obtenus par courants pulse´s: corre´lation entre plage faradique et certaines caracte´ristiques de tels reveˆtements, Thesis, Universite´ de Franche-Comte´, Besanc¸on, 1988. [3] P.R. Ebdon, J. Mater. Product Technol. 1 (2) (1986) 290. [4] P.R. Ebdon, Plat. Surf. Finish. September (1988) 65. [5] R.N. Duncan, Metal Finish. September (1989) 33. [6 ] S.S. Tulsi, Trans. Inst. Metal Finish. 61 (1983) 147. [7] K. Helle, A. Opschoor, Proc. Interfinish 80 (1980) 234. [8] R. Tournier, Bain au nickel sulfamate, Les Fiches Techniques G.O.T.S. [9] M. Guglielmi, J. Electrochem. Soc. 119 (8) (1972) 1009. [10] F. Eba, Contribution a` l’e´tude du me´canisme d’incorporation de particules de PTFE dans un de´poˆt e´lectrolytique de nickel: influence d’une agitation ultrasonore, Thesis, Universite´ de FrancheComte´, Besanc¸on, 1998. [11] W. Metzger, T.H. Florian, Metalloberfla¨che 34 (7) (1980) 67. [12] D. Chicot, J. Lesage, Durete´ de mate´riaux de´pose´s en couches minces, Mate´riaux et Techniques 1011 (1994) 19.
Electrolytic and electroless coatings of Ni–PTFE composites Study of some characteristics E. Pena-Munoz a, P. Berc¸ot b,*, A. Grosjean b, M. Rezrazi b, J. Pagetti b a 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 Me´gevand, 25000 Besanc¸on, France Received 14 April 1998; accepted 12 June 1998
Abstract This work consists of the realization and characterization of composite coatings of Ni–PTFE carried out by electroless deposition on the one hand and electrolytic deposition on the other hand, obtained by pulsed current (CPS) and d.c. current (DC ). The evolution of some properties of these coatings generated by various procedures, properties such as morphology, hardness, ductility and wear resistance, are highlighted. © 1998 Elsevier Science S.A. All rights reserved. Keywords: D.c. current; Electrolytic and electroless coatings of composites Ni–PTFE; Physical characteristics; Pulsed current
1. Introduction The deposition of particles finely dispersed in a metal matrix by the process of electrocodeposition led to a new generation of composites, which present particular chemical and physical properties. These properties depend not only on the concentration, size, distribution inside deposited material, nature and morphology of the particles, but also on the type of solution used and physicochemical parameters (pH, temperature, density of current, etc.). One recalls that a composite is a polyphase solid in which two or several components are associated in order to obtain, on a macroscopic scale and at least in certain directions, an original whole of properties which the components taken separately do not make it possible to reach [1]. Among the shapes of current most employed for the implementation of the electrolytic coatings, we used d.c. current and pulsed currents. One of the most important differences between these shapes of current is the maximum value which the density of current can reach. Indeed, the reaction of deposition consumes metal ions and therefore tends to impoverish the solution in the immediate vicinity of the cathode. This impoverishment is naturally compensated by the diffusion of metal ions
* Corresponding author.
of comparable nature which move from the centre of the solution towards the impoverished area near the electrode and which, in this way, feed the reaction. However, the speed with which the ions can diffuse in the electrolyte is limited, which restricts the acceptable intensity of current in DC. The technique of electrodeposition, called ‘‘pulsed currents’’, consists in the use of discontinuous currents, the most used according to the literature have the shape of rectangular pulses, as shown in Fig. 1. The use of pulsed currents enables very high current densities by the application of impulses of current following a rest period. The duration of the impulses must be limited in order not to drastically impoverish the cathode/solution interface in metal cations, which makes it possible to avoid the problems involved in diffusion. The rest period must be long enough to allow a sufficient restocking of this zone. The amplitude of current thus reached involves an important modification of the deposit microstructure and thus properties. A plot of transitory curves V=f(t) makes it possible to follow the evolution of the various electrochemical processes which are established on the surface of the work electrode [2]. From these curves, it is possible to define the lower limits and higher Faradaic ranges to locate the Faradaic reaction where only the transfer of loads corresponding to the deposit occurs. Fig. 2 shows the Faradaic range obtained from the nickel sulphamate bath. This makes
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Fig. 1. Form of the pulsed currents.
it possible to fix the parameters related to the shapes of current. Concerning electroless coatings, they are well mastered and used: commercial processes exist, e.g. NiflorA [3]. Many articles cover the performance of this kind of deposition [4–6 ]. Concerning electrolytic coatings, they are studied but still remain today a curiosity of the laboratory. Helle and Opschoor [7] were pioneers in showing that incorporation of PTFE particles by electroplating deposition depends upon two vital factors: the mode of agitation to keep the particles buoyant and the use of surfactants to keep the particles from agglomeration. The aim of this study is a better knowledge of the influence of PTFE on some characteristics of coatings (morphology, hardness, ductility, coefficient of friction and distribution of thickness). Coatings are obtained in an electroless way and an electrolytic way. In this latter case, the form of current, namely DC and CPS, is allowed to vary.
The PTFE particles are added in the form of emulsion. They have an average size lower than 0.5 mm. Dispersion is in pseudo-equilibrium, stabilized using a non-ionic dampening agent. In order to make a comparative study, pure nickel deposits are also carried out under the same operating conditions of work. The experimental device is constituted of the following elements ( Fig. 3): – a cell containing the electrolysis bath with a magnetic stirrer; – electrodes immersed in the bath: an anode made up of nickel ‘‘rounds’’ and a 12 cm2 plate of copper placed as a cathode (substrate); – the electric device made up of conductors supplying the electrodes connected to a generator of current: the whole device is controlled by a computer; – an oscilloscope to visualize the transitory curves V=f(t) corresponding to the current impulses. For a better comparison of the results obtained with DC and CPS, we allow the parameters J and T to c c
2. Experimental conditions 2.1. Electrolytic coating A bath containing some nickel sulphamate [8], without either brightener or antipitting agent, is used in the pH range from 4 to 5 and at a temperature of 55 °C.
Fig. 2. Faradaic range.
Fig. 3. Electrochemical device.
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Fig. 4. Micrography on the surface of a pure nickel coating, carried out in DC with J=3 A dm−2.
Fig. 5. Micrography on the surface of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in DC with J=3 A dm−2.
vary, in order to always keep the same average density of current, namely J =3 A dm−2. This value is equal m to the current density of the DC experiment. 2.2. Electroless coatings The bath used contains nickel sulphate and sodium hypophosphite and works within the pH range from 4 to 5, at a temperature of 85 °C. It enables us to obtain a coating of nickel–phosphorus containing phosphorus at 10% level. The %P was measured by means of quantitative X-ray microanalysis. A cell with agitation induced by fluid circulation with a centrifugal pump is used. A ‘‘circulation overflowing’’ cell with two compartments, one regulating the temper-
ature and the other doing the plating, was chosen. This kind of cell, with upward circulation, has the advantage of keeping the particles suspended in the solution and of ensuring the speed of particles in the solution, providing the agitation.
3. Experimental results 3.1. Morphology Traditional optical microscopy and electronic scanning microscopy are used to realize the various stereotypes.
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Fig. 6. Micrography on the surface of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in electroless plating.
Fig. 7. Micrography out-of-cut of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in DC with J=3 A dm−2.
3.2. Study of the surface morphology Micrographies presented allow comparison between a pure nickel coating (Fig. 4) and nickel–PTFE coatings carried out with 10 g l−1 of PTFE in the bath, one electrolytic (Fig. 5) and the other electroless ( Fig. 6). The pure nickel deposit has a rather regular surface, whereas the electrolytic coating develops in a nodular way as PTFE is introduced into the solution. Moreover, we noticed that the size of the nodules increases with PTFE concentration. The morphology of the electroless coating nickel–PTFE is uniform and the PTFE particles are clearly visible on the surface with a homogeneous distribution. The surface analysis of the Ni–PTFE coatings is not
the most suitable method to determine the presence of PTFE in the nickel matrix, but it is appropriate to observe the morphology and brightness changes. Conversely, one of the most reliable methods to observe the distribution of the PTFE particles in the nickel matrix consists of taking photographs of out-of-cut deposits. 3.3. Study of morphological cuts The observation requires three successive stages: the first consists in coating a part of the covered copper plate in a chemically inert resin; the second in polishing it with various granulometries; and finally a chemical attack of the surface is necessary to reveal the presence of PTFE particles incorporated in the coating. A nitric
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Fig. 8. Micrography out-of-cut of a composite coating Ni–PTFE with 10 g l−1 PTFE, carried out in pulsed currents with J =30 A dm−2 and c J =3 A dm−2. m
Fig. 9. Inclusion rate according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic ones for DC and CPS with J=J =3 A dm−2. m
Fig. 10. Hardness measurement according to the PTFE concentration in the bath. Comparison between electroless coatings and electrolytic ones for DC and CPS with J=J =3 A dm−2. m
acid 12 N solution was used during 5 s for our experiments. The micrographies performed show the presence and distribution of the PTFE particles in the metal matrix. They appear as black points, of size at least regular.
The micrographies presented allow comparison between nickel–PTFE coatings carried out with 10 g l−1 PTFE in the bath: two electrolytic coatings, one DC ( Fig. 7) and the other CPS (Fig. 8), and an electroless coating.
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From the coatings obtained, the PTFE particles seem to be irregularly distributed in the layer of nickel. We noticed that for a level of 30 g l−1, the presence of PTFE in the nickel matrix is most important, while it is relatively weaker at 50 g l−1. The particles then have a tendency to agglomerate during their incorporation. 3.4. Inclusion rate The processing of preceding images allows us to carry out measurements of the rate of PTFE inclusion in the layer of nickel. The technique used proceeds in three quite distinct stages, described as follows. (1) Acquisition and digitalization of the image using a miniature camera of type C.C.V. MICAM provided with an objective of 16 mm. (2) Storage of the image in a 256×64 matrix by acquisition on computer. The image is then memorized under 64 nuances of grey. (3) A thresholding operation which consists in bringing back the image from 64 to two nuances of grey: one corresponding to the ‘‘black’’ points, and the other at the ‘‘white’’ bottom. Consequently, a counting of the inclusion forms is carried out right before calculations of surface for various levels of pixels; the pixel being a measurement representing an elementary point of the image. A percentage of black points is then estimated; this corresponds in fact to the rate of built-in particles. Statistical processing of several images is carried out. The results presented depend on the conditions of development of the deposit, like the density of current, PTFE concentration in the bath, shape of current employed, etc. Fig. 9 reproduces the rate of incorporation (according to the PTFE concentration in the bath) and allows comparison of the three techniques of development (DC, CPS and electroless). The results, in agreement with the literature [9–11], indicate that the rate of inclusion has a maximum in the vicinity of 30 g l−1 PTFE in the bath. Concerning the electrolytic coatings, the use of pulsed currents makes it possible to reach rates of inclusion up to 1.7 times higher than those obtained in DC, with a maximum close to 12% under the best conditions. Nevertheless, electroless coatings have the most important rates, able to reach 30% for a concentration of 30 g l−1 PTFE in the solution.
Fig. 11. Ductility test in accordance with the standard ISO 4524/5.
of hardness is always influenced by the substrate, we used the Jo¨nsson and Hogmark method [12], able to dissociate the contributions of the substrate and coating on measured hardness. In addition, we noted that the hardness measurement is not influenced by the thickness, which shows that the result of measurement is not influenced by the substrate and that the selected method is thus valid. Fig. 10 presents hardness measurements corrected with this method for coatings carried out with the three techniques of deposition. In the case of electrolytic coatings, the values show that the hardness of the coating is slightly influenced by
3.5. Measurements of Vickers’ hardness These measurements are taken on samples from approximately 10 mm thickness by using a microhardness instrument of Vickers’ type, calibrated with a load of 300 g to obtain a print of sufficient size in order to improve the measuring accuracy. Since the measurement
Fig. 12. Formation of the cracks in the composite coatings.
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Fig. 13. Thickness distribution of the composite coatings.
the presence of PTFE, increasing when PTFE is added to the bath and then incorporated in the coating. For the samples carried out in CPS, the same behaviour as in DC can be observed. Hardness is generally larger for a given PTFE concentration in the bath. Hardness varies with the rate of incorporation of particles in the metal matrix which, as we showed previously, varies with the PTFE concentration in the bath (Fig. 10). In the case of electroless plating, the presence of PTFE involves a significant reduction in hardness, indeed the nickel–phosphorus coating without PTFE has a hardness higher than that obtained with electrolytic coatings, and decreases strongly to become very much lower than those in the presence of PTFE. 3.6. Measurements of ductility In order to study the ductility of coatings, we carried out a test in accordance with the standard ISO 4524/5, which consists in folding the samples as indicated in Fig. 11. This test does not allow us to give a quantitative measurement, but only a qualitative estimate of the coating behaviour with respect to this property. The bar on which the samples are folded has a diameter of 4 mm, and the folding angle h is 30°. The operation is carried out three consecutive times. Then, an observation under microscope (50×) enables us to detect the formation of some cracks on the sample surface. The number and size of created cracks depend on the operating conditions, like the density of current, concentration of PTFE particles in the bath, shape of current employed, etc. In all cases, ductility is improved by the presence of PTFE in the coating. Moreover, tests carried out in CPS show that the coatings generally have a good ductility. In conclusion, usually the application of CPS appears to improve ductility. However, as the difficulty in quantifying this characteristic is concerned, this test remains
qualitative and does not allow us to know the influence of the other influential parameters. Only the PTFE influence is strong enough to be highlighted. Fig. 12 shows some micrographies of the surface quality of the samples after the standardized tests have been performed. 3.7. Distribution thickness Here only electrolytic coatings (DC and CPS) are compared. The distribution thickness is directly related to the problems of edge effects. These effects do not exist in the case of electroless coatings. The thicknesses of the coatings are measured with an X-ray fluorescence apparatus Fisher 1600. A series of 100 measurements (10×10) is carried out on each sample to allow a more complete statistical analysis. The results obtained with this technique allow us to carry out a 3D representation of the measured thicknesses, and hence the general form of the surface. In this form the edge effects are clearly revealed according to the various parameters of electrolysis. The use of pulsed currents greatly reduces edge effects, conversely to the use of DC. This is checked by the fact that the standard deviation calculated on the whole of the measurement thicknesses reaches a minimal value of 1.11 in CPS, whereas it reaches 3.84 in DC. The 3D representation of thickness distribution shows a significant difference when one compares pulsed current and DC. Indeed, the observations carried out ( Fig. 13) reveal that, on DC, the surfaces present valleys, which cause rather irregular thicknesses. On the other hand, with the use of CPS, a more planar surface tends to be obtained, thus leading to more regular thicknesses and close to the awaited thickness. These results are obtained in CPS when cathodic currents T are imposed c for short times. 3.8. Coefficients of friction A standard tribometer ball/plan is used to test the tribological properties of the composite coatings
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system, relatively important with respect to the traditional tests of pawn-plan, makes it possible to increase the phenomenon of stick–slip, which indicates the modifications of tribological conditions. The signal delivered by the gauges is digitized and stored in the form of a file by microcomputer. The evolution of the average friction coefficient is then measured, according to the number of cycles. The measurement is always done at the same place with each passage of the wiper (detected by a position encoder). Fig. 14 shows the evolution of the coefficient of friction according to the number of cycles of wear for various samples. The first test realized on the coatings without PTFE shows an important difference between the electroless coatings and the electrolytic coatings. This is probably due to the presence of phosphorus in the electroless coating. The difference between DC and CPS for the conditions of pulsation used is not significant. The friction coefficient m decreases considerably with PTFE in the bath, and the number of cycles N is higher when the PTFE concentration increases. This behaviour is explained by the fact that the inclusion rate in selflubricating particles in the coating is higher for strong PTFE concentrations in the bath, at least until a limit of saturation which is in the vicinity of 30 g l−1. Indeed, for the coatings carried out in DC, the rates of inclusion are respectively 4.1, 6.2 and 7.2% for concentrations of 10, 20 and 30 g l−1, and N then varies from 320 to 550 cycles. In the case of pure nickel, wear is much faster and appears even for the first cycles. It is also noted that, in all cases, the friction coefficient becomes stable in the vicinity of 0.5 for a number of cycles higher than N.
4. Conclusion
Fig. 14. Coefficients of friction.
Ni–PTFE. The studied surface is moved with a rotary movement at a controlled angular velocity lower than 300 tr min−1. The wiper resting against the surface is a steel ball 100 C6 having a diameter of 10 mm. The normal load applied to the ball is obtained by fixing weights above the wiper. This system has the advantage of guaranteeing a constant load during the test. The force sensor is a stainless steel beam embedded in a bracket and carrying the wiper on the other end. Gauges of deformation stuck on the blade measure the inflection of the blade, which is proportional to the effort of friction generated in the contact. The elasticity of the
The type of coating (electroless or electrolytic), the shape of the current employed as well as the effect of the incorporation of PTFE particles appears to have an influence on certain characteristics of the deposits. Deposits in DC, CPS and electroless plating were compared. The pure nickel deposit has a regular surface. On the other hand, with PTFE introduction into the solution, the surface of the electrolytic coatings presents some nodules whose size increases with PTFE concentration. Besides, the morphology of the electroless nickel–PTFE coatings is uniform and the PTFE particles are definitely visible on the surface. They are distributed in a homogeneous way. The rate of inclusion increases with concentration of particles in the bath, and has a maximum in the vicinity of 30 g l−1 PTFE in the bath. The most important rate is reached with the electroless coatings and the pulsed currents allow an increase compared to the DC.
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With regard to the hardness of the coatings, it is higher in CPS. In the case of the electroless coatings, the hardness decreases with concentration of particles. For ductility measurements, the samples carried out in DC show poor characteristics. But these properties improve with the application of pulsed currents. In all cases, ductility is improved by the presence of PTFE. The same holds for wear resistance, with a much greater effect in the case of electroless coatings. Finally, the use of pulsed currents allows us to obtain coatings with more homogeneous thickness distribution.
References [1] M. Ruimi, K.V. Quang, Reveˆtements composites e´lectrode´pose´s, Techniques de l’Inge´nieur M1626 (10) (1990) 2.
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[2] P. Bercot, E´tude des reveˆtements d’or obtenus par courants pulse´s: corre´lation entre plage faradique et certaines caracte´ristiques de tels reveˆtements, Thesis, Universite´ de Franche-Comte´, Besanc¸on, 1988. [3] P.R. Ebdon, J. Mater. Product Technol. 1 (2) (1986) 290. [4] P.R. Ebdon, Plat. Surf. Finish. September (1988) 65. [5] R.N. Duncan, Metal Finish. September (1989) 33. [6 ] S.S. Tulsi, Trans. Inst. Metal Finish. 61 (1983) 147. [7] K. Helle, A. Opschoor, Proc. Interfinish 80 (1980) 234. [8] R. Tournier, Bain au nickel sulfamate, Les Fiches Techniques G.O.T.S. [9] M. Guglielmi, J. Electrochem. Soc. 119 (8) (1972) 1009. [10] F. Eba, Contribution a` l’e´tude du me´canisme d’incorporation de particules de PTFE dans un de´poˆt e´lectrolytique de nickel: influence d’une agitation ultrasonore, Thesis, Universite´ de FrancheComte´, Besanc¸on, 1998. [11] W. Metzger, T.H. Florian, Metalloberfla¨che 34 (7) (1980) 67. [12] D. Chicot, J. Lesage, Durete´ de mate´riaux de´pose´s en couches minces, Mate´riaux et Techniques 1011 (1994) 19.