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Pulse-electrodeposited NiP–SiC composite coatings
Electrochimica Acta 114 (2013) 851–858
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pulse-electrodeposited NiP–SiC composite coatings W.E.G. Hansal a,∗ , G. Sandulache a , R. Mann a , P. Leisner b a b
Happy Plating GmbH, Surface Finishing, Viktor Kaplan Straße 2, A-2700 Wiener Neustadt, Austria School of Engineering, Jönköping University, SE-551 11 Jönköping, Sweden
a r t i c l e
i n f o
Article history: Received 28 March 2013 Received in revised form 22 August 2013 Accepted 22 August 2013 Available online 9 September 2013 Keywords: Composite coating Ni–P matrix SiC particles Pulse plating
a b s t r a c t This paper describes the effect of modulated bipolar current (pulse reverse plating) on the incorporation of micron and submicron sized SiC particles within an electrodeposited Ni–P alloy matrix (dispersion coating). Based on electrochemical measurements, a pulse plating process has been defined and the effects of pulse parameters (type of current, frequency of current pulses and current density), the electrolyte composition and the size of the silicon carbide on the particles incorporation rate, phosphorus co-deposition rate, surface morphology, structure, micro hardness and wear resistance of the deposits has been investigated. The experimental results show that the phosphorus co-deposition and the particles incorporation rate decrease applying higher current density. The reduction of particle size decreases the co-deposition content of the particles within the coating. Application of pulsed current leads to a more compact composite coating, significantly improving the hardness and the tribological behaviour of the Ni/SiC deposits, mainly at higher frequency of the applied current pulses. DC and bipolar pulses generate unfavourable higher codeposition rate of phosphorus, hence a loss in hardness has been observed. Tailored shift of the properties and alloy composition during the deposition process can be achieved by change of matrix properties via alternation of the pulse sequences. © 2013 Published by Elsevier Ltd.
1. Introduction Electrochemically produced dispersion coatings within a nickel matrix have proven their technical importance as wear resistant coatings in many applications over the last decades. With increasing demands on multi-functionality e.g. additional excellent corrosion resistance, the transfer from a single metal matrix towards metal alloys becomes more and more important. Beside alloys such as Ni–Co [1] the “classical” Ni–P alloys, providing hardness as well as an increased corrosion resistance, are suggested for this purpose. Electrolytic Ni–P systems are favored over chemical (electroless) deposition systems, since electrolytic process routes are providing higher deposition rates, enhanced electrolyte stability and in case of dispersion coatings a more precise control of particle incorporation. Applying pulse deposition techniques allows tailoring the alloy matrix structure and composition a well as the particle incorporation. Dispersion layer systems produced in this way are extremely compact and durable [2–4]. Metal matrix composite coatings produced by electrochemical methods consist in embedding of small particles into the electrodeposited matrix during the electroplating process. The major
∗ Corresponding author. Tel.: +4369915111555; fax: +4326222384240. E-mail address: [email protected] (W.E.G. Hansal). 0013-4686/$ – see front matter © 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2013.08.182
challenge for the co-deposition of the solid particles appears to be the incorporation of sufficient amount of the particles combined with a homogeneously distribution without agglomeration of the particles. Electrodeposited composite coatings based on a Ni–P alloy matrix containing different fine particles of ceramic or polymer compounds have attracted attention due to their good mechanical and chemical properties including high hardness and enhanced wear resistance combined with a good corrosion resistance, especially when compared to pure nickel coatings. Ni–P alloys with phosphorus content over 9 wt% are considered amorphous [5,6]. After heat treatment at 400 ◦ C the amorphous phase is crystallized in Ni, Ni3 P and Ni2 P phases [7]. The effect of sulphur-containing organic additives on the electrodeposition of Ni–P alloy was also studied [8]. Introduction of saccharin into the electrolyte resulted in an enhancement of the current efficiency to about 80%, incorporation of sulphur in the deposit and decrease in both SiC and phosphorus amount. Ni–P electrodeposition is usually performed in the electrolytes containing Ni2+ ions and phosphorus acid, which provides the source of phosphorus in the deposit. A strong pH dependency is determining this deposition mechanism, since hydrogen has a significant role for the reduction of the P to form the nickel–phosphorous alloy. Two reaction mechanisms, direct and indirect have been proposed for the co-deposition of phosphorous
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into nickel matrix [9–11]. According to the half reaction, Ni2+ and protons can be reduced directly at the cathode in the nickel electrolyte containing phosphorus acid (reactions (1)–(3)). Ni2+ + 2e− → Ni
(1)
H+ + e− → Hads
(2)
Hads + Hads → H2
(3)
In the direct Ni–P deposition mechanism, H3 PO3 is electrochemically reduced to elemental phosphorus, either directly or via Hads (overall reaction (4)). H3 PO3 + 3H+ + 3 e− → P + 3H2 O
(4)
For the indirect reaction mechanism, H3 PO3 is reduced to phosphine (PH3 ), which in the presence of Ni2+ is oxidized to phosphorus, as shown in reactions ((5)–(7)) 6H+ + 6e− → 6 Hads
(5)
H3 PO3 + 6Hads → PH3 + 3H2 O
(6)
2PH3 + 3Ni2+ → 3Ni + 2P + 6H+
(7)
According to reactions (4)–(7), the reduction of protons at the surface of the growing layer is necessary for the co-deposition of phosphorus for both proposed reaction mechanisms. For example, more H+ adsorbed on the surface of the electrode (cathode) promotes the reduction of H3 PO3 to phosphorus via reaction (4). While H+ reduction is necessary for co-deposition of phosphorus according to both mechanisms, the concurrent recombination of Hads (reaction (3)) limits the amount of phosphorus that can be deposited. At higher current densities the recombination becomes dominant and the rate of phosphorous deposition decreases. Therefore, it can be expected that the surface H+ concentration on the growing deposit affects both the phosphorus content of the deposit and the current efficiency. The efficiency of such systems therefore will never reach values close to 100% as long as phosphorous will be co-deposited. Due to this special deposition mechanism the evolution of hydrogen gas is unavoidable. The resulting coverage of the electrode surface by the hydrogen bubbles can hinder the incorporation of SiC particles [2]. Pulse plating techniques can mitigate this effect by allowing the hydrogen gas to leave the electrode surface during off-time. Within this work, performed within a multinational MNT Eranet project, both micron and submicron sized SiC particles were incorporated into nickel–phosphorus (Ni–P) alloy matrix systems deposited by direct current and pulse plating from a sulphate based electrolyte. The change of particle and phosphorus content within the composite coating was set in relationship to the pulse parameters applied. 2. Experimental NiP matrix composite coatings were produced from a sulphatebased electrolyte with addition of micron and submicron sized silicon carbide as reinforcing agent. The plating electrolyte contained nickel sulphate as the Ni source, phosphorous acid as the P source and nickel chloride to support the dissolution of the anode. The plating electrolyte was mixed with SiC powder with mean size values (D90) of 0.8 m and 1.8 m and magnetically stirred for 24 h before electroplating. The zeta potential measurements were performed to investigate the stability of the particle dispersion within the electrolyte and the affinity of the dispersed particles towards the cathode in the applied field considering the
Table 1 Electro-deposition conditions. Temperature pH Particle load Current density Type of current Pulse frequency Substrate Anode
50 ◦ C 1–2 150 g/l 5–25 A/dm2 DC, PP 1–100 Hz Steel platelet Nickel
full pH range investigated. These measurements have been carried out using a Colloidal Dynamics ZetaProbe device, which uses a purpose-built ultrasonic and an electro-acoustic sensor. An electrochemical cell with a three-electrode configuration was used for the electrochemical characterization of the electrolyte. A steel rotating disk electrode, Hg/Hg2 SO4 /SO4 2− and a nickel platelet was used as working, reference and counter electrode, respectively. A Jaissle PGU 20V-2A-E potentiostat/galvanostat system controlled by a EcmWin software package was used for this purpose. A portable PCbased digital SDS 200/SoftScope oscilloscope was used to observe the signal voltages for the applied pulses at the pulse-potential measurements. A computer controlled pulse reverse power supply system (Plating Electronic pe86) was used for the plating experiments. Samples for surface morphology, composition investigation, structure examination and micro-hardness measurements were electrodeposited on steel platelets with an area of 9 cm2 (3 cm × 3 cm). Samples for wear resistance tests were electrodeposited on steel disks. Prior to deposition, the steel substrates were electrolytically degreased in a Ekasit 2030 (Metallchemie) solution, rinsed with the distilled water, activated in 10% hydrochloric acid solution and rinsed again with distilled water. To avoid any possible influence of the substrate the thickness of the deposits was about 30 m or more. To evaluate the cathode current efficiency, the samples were weighed before and after electroplating. Once the mass and the composition of the dispersion coatings plated with a given charge were known, the current efficiency could be calculated on the assumption that reduction of one Ni2+ and H3 PO3 to nickel and phosphorous involves the transfer of two and three electrons, respectively. The deposited sample was also cross-sectioned and metallographically mounted with conductive phenolic resin powder. After mechanical grinding and polish, the specimen was chemically etched to reveal the microstructure. The etching solution contained 10 g copper sulphate, 20 ml nitric acid 65% and 10 ml distilled water. Additionally the layer thickness was measured by examination of these cross sections. An overview of the deposition parameter for the preparation of the composite coatings is presented in Table 1. The composition and the particle distribution of the composite coating were examined using a scanning electron microscope (FESEM, Hitachi S4800) in secondary electrons (SE) mode. Additionally, the amount of the particles in the coating was gravimetrically verified after dissolution of the coating. The surface hardness of the deposits was measured by a Vickers micro-hardness tester under a given load of 50 g (HV0.05) and a duration of 15 s. Ten measurements were made for every sample and averaged. Cylinder (100Cr bearing steel) on disk (line contact) wear resistance tests were carried out by a SRV wear tester at normal load of 50 N, oscillating movement at the frequency of 1 Hz at room temperature. The electrochemical depositions were performed applying pulse plating techniques, since apart from the electrolyte composition, shape, amount and type of dispersed particles, the composite coating properties can be adjusted by the variation of the pulse plating parameters such as current density, pulse frequency and duty cycle.
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The observed resulting potentials varied with the applied current densities. The system responds within reasonable time intervals to the pulses applied due to relatively low charging and discharging times of the electrolytic double layer. While at a pulse length of 20 ms the pulse will be slightly dampened, at 50 ms pulse time, this dampening effect for the pulses can be neglected [3,12] (Fig. 2). 3.2. Zeta potential measurements
Fig. 1. Cathodic polarization curves for NiP/SiC system vs. MSE (mercurous sulfate electrode) measured on a rotating disk electrode at 50 ◦ C and pH = 1.2 at different electrode rotation speeds: 0 rpm, 300 rpm, 600 rpm and 1000 rpm with a sweep rate of 10 mV/s.
DC plating was performed within this system as a reference deposition. 3. Results and discussion 3.1. Electrochemical electrolyte characterization The development of an applicable pulse sequence demands a thorough evaluation of the electrolyte system on kinetics, passivation, stability, as well as consideration of substrate material and geometry. The polarization curves contain the most valuable information since they reflect the nature of the whole deposition process and provide information on the deposition mechanism. Fig. 1 shows the polarization curves for the cathodic regime measured on a rotating disk electrode for the NiP/SiC dispersion electrolyte investigated at different rotation speeds at a sweep rate of 10 mV/s. At a stationary electrode, NiP deposition starts at −950 mV and shifts to less cathodic potentials with increasing rotation speed. The diffusion limited current, which occurs when every ion that reaches the electrode surface is reduced and normally shows itself as a plateau in the polarization curve, is not visible due to the overlapping of metal reduction with the hydrogen gas evolution. According to both proposed indirect and direct reaction mechanisms [9–11], the H+ at the surface of the growing deposit substantially influences the co-deposition of phosphorus. A proper electrolyte agitation during the electrodeposition has to be sustained so that all particles are well dispersed. A compromise between sufficient transport of the particles toward the electrode surface and the disturbance of the particles due to strong agitation usually has to be found. Pulse potential measurements were carried out to investigate the response of the dispersion electrolyte on pulse plating. These measurements enable a preliminary estimation of the maximum applicable pulse frequency. The speed of the potential changes at the electrode surface at fast current pulses and the time of the charge/discharge of the electrolytic double layer are crucial system properties that determine the response to applied current pulses. The time required for discharge of the double layer should be much shorter than the on-time and the off-time between the pulses. Pulses in the frequency range, where capacitive effects are relevant affect the amplitude of the pulses and hence, the structure and the properties of the deposit.
The electrochemical reaction mechanism of the dispersion coating process can be related to the zeta potential, which depends on the nature of the particles as well as the electrolyte composition [13–15]. The zeta potential is the scientific term for the electrokinetic potential of a non-conductive surface within a liquid medium. If the particles in suspension have a large negative or positive zeta potential than they will tend to repel each other and there is no tendency to flocculate and the dispersions are electrically stabilized. Zeta potential is not measurable directly but it can be calculated using theoretical models. Using ultrasonic and electroacoustic techniques it was possible to determine the zeta potential for the SiC micron and submicron sized particles in the NiP electrolyte. Electroacoustic techniques have the advantage of being able to perform measurements in intact samples of conductive electrolyte, without need of high dilution. A standard potentiometric titration method was used for determination of the iso-electric point for the micron and submicron sized SiC particles at ambient temperature. The results show that the particles are positively charged at the operating pH value range, which imply that the SiC particles can migrate toward the cathode during the electroplating. An important parameter, which influences the zeta potential and derives from the mathematical treatment of Stern is the thickness of the double layer. Since the investigated NiP dispersion electrolyte is highly concentrated, the electrical double layer thickness of SiC particle will be compressed and this has a major impact on the zeta potential. For micron sized SiC particles a zeta potential of about 7 mV at a pH value of 1.2 was obtained. Submicron sized SiC particles exhibit slightly lower zeta potentials with a determined value of about 6 mV. Positive zeta potential values at lower pH may be correlated to a high activity of protons, which can adsorb on specific chemical groups on the SiC particles producing a positive surface charge [16]. Dablé et al. [16] investigated the electrochemical behaviour of the SiC particles surface in salt solution at pH range of 1.5–4.0 (chloride and perchlorate solutions, nickel chloride solution, nickel sulfate solution) and found out that no nickel ion adsorbs at the particles surface in the pH range considered. In the papers dealing with the adsorption of nickel ions on SiC, the working pH was higher than 4. Thus the increase in zeta potential with increased pH of the electrolyte can be explained by the adsorption of Ni2+ ions on the SiC particles. Increasing the pH of the electrolyte by addition of sodium hydroxide led to a slight increase in zeta potential for both micron and submicron sized particles (Fig. 3) up to a pH value of 5.5. In the pH range of 4–5.5 there is a plateau with the maximal zeta potential, further increasing the pH leads to a drop of the zeta potential. These measurements suggested that SiC powder has a tendency to adsorbs the Ni2+ ions in the pH range of 4–6 of the dispersion solution. When the pH value is higher than 6.0, Ni2+ begins to precipitate as hydrated Ni(OH)2 and the zeta potential decreases. The iso-electric point (IEP) of the SiC, when the particle surface carries no net electrical charge is at pH 7.1 for micron sized SiC and at pH 7.4 for submicron sized SiC (Fig. 3). At even higher pH values the SiC particles are negatively charged and the dispersion deposition will be hindered. At such pH values, some additive for particles surface modification would be needed. Since these additives might
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Fig. 2. Potential–time curves obtained during a pulsed current sequence vs. MSE (mercurous sulfate electrode) at 50 ◦ C and pH 1.2 for NiP/SiC system, plotted for different current densities for: (a) 20 ms on and 20 ms off and (b) 50 ms on and 50 ms off.
interfere with the phosphorous co-deposition, the pH of the electrolyte was kept lower without surface-active agents. The developed NiP/SiC dispersion electrolyte is based on an electrolyte designed to produce thin nickel–phosphorus deposits under DC plating conditions. The specified pH operating range of this electrolyte is between 1.0 and 1.6. At values of pH higher than 2.0 the phosphor content in the deposit drops markedly and the properties of the layer downgrades (embrittlement). The composition of the commercial electrolyte was modified so that pulse plating can be applied and in combination with particle incorporation (SiC) we were able to obtain thick deposits, which exhibit higher hardness and enhanced wear resistance. Plating at pH 4–5.5 may favour the incorporation of the SiC particles but the co-deposition of phosphorus is hindered. 3.3. SiC content, P-content and hardness The phosphorus content of NiP/SiC composits decreases sharply with increase of pH of the electrolyte while for SiC particles the obtained weight percentage in the composite coating shows no definite trend with alteration of pH in the region of 1 to 2 (Fig. 4). The current efficiency as a function of pH value of the dispersion electrolyte is plotted in Fig. 5 for deposition under pulse plating conditions. The dependency of current efficiency with temperature has been not investigated so far but it will be subject of further research. According to Fig. 5 the plating efficiency seems to be enhanced by applying unipolar pulses in comparison with bipolar pulses and DC plating.
Fig. 3. Dependency of zeta potential on the pH for submicron sized SiC particles within the used nickel–phosphorus electrolyte (sulphate based) at ambient temperature.
Fig. 4. Micron sized SiC content and the P content vs. pH for the composite coatings electroplated at 50 ◦ C applying bipolar pulse sequence PP3 at 83 Hz with average current density of 10 A/dm2 .
The current efficiency seems to mainly depend on the two parameters pH value and pulse current density. With decreasing pH value the current efficiency drops accordingly. Bipolar pulse plating at the same average current density and pulse frequency as unipolar pulse plating requires a higher cathodic current density resulting in an efficiency drop. DC values were added as reference and are at
Fig. 5. Cathode current efficiency vs. pH value for NiP/SiC micron sized pulse deposition with unipolar pulse being pulse sequence PP2 at 83 Hz and bipolar pulse being pulse sequence PP3 at 83 Hz.
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Fig. 6. Effect of the pulse frequency on SiC and P content for the composite coatings electroplated at 50 ◦ C and pH 1.6 applying unipolar pulses with 20% duty cycle at 25 A/dm2 pulse height.
lower pH values between unipolar and bipolar. At higher pH values, the DC reference showed a much lower relative efficiency. The current efficiency of the Ni–P deposits with high phosphorus content is typically less than 50% [17,18]. This loss in current efficiency is associated with the hydrogen gas evolution according to reaction (3). A lower and more stabile surface pH can be achieved using the pulse current with low duty cycle and high frequency [11]. A decrease in frequency below 5 Hz with 20% duty cycle results in a higher particle incorporation rate (Fig. 6). This might be due to the fact that a lower duty cycle and a lower frequency means a relatively longer (off-)time that gives the system sufficient time for particle diffusion to and adsorption at the electrode (cathode) surface and thus leads to enhanced incorporation rate of the particles within the growing metal matrix. The P-content and SiC incorporation rate obtained at higher frequencies than 5 Hz for 20% duty cycle appears to be constant in the analyzed frequency range. Fig. 7 displays the appearance of the surface of NiP/SiC composite coatings using unipolar pulse plating at different pulse frequencies. At a lower frequency (b) the composite coating is smooth and exhibits higher SiC particle content and the phosphorus amount is also increased. At high pulse frequencies (a) the surface roughness is increased and the particle incorporation rate significantly decreased. While hardness of the metal matrix will increase with increasing pulse frequency (see below), the incorporation of the particles seems to be hindered at short pulse times. As noticed for the length of the off-time above, the systems seems to require a certain time period for the particles to be incorporated into the metal matrix. If the pulses (and off-times) are below a threshold value,
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Fig. 8. Micron sized SiC content, P content and hardness for composite coatings plated under different current condition at 50 ◦ C, pH 1.2 and 10 A/dm2 (average) current density, sample 1 being DC, sample 2 and sample 3 unipolar pulses, sample 4 and sample 5 bipolar pulses.
the entrapment of particles cannot follow and particles will not be incorporated. Fig. 8 shows that DC Plating (sample 1) and bipolar pulse plating (sample 4 and sample 5) generates more phosphorous in the composite coating, which leads to a decrease in hardness. The microhardness increases to a maximum of 706HV 0.05 for the composite coatings with lower phosphorous content and a medium particle incorporation rate electrodeposited applying unipolar pulses (sample 2 and sample 3). Decrease of the average particle size resulted in a decrease of the weight percentage of the embedded SiC. These composite coatings exhibit slightly lower hardness than the Ni–P deposits with micron sized SiC particles. The variation of the electrolyte composition showed that less phosphorus (in form of phosphorous acid) in the electrolyte will lead to less phosphorus in the deposit. The cross section of polished NiP/SiC (micron sized particles) composite coatings obtained at different plating pulses (DC, unipolar, bipolar) at higher (average) current density are shown in Fig. 9. The particle distribution through the Ni–P–SiC coatings electrodeposited under both DC and PP conditions appears to be homogeneous. By modifying the current parameters different percentages of embedded particles were obtained. Applying unipolar pulses (sample PP1 and PP2) leads to an increased incorporation rate of the particles from 2.6 wt% SiC at DC to 3.4 wt% SiC for the pulse sequence PP1 and to 4.3 wt% SiC for the pulse sequence PP2. The difference between PP1 and PP2 has been the duty cycle (the ratio between the length of the cathodic pulse and the length of
Fig. 7. SEM images of the surface topography obtained at different pulse frequency at 50 ◦ C and pH 1.6: (a) 10 Hz and (b) 1 Hz.
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Fig. 9. SEM micrographs of the polished cross section of NiP/SiC composite layers electroplated under different current condition at 20 A/dm2 , 50 ◦ C and pH 1.6, DC being direct current, PP1 unipolar pulse at 83 Hz and 33% duty cycle, PP2 unipolar pulse at 83 Hz and 50% duty cycle and PP3 bipolar pulse at 83 Hz.
Fig. 10. Etched dispersion coatings electroplated under different current conditions at 50 ◦ C and pH 1.6: (a) columnar microstructure for unipolar pulse at 83 Hz and 33% duty cycle (PP1) and (b) lamellar microstructure for bipolar pulse at 83 Hz (PP3).
the total pulse sequence), which has been 33% and 50%, respectively. The application of bipolar pulses (PP3) generates comparable embedding percentage to DC. Metallographic etching of the dispersion coatings indicates differences in the microstructure depending on the applied current signal (see Fig. 10). While a simple unipolar pulse (PP1) will favour a columnar growth, the application of bipolar pulses (including anodic reverse pulses) changes the substructure to a lamellar type of microstructure. Fig. 11 shows that both the phosphorus content and the particles incorporation rate in the deposit decreased with increasing the applied current density. The dispersion coatings with low phosphorus and medium amount of particles exhibit excellent hardness. It seems that higher current densities support the formation of hydrogen bubbles according to reaction (3) and thus less H+ , which promotes the reduction of phosphorous acid to elemental phosphorus is adsorbed on the cathode surface. Additionally, an interference of particle incorporation with the increased hydrogen evolvement might occur so that the particles do not have enough time to be
Fig. 11. SiC particle (micron sized) content, P content in the layer and micro hardness applying unipolar pulse deposition (PP1 at 83 Hz and 33% duty cycle) at 50 ◦ C and pH 1.6 for various average current densities.
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Fig. 12. Profile of the wear tracks after tribo tests for the Ni–P and NiP/SiC coatings: (a) DC Ni–P layer without SiC particles, (b) pulsed Ni–P layer without SiC particles, (c) pulsed Ni–P layer with submicron sized SiC particles, (d) pulsed Ni–P layer with micron sized SiC particles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
incorporated after being loosely adsorbed on the electrode surface and hence the particle incorporation rate decreases with higher current densities [2]. 3.4. Tribological behaviour The modification of metal coatings by the incorporation of SiC particles will enhance the hardness and wear resistance of such coatings. Pulse plating is expected to support this effect [3]. For evaluation of the change in wear resistance due to the SiC incorporation, the obtained layers were analysed using the cylinder on disc method (see Section 2). For this evaluation standardized steel cylinders were coated with a 20 m NiP layer including SiC particles. DC and pulse plating NiP layers without particles were used as reference and for evaluation of the impact of the pulse plating process on the stability of the pure Ni–P matrix. The tested deposits were produced at 50 ◦ C and pH 1.6. Sample (b), sample (c) and sample (d) were deposited applying unipolar pulse sequence PP2 at 83 Hz and 50% duty cycle. Fig. 12 shows the wear performance of the NiP and NiP/SiC deposits. The wear track profile appears coloured with dark scars. The Ni–P deposit obtained at direct current condition (sample a) shows the highest abrasion with wear tracks of about 6 m in depth, being twice as high as the measured track depth for the pulsed Ni–P deposit (sample b). Sample c (NiP with 1.5 wt% SiC submicron sized particles) and sample d (NiP with 2.6 wt% SiC micron sized particles) electrodeposited under pulsed current (PP2) at 83 Hz with 50% duty cycle show no measurable wear along the track. The pulsed composite coatings are more compact and exhibit better tribological performance. Incorporation of the SiC particles into Ni–P matrix applying pulsed current has significantly enhanced the tribological properties of the obtained composite coatings. 4. Conclusions In this work the impact of pulse plating on the particle incorporation, phosphorus content, microstructure and tribological properties of NiP/SiC dispersion coatings was investigated. Basic electrochemical measurements revealed a fast electrolyte response to applied current pulses, showing no damping effects of the signal within acceptable pulse times. When phosphorus shall
be incorporated into the nickel matrix, forming a NiP alloy layer, the efficiency of the deposition drops due to hydrogen evolution. Lower pH values of the dispersion electrolyte generate higher content of phosphorus in the deposited composite coating. Higher amount of phosphorus in the deposit goes hand in hand with decrease in current efficiency observed at these lower pH values. Particles incorporation rate shows no significantly dependency within the investigated pH range. Applying pulsed current at higher pH values leads to an enhanced current efficiency compared to direct current deposition. Tailored shift of the alloy composition and coating properties during the deposition process can be achieved via alternation of the pulse sequences, the pulse parameters have a direct influence on alloy composition (phosphorous content) and particle incorporation rate. Pulse electrodeposition at low frequencies and low duty cycles favors the incorporation of the particles into the Ni–P metal matrix. Applying unipolar pulses, especially at higher frequencies leads to improved composite hardness and wear resistance. Bipolar pulses generate higher amount of phosphorus in the deposit, thus a decrease in hardness was noticed. Generally, an increase in current density will result in a decrease in both phosphorus content in the layer and particle incorporation rate. Decrease of the average particle size resulted in a decrease of the weight percentage of the SiC particles embedded. Composite NiP/SiC deposits prepared under pulse current condition exhibit higher hardness values than DC layers. Hardness of the layers does strongly depend on pulse waveform and Pcontent. The resulting wear resistance does basically go with the micro hardness for pure NiP coatings (no particles incorporated). When particles are present within the coating, the particle load will have the major effect on the wear resistance as long as the coatings are homogenous and the particles are well embedded. Best results regarding higher hardness and enhanced wear resistance were obtained applying unipolar pulses with 83 Hz at 30–50% duty cycle at 50 ◦ C and pH 1.6. The resulting optimal deposits contained 6–8 wt% P and 2.5 wt% SiC particles. Acknowledgment This work was financially supported by the European MNT Eranet program, VINOVA and FFG.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Pulse-electrodeposited NiP–SiC composite coatings W.E.G. Hansal a,∗ , G. Sandulache a , R. Mann a , P. Leisner b a b
Happy Plating GmbH, Surface Finishing, Viktor Kaplan Straße 2, A-2700 Wiener Neustadt, Austria School of Engineering, Jönköping University, SE-551 11 Jönköping, Sweden
a r t i c l e
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Article history: Received 28 March 2013 Received in revised form 22 August 2013 Accepted 22 August 2013 Available online 9 September 2013 Keywords: Composite coating Ni–P matrix SiC particles Pulse plating
a b s t r a c t This paper describes the effect of modulated bipolar current (pulse reverse plating) on the incorporation of micron and submicron sized SiC particles within an electrodeposited Ni–P alloy matrix (dispersion coating). Based on electrochemical measurements, a pulse plating process has been defined and the effects of pulse parameters (type of current, frequency of current pulses and current density), the electrolyte composition and the size of the silicon carbide on the particles incorporation rate, phosphorus co-deposition rate, surface morphology, structure, micro hardness and wear resistance of the deposits has been investigated. The experimental results show that the phosphorus co-deposition and the particles incorporation rate decrease applying higher current density. The reduction of particle size decreases the co-deposition content of the particles within the coating. Application of pulsed current leads to a more compact composite coating, significantly improving the hardness and the tribological behaviour of the Ni/SiC deposits, mainly at higher frequency of the applied current pulses. DC and bipolar pulses generate unfavourable higher codeposition rate of phosphorus, hence a loss in hardness has been observed. Tailored shift of the properties and alloy composition during the deposition process can be achieved by change of matrix properties via alternation of the pulse sequences. © 2013 Published by Elsevier Ltd.
1. Introduction Electrochemically produced dispersion coatings within a nickel matrix have proven their technical importance as wear resistant coatings in many applications over the last decades. With increasing demands on multi-functionality e.g. additional excellent corrosion resistance, the transfer from a single metal matrix towards metal alloys becomes more and more important. Beside alloys such as Ni–Co [1] the “classical” Ni–P alloys, providing hardness as well as an increased corrosion resistance, are suggested for this purpose. Electrolytic Ni–P systems are favored over chemical (electroless) deposition systems, since electrolytic process routes are providing higher deposition rates, enhanced electrolyte stability and in case of dispersion coatings a more precise control of particle incorporation. Applying pulse deposition techniques allows tailoring the alloy matrix structure and composition a well as the particle incorporation. Dispersion layer systems produced in this way are extremely compact and durable [2–4]. Metal matrix composite coatings produced by electrochemical methods consist in embedding of small particles into the electrodeposited matrix during the electroplating process. The major
∗ Corresponding author. Tel.: +4369915111555; fax: +4326222384240. E-mail address: [email protected] (W.E.G. Hansal). 0013-4686/$ – see front matter © 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2013.08.182
challenge for the co-deposition of the solid particles appears to be the incorporation of sufficient amount of the particles combined with a homogeneously distribution without agglomeration of the particles. Electrodeposited composite coatings based on a Ni–P alloy matrix containing different fine particles of ceramic or polymer compounds have attracted attention due to their good mechanical and chemical properties including high hardness and enhanced wear resistance combined with a good corrosion resistance, especially when compared to pure nickel coatings. Ni–P alloys with phosphorus content over 9 wt% are considered amorphous [5,6]. After heat treatment at 400 ◦ C the amorphous phase is crystallized in Ni, Ni3 P and Ni2 P phases [7]. The effect of sulphur-containing organic additives on the electrodeposition of Ni–P alloy was also studied [8]. Introduction of saccharin into the electrolyte resulted in an enhancement of the current efficiency to about 80%, incorporation of sulphur in the deposit and decrease in both SiC and phosphorus amount. Ni–P electrodeposition is usually performed in the electrolytes containing Ni2+ ions and phosphorus acid, which provides the source of phosphorus in the deposit. A strong pH dependency is determining this deposition mechanism, since hydrogen has a significant role for the reduction of the P to form the nickel–phosphorous alloy. Two reaction mechanisms, direct and indirect have been proposed for the co-deposition of phosphorous
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into nickel matrix [9–11]. According to the half reaction, Ni2+ and protons can be reduced directly at the cathode in the nickel electrolyte containing phosphorus acid (reactions (1)–(3)). Ni2+ + 2e− → Ni
(1)
H+ + e− → Hads
(2)
Hads + Hads → H2
(3)
In the direct Ni–P deposition mechanism, H3 PO3 is electrochemically reduced to elemental phosphorus, either directly or via Hads (overall reaction (4)). H3 PO3 + 3H+ + 3 e− → P + 3H2 O
(4)
For the indirect reaction mechanism, H3 PO3 is reduced to phosphine (PH3 ), which in the presence of Ni2+ is oxidized to phosphorus, as shown in reactions ((5)–(7)) 6H+ + 6e− → 6 Hads
(5)
H3 PO3 + 6Hads → PH3 + 3H2 O
(6)
2PH3 + 3Ni2+ → 3Ni + 2P + 6H+
(7)
According to reactions (4)–(7), the reduction of protons at the surface of the growing layer is necessary for the co-deposition of phosphorus for both proposed reaction mechanisms. For example, more H+ adsorbed on the surface of the electrode (cathode) promotes the reduction of H3 PO3 to phosphorus via reaction (4). While H+ reduction is necessary for co-deposition of phosphorus according to both mechanisms, the concurrent recombination of Hads (reaction (3)) limits the amount of phosphorus that can be deposited. At higher current densities the recombination becomes dominant and the rate of phosphorous deposition decreases. Therefore, it can be expected that the surface H+ concentration on the growing deposit affects both the phosphorus content of the deposit and the current efficiency. The efficiency of such systems therefore will never reach values close to 100% as long as phosphorous will be co-deposited. Due to this special deposition mechanism the evolution of hydrogen gas is unavoidable. The resulting coverage of the electrode surface by the hydrogen bubbles can hinder the incorporation of SiC particles [2]. Pulse plating techniques can mitigate this effect by allowing the hydrogen gas to leave the electrode surface during off-time. Within this work, performed within a multinational MNT Eranet project, both micron and submicron sized SiC particles were incorporated into nickel–phosphorus (Ni–P) alloy matrix systems deposited by direct current and pulse plating from a sulphate based electrolyte. The change of particle and phosphorus content within the composite coating was set in relationship to the pulse parameters applied. 2. Experimental NiP matrix composite coatings were produced from a sulphatebased electrolyte with addition of micron and submicron sized silicon carbide as reinforcing agent. The plating electrolyte contained nickel sulphate as the Ni source, phosphorous acid as the P source and nickel chloride to support the dissolution of the anode. The plating electrolyte was mixed with SiC powder with mean size values (D90) of 0.8 m and 1.8 m and magnetically stirred for 24 h before electroplating. The zeta potential measurements were performed to investigate the stability of the particle dispersion within the electrolyte and the affinity of the dispersed particles towards the cathode in the applied field considering the
Table 1 Electro-deposition conditions. Temperature pH Particle load Current density Type of current Pulse frequency Substrate Anode
50 ◦ C 1–2 150 g/l 5–25 A/dm2 DC, PP 1–100 Hz Steel platelet Nickel
full pH range investigated. These measurements have been carried out using a Colloidal Dynamics ZetaProbe device, which uses a purpose-built ultrasonic and an electro-acoustic sensor. An electrochemical cell with a three-electrode configuration was used for the electrochemical characterization of the electrolyte. A steel rotating disk electrode, Hg/Hg2 SO4 /SO4 2− and a nickel platelet was used as working, reference and counter electrode, respectively. A Jaissle PGU 20V-2A-E potentiostat/galvanostat system controlled by a EcmWin software package was used for this purpose. A portable PCbased digital SDS 200/SoftScope oscilloscope was used to observe the signal voltages for the applied pulses at the pulse-potential measurements. A computer controlled pulse reverse power supply system (Plating Electronic pe86) was used for the plating experiments. Samples for surface morphology, composition investigation, structure examination and micro-hardness measurements were electrodeposited on steel platelets with an area of 9 cm2 (3 cm × 3 cm). Samples for wear resistance tests were electrodeposited on steel disks. Prior to deposition, the steel substrates were electrolytically degreased in a Ekasit 2030 (Metallchemie) solution, rinsed with the distilled water, activated in 10% hydrochloric acid solution and rinsed again with distilled water. To avoid any possible influence of the substrate the thickness of the deposits was about 30 m or more. To evaluate the cathode current efficiency, the samples were weighed before and after electroplating. Once the mass and the composition of the dispersion coatings plated with a given charge were known, the current efficiency could be calculated on the assumption that reduction of one Ni2+ and H3 PO3 to nickel and phosphorous involves the transfer of two and three electrons, respectively. The deposited sample was also cross-sectioned and metallographically mounted with conductive phenolic resin powder. After mechanical grinding and polish, the specimen was chemically etched to reveal the microstructure. The etching solution contained 10 g copper sulphate, 20 ml nitric acid 65% and 10 ml distilled water. Additionally the layer thickness was measured by examination of these cross sections. An overview of the deposition parameter for the preparation of the composite coatings is presented in Table 1. The composition and the particle distribution of the composite coating were examined using a scanning electron microscope (FESEM, Hitachi S4800) in secondary electrons (SE) mode. Additionally, the amount of the particles in the coating was gravimetrically verified after dissolution of the coating. The surface hardness of the deposits was measured by a Vickers micro-hardness tester under a given load of 50 g (HV0.05) and a duration of 15 s. Ten measurements were made for every sample and averaged. Cylinder (100Cr bearing steel) on disk (line contact) wear resistance tests were carried out by a SRV wear tester at normal load of 50 N, oscillating movement at the frequency of 1 Hz at room temperature. The electrochemical depositions were performed applying pulse plating techniques, since apart from the electrolyte composition, shape, amount and type of dispersed particles, the composite coating properties can be adjusted by the variation of the pulse plating parameters such as current density, pulse frequency and duty cycle.
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The observed resulting potentials varied with the applied current densities. The system responds within reasonable time intervals to the pulses applied due to relatively low charging and discharging times of the electrolytic double layer. While at a pulse length of 20 ms the pulse will be slightly dampened, at 50 ms pulse time, this dampening effect for the pulses can be neglected [3,12] (Fig. 2). 3.2. Zeta potential measurements
Fig. 1. Cathodic polarization curves for NiP/SiC system vs. MSE (mercurous sulfate electrode) measured on a rotating disk electrode at 50 ◦ C and pH = 1.2 at different electrode rotation speeds: 0 rpm, 300 rpm, 600 rpm and 1000 rpm with a sweep rate of 10 mV/s.
DC plating was performed within this system as a reference deposition. 3. Results and discussion 3.1. Electrochemical electrolyte characterization The development of an applicable pulse sequence demands a thorough evaluation of the electrolyte system on kinetics, passivation, stability, as well as consideration of substrate material and geometry. The polarization curves contain the most valuable information since they reflect the nature of the whole deposition process and provide information on the deposition mechanism. Fig. 1 shows the polarization curves for the cathodic regime measured on a rotating disk electrode for the NiP/SiC dispersion electrolyte investigated at different rotation speeds at a sweep rate of 10 mV/s. At a stationary electrode, NiP deposition starts at −950 mV and shifts to less cathodic potentials with increasing rotation speed. The diffusion limited current, which occurs when every ion that reaches the electrode surface is reduced and normally shows itself as a plateau in the polarization curve, is not visible due to the overlapping of metal reduction with the hydrogen gas evolution. According to both proposed indirect and direct reaction mechanisms [9–11], the H+ at the surface of the growing deposit substantially influences the co-deposition of phosphorus. A proper electrolyte agitation during the electrodeposition has to be sustained so that all particles are well dispersed. A compromise between sufficient transport of the particles toward the electrode surface and the disturbance of the particles due to strong agitation usually has to be found. Pulse potential measurements were carried out to investigate the response of the dispersion electrolyte on pulse plating. These measurements enable a preliminary estimation of the maximum applicable pulse frequency. The speed of the potential changes at the electrode surface at fast current pulses and the time of the charge/discharge of the electrolytic double layer are crucial system properties that determine the response to applied current pulses. The time required for discharge of the double layer should be much shorter than the on-time and the off-time between the pulses. Pulses in the frequency range, where capacitive effects are relevant affect the amplitude of the pulses and hence, the structure and the properties of the deposit.
The electrochemical reaction mechanism of the dispersion coating process can be related to the zeta potential, which depends on the nature of the particles as well as the electrolyte composition [13–15]. The zeta potential is the scientific term for the electrokinetic potential of a non-conductive surface within a liquid medium. If the particles in suspension have a large negative or positive zeta potential than they will tend to repel each other and there is no tendency to flocculate and the dispersions are electrically stabilized. Zeta potential is not measurable directly but it can be calculated using theoretical models. Using ultrasonic and electroacoustic techniques it was possible to determine the zeta potential for the SiC micron and submicron sized particles in the NiP electrolyte. Electroacoustic techniques have the advantage of being able to perform measurements in intact samples of conductive electrolyte, without need of high dilution. A standard potentiometric titration method was used for determination of the iso-electric point for the micron and submicron sized SiC particles at ambient temperature. The results show that the particles are positively charged at the operating pH value range, which imply that the SiC particles can migrate toward the cathode during the electroplating. An important parameter, which influences the zeta potential and derives from the mathematical treatment of Stern is the thickness of the double layer. Since the investigated NiP dispersion electrolyte is highly concentrated, the electrical double layer thickness of SiC particle will be compressed and this has a major impact on the zeta potential. For micron sized SiC particles a zeta potential of about 7 mV at a pH value of 1.2 was obtained. Submicron sized SiC particles exhibit slightly lower zeta potentials with a determined value of about 6 mV. Positive zeta potential values at lower pH may be correlated to a high activity of protons, which can adsorb on specific chemical groups on the SiC particles producing a positive surface charge [16]. Dablé et al. [16] investigated the electrochemical behaviour of the SiC particles surface in salt solution at pH range of 1.5–4.0 (chloride and perchlorate solutions, nickel chloride solution, nickel sulfate solution) and found out that no nickel ion adsorbs at the particles surface in the pH range considered. In the papers dealing with the adsorption of nickel ions on SiC, the working pH was higher than 4. Thus the increase in zeta potential with increased pH of the electrolyte can be explained by the adsorption of Ni2+ ions on the SiC particles. Increasing the pH of the electrolyte by addition of sodium hydroxide led to a slight increase in zeta potential for both micron and submicron sized particles (Fig. 3) up to a pH value of 5.5. In the pH range of 4–5.5 there is a plateau with the maximal zeta potential, further increasing the pH leads to a drop of the zeta potential. These measurements suggested that SiC powder has a tendency to adsorbs the Ni2+ ions in the pH range of 4–6 of the dispersion solution. When the pH value is higher than 6.0, Ni2+ begins to precipitate as hydrated Ni(OH)2 and the zeta potential decreases. The iso-electric point (IEP) of the SiC, when the particle surface carries no net electrical charge is at pH 7.1 for micron sized SiC and at pH 7.4 for submicron sized SiC (Fig. 3). At even higher pH values the SiC particles are negatively charged and the dispersion deposition will be hindered. At such pH values, some additive for particles surface modification would be needed. Since these additives might
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Fig. 2. Potential–time curves obtained during a pulsed current sequence vs. MSE (mercurous sulfate electrode) at 50 ◦ C and pH 1.2 for NiP/SiC system, plotted for different current densities for: (a) 20 ms on and 20 ms off and (b) 50 ms on and 50 ms off.
interfere with the phosphorous co-deposition, the pH of the electrolyte was kept lower without surface-active agents. The developed NiP/SiC dispersion electrolyte is based on an electrolyte designed to produce thin nickel–phosphorus deposits under DC plating conditions. The specified pH operating range of this electrolyte is between 1.0 and 1.6. At values of pH higher than 2.0 the phosphor content in the deposit drops markedly and the properties of the layer downgrades (embrittlement). The composition of the commercial electrolyte was modified so that pulse plating can be applied and in combination with particle incorporation (SiC) we were able to obtain thick deposits, which exhibit higher hardness and enhanced wear resistance. Plating at pH 4–5.5 may favour the incorporation of the SiC particles but the co-deposition of phosphorus is hindered. 3.3. SiC content, P-content and hardness The phosphorus content of NiP/SiC composits decreases sharply with increase of pH of the electrolyte while for SiC particles the obtained weight percentage in the composite coating shows no definite trend with alteration of pH in the region of 1 to 2 (Fig. 4). The current efficiency as a function of pH value of the dispersion electrolyte is plotted in Fig. 5 for deposition under pulse plating conditions. The dependency of current efficiency with temperature has been not investigated so far but it will be subject of further research. According to Fig. 5 the plating efficiency seems to be enhanced by applying unipolar pulses in comparison with bipolar pulses and DC plating.
Fig. 3. Dependency of zeta potential on the pH for submicron sized SiC particles within the used nickel–phosphorus electrolyte (sulphate based) at ambient temperature.
Fig. 4. Micron sized SiC content and the P content vs. pH for the composite coatings electroplated at 50 ◦ C applying bipolar pulse sequence PP3 at 83 Hz with average current density of 10 A/dm2 .
The current efficiency seems to mainly depend on the two parameters pH value and pulse current density. With decreasing pH value the current efficiency drops accordingly. Bipolar pulse plating at the same average current density and pulse frequency as unipolar pulse plating requires a higher cathodic current density resulting in an efficiency drop. DC values were added as reference and are at
Fig. 5. Cathode current efficiency vs. pH value for NiP/SiC micron sized pulse deposition with unipolar pulse being pulse sequence PP2 at 83 Hz and bipolar pulse being pulse sequence PP3 at 83 Hz.
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Fig. 6. Effect of the pulse frequency on SiC and P content for the composite coatings electroplated at 50 ◦ C and pH 1.6 applying unipolar pulses with 20% duty cycle at 25 A/dm2 pulse height.
lower pH values between unipolar and bipolar. At higher pH values, the DC reference showed a much lower relative efficiency. The current efficiency of the Ni–P deposits with high phosphorus content is typically less than 50% [17,18]. This loss in current efficiency is associated with the hydrogen gas evolution according to reaction (3). A lower and more stabile surface pH can be achieved using the pulse current with low duty cycle and high frequency [11]. A decrease in frequency below 5 Hz with 20% duty cycle results in a higher particle incorporation rate (Fig. 6). This might be due to the fact that a lower duty cycle and a lower frequency means a relatively longer (off-)time that gives the system sufficient time for particle diffusion to and adsorption at the electrode (cathode) surface and thus leads to enhanced incorporation rate of the particles within the growing metal matrix. The P-content and SiC incorporation rate obtained at higher frequencies than 5 Hz for 20% duty cycle appears to be constant in the analyzed frequency range. Fig. 7 displays the appearance of the surface of NiP/SiC composite coatings using unipolar pulse plating at different pulse frequencies. At a lower frequency (b) the composite coating is smooth and exhibits higher SiC particle content and the phosphorus amount is also increased. At high pulse frequencies (a) the surface roughness is increased and the particle incorporation rate significantly decreased. While hardness of the metal matrix will increase with increasing pulse frequency (see below), the incorporation of the particles seems to be hindered at short pulse times. As noticed for the length of the off-time above, the systems seems to require a certain time period for the particles to be incorporated into the metal matrix. If the pulses (and off-times) are below a threshold value,
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Fig. 8. Micron sized SiC content, P content and hardness for composite coatings plated under different current condition at 50 ◦ C, pH 1.2 and 10 A/dm2 (average) current density, sample 1 being DC, sample 2 and sample 3 unipolar pulses, sample 4 and sample 5 bipolar pulses.
the entrapment of particles cannot follow and particles will not be incorporated. Fig. 8 shows that DC Plating (sample 1) and bipolar pulse plating (sample 4 and sample 5) generates more phosphorous in the composite coating, which leads to a decrease in hardness. The microhardness increases to a maximum of 706HV 0.05 for the composite coatings with lower phosphorous content and a medium particle incorporation rate electrodeposited applying unipolar pulses (sample 2 and sample 3). Decrease of the average particle size resulted in a decrease of the weight percentage of the embedded SiC. These composite coatings exhibit slightly lower hardness than the Ni–P deposits with micron sized SiC particles. The variation of the electrolyte composition showed that less phosphorus (in form of phosphorous acid) in the electrolyte will lead to less phosphorus in the deposit. The cross section of polished NiP/SiC (micron sized particles) composite coatings obtained at different plating pulses (DC, unipolar, bipolar) at higher (average) current density are shown in Fig. 9. The particle distribution through the Ni–P–SiC coatings electrodeposited under both DC and PP conditions appears to be homogeneous. By modifying the current parameters different percentages of embedded particles were obtained. Applying unipolar pulses (sample PP1 and PP2) leads to an increased incorporation rate of the particles from 2.6 wt% SiC at DC to 3.4 wt% SiC for the pulse sequence PP1 and to 4.3 wt% SiC for the pulse sequence PP2. The difference between PP1 and PP2 has been the duty cycle (the ratio between the length of the cathodic pulse and the length of
Fig. 7. SEM images of the surface topography obtained at different pulse frequency at 50 ◦ C and pH 1.6: (a) 10 Hz and (b) 1 Hz.
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Fig. 9. SEM micrographs of the polished cross section of NiP/SiC composite layers electroplated under different current condition at 20 A/dm2 , 50 ◦ C and pH 1.6, DC being direct current, PP1 unipolar pulse at 83 Hz and 33% duty cycle, PP2 unipolar pulse at 83 Hz and 50% duty cycle and PP3 bipolar pulse at 83 Hz.
Fig. 10. Etched dispersion coatings electroplated under different current conditions at 50 ◦ C and pH 1.6: (a) columnar microstructure for unipolar pulse at 83 Hz and 33% duty cycle (PP1) and (b) lamellar microstructure for bipolar pulse at 83 Hz (PP3).
the total pulse sequence), which has been 33% and 50%, respectively. The application of bipolar pulses (PP3) generates comparable embedding percentage to DC. Metallographic etching of the dispersion coatings indicates differences in the microstructure depending on the applied current signal (see Fig. 10). While a simple unipolar pulse (PP1) will favour a columnar growth, the application of bipolar pulses (including anodic reverse pulses) changes the substructure to a lamellar type of microstructure. Fig. 11 shows that both the phosphorus content and the particles incorporation rate in the deposit decreased with increasing the applied current density. The dispersion coatings with low phosphorus and medium amount of particles exhibit excellent hardness. It seems that higher current densities support the formation of hydrogen bubbles according to reaction (3) and thus less H+ , which promotes the reduction of phosphorous acid to elemental phosphorus is adsorbed on the cathode surface. Additionally, an interference of particle incorporation with the increased hydrogen evolvement might occur so that the particles do not have enough time to be
Fig. 11. SiC particle (micron sized) content, P content in the layer and micro hardness applying unipolar pulse deposition (PP1 at 83 Hz and 33% duty cycle) at 50 ◦ C and pH 1.6 for various average current densities.
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Fig. 12. Profile of the wear tracks after tribo tests for the Ni–P and NiP/SiC coatings: (a) DC Ni–P layer without SiC particles, (b) pulsed Ni–P layer without SiC particles, (c) pulsed Ni–P layer with submicron sized SiC particles, (d) pulsed Ni–P layer with micron sized SiC particles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
incorporated after being loosely adsorbed on the electrode surface and hence the particle incorporation rate decreases with higher current densities [2]. 3.4. Tribological behaviour The modification of metal coatings by the incorporation of SiC particles will enhance the hardness and wear resistance of such coatings. Pulse plating is expected to support this effect [3]. For evaluation of the change in wear resistance due to the SiC incorporation, the obtained layers were analysed using the cylinder on disc method (see Section 2). For this evaluation standardized steel cylinders were coated with a 20 m NiP layer including SiC particles. DC and pulse plating NiP layers without particles were used as reference and for evaluation of the impact of the pulse plating process on the stability of the pure Ni–P matrix. The tested deposits were produced at 50 ◦ C and pH 1.6. Sample (b), sample (c) and sample (d) were deposited applying unipolar pulse sequence PP2 at 83 Hz and 50% duty cycle. Fig. 12 shows the wear performance of the NiP and NiP/SiC deposits. The wear track profile appears coloured with dark scars. The Ni–P deposit obtained at direct current condition (sample a) shows the highest abrasion with wear tracks of about 6 m in depth, being twice as high as the measured track depth for the pulsed Ni–P deposit (sample b). Sample c (NiP with 1.5 wt% SiC submicron sized particles) and sample d (NiP with 2.6 wt% SiC micron sized particles) electrodeposited under pulsed current (PP2) at 83 Hz with 50% duty cycle show no measurable wear along the track. The pulsed composite coatings are more compact and exhibit better tribological performance. Incorporation of the SiC particles into Ni–P matrix applying pulsed current has significantly enhanced the tribological properties of the obtained composite coatings. 4. Conclusions In this work the impact of pulse plating on the particle incorporation, phosphorus content, microstructure and tribological properties of NiP/SiC dispersion coatings was investigated. Basic electrochemical measurements revealed a fast electrolyte response to applied current pulses, showing no damping effects of the signal within acceptable pulse times. When phosphorus shall
be incorporated into the nickel matrix, forming a NiP alloy layer, the efficiency of the deposition drops due to hydrogen evolution. Lower pH values of the dispersion electrolyte generate higher content of phosphorus in the deposited composite coating. Higher amount of phosphorus in the deposit goes hand in hand with decrease in current efficiency observed at these lower pH values. Particles incorporation rate shows no significantly dependency within the investigated pH range. Applying pulsed current at higher pH values leads to an enhanced current efficiency compared to direct current deposition. Tailored shift of the alloy composition and coating properties during the deposition process can be achieved via alternation of the pulse sequences, the pulse parameters have a direct influence on alloy composition (phosphorous content) and particle incorporation rate. Pulse electrodeposition at low frequencies and low duty cycles favors the incorporation of the particles into the Ni–P metal matrix. Applying unipolar pulses, especially at higher frequencies leads to improved composite hardness and wear resistance. Bipolar pulses generate higher amount of phosphorus in the deposit, thus a decrease in hardness was noticed. Generally, an increase in current density will result in a decrease in both phosphorus content in the layer and particle incorporation rate. Decrease of the average particle size resulted in a decrease of the weight percentage of the SiC particles embedded. Composite NiP/SiC deposits prepared under pulse current condition exhibit higher hardness values than DC layers. Hardness of the layers does strongly depend on pulse waveform and Pcontent. The resulting wear resistance does basically go with the micro hardness for pure NiP coatings (no particles incorporated). When particles are present within the coating, the particle load will have the major effect on the wear resistance as long as the coatings are homogenous and the particles are well embedded. Best results regarding higher hardness and enhanced wear resistance were obtained applying unipolar pulses with 83 Hz at 30–50% duty cycle at 50 ◦ C and pH 1.6. The resulting optimal deposits contained 6–8 wt% P and 2.5 wt% SiC particles. Acknowledgment This work was financially supported by the European MNT Eranet program, VINOVA and FFG.
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