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Electrodeposited nickel-phosphorous (Ni-P) alloy coating: an in-depth study of its preparation, properties, and structural transitions.

Abstract Ni-P deposits with a phosphorous content of up to 20% (wt) were obtained on AA6061 substrates by direct current electrodeposition technique from a solution containing nickel sulfate, nickel chloride, phosphorous acid, phosphoric acid, and a wetting agent (sodium lauryl sulfate). The effect of various plating parameters like current density, concentration of phosphorous acid, concentration of phosphoric acid and plating temperature on the P content of the coating as well as the rate of deposition was investigated systematically. It has been observed that the influence of current density on the P content of the deposit is largely dependent on the concentration of phosphorous acid in the plating bath. Composition, surface morphology, microstructure, and mechanical properties of the Ni-P deposits were studied using SEM, EDAX, XRD, and nanoindentation techniques. Ni-P electrodeposits with low P content in the range of 4-7 wt% of P exhibited superior microhardness of 7.74-8.57 GPa. With increasing P content in the deposit, the structure undergoes transition from crystalline to nanocrystalline and becomes amorphous above 9.14 wt% of P. Ni-P alloys with some selected compositions were subjected to beat treatment at 400[degrees] C for 1 h in a hot air oven and the resulting variation in mechanical properties was studied using nanoindentation technique.

Keywords Electrocleposition, Nickel-phosphorous alloy, Microstructure, Nanoindentation, Heat treatment

Introduction

Among the various transition metal-metalloid alloys, nickel-phosphorous (Ni-P) alloys have received considerable interest owing to their interesting functional properties. (1-5) Ni-P coatings exhibit good corrosion and wear resistance. (6), (7) The high hardness and excellent machineability of this material makes it suitable for diamond turning applications such as fabrication of large optics and other high precision parts. (8) Furthermore, Ni-P is a promising material as a diffusion barrier and as an undercoat for gold-plated components, which enables a drastic decrease in the gold layer thickness. (9), (10) Additional technological applications of Ni-P coatings involve their use as catalytic coatings for hydrogen evolution reactions, (11), (12) as thin film resistors and unclerlayers of thin film magnetic discs, (13), (14) in microgalvanics applicationsI5 and for decorative applications in automotive industries. (16)

There are several methods for producing Ni-P coatings, including vacuum deposition, chemical reduction (autocatalytic/electroless deposition), rapid quenching, melt spinning, vapor deposition, and eleetrodeposition. (17-21) One of the most frequently employed methods is the clectroless deposition owing to its excellent throwing power, making it possible to obtain a uniform deposit even into deep recesses. However, electroless nickel plating solutions are laborious to control and have relatively short life span owing to the frequent solution phase precipitation, thereby generating relatively high amounts of waste. The solution composition is quite complex including the use of complexing agents, buffering agents, stabilizers and expensive reducing agents (e.g. sodium hypophosphite), hence making production, maintenance, and disposal of solutions expensive. Other disadvantages of electroless plating include the low deposition rate of about 10 [micro]m [h.sup.-1] and high operating temperatures. This requires long process times, again making the process more expensive. To overcome these difficulties Brenner et al. (22), (23) developed baths for the electrodeposition of Ni-P alloy coatings. Since then, increased attention has been directed at electro-deposition as an alternate to electroless deposition for producing Ni-P alloys. One principal advantage of the electrodeposition process is that thick deposits of alloys can be obtained in a relatively short time by means of a simple process. From a conservation point of view, the use of electrolytic processes will save time, conserve energy and eliminate costly down time for cleaning and rejuvenating the electroless processes. (17), (24), (25)

Phosphorous cannot be electrodeposited as a pure phase but can be readily co-deposited with iron group metals such as nickel, from solutions containing both Ni and P ions. It is believed that the strong atomic interaction between Ni and P makes the induced co-deposition of Ni-P alloy with stoichiometric composition possible. Electrolytic formation of nickel takes place in the face centered cubic (fee) system and the co-deposition of phosphorous occurs in the octahedral interstitial sites. It is generally accepted that the crystallographic structure of Ni-P alloys is influenced by the amount of P present in the alloy and undergoes transitions from crystalline to nanocrystalline and eventually becomes amorphous with increasing P content. In the co-deposition process, the reduction of [Ni.sup.2+] ion at an active center on the cathode surface is followed by the surface diffusion of the Ni ad atom to a suitable crystal lattice site. Reduction and co-deposition of phosphorous inhibit this surface diffusion of the Ni atoms and hence growth of crystal nuclei. With increasing P content in the deposit, the rate of fresh nucleus formation becomes higher than the rate of growth of existing crystal nucleus, thus refining the deposit grain size. Finally, when a critical P content is achieved, nucleus growth effectively ceases and an amorphous structure having short range order over a few atomic distances results. (17) There exists a considerable inconsistency in the available literature about the phosphorous content of the deposit at which crystalline to amorphous transition occurs and researchers have come to the conclusion that the transition from crystalline to amorphous structure takes place over a range of concentration of P rather than in an abrupt way at a certain composition. (26-28)

Since the pioneering work of Brenner et al., (22), (23) variations in solution composition and operating parameters have resulted in a number of stable solutions clable of producing good electrodeposits of Ni-P. (29-33) In the present study we have used a solution containing nickel sulfate, nickel chloride, phosphorous acid, phosphoric acid, and a wetting agent. Nickel sulfate serves as the primary source of nickel while nickel chloride is added to improve anode corrosion, solution conductivity, and uniformity of the coating thickness distribution. Phosphorous acid acts as the phosphorous source in the solution, while phosphoric acid acts as a leveling and brightening agent. The bath is generally operated at a solution pH less than 1.5 and phosphoric acid is acting as a buffering agent also. A wetting agent is added to control pitting.

Though many researchers have investigated the structure and mechanism of induced co-deposition of Ni-P alloys, there is still a lack of information about the relationship between the electrolytic plating parameters and the coating composition and the available information often seems to be contradictory. The objective of the present work is to do a systematic study of these effects. Being the most commonly used material in space and aerospace applications, we have chosen aluminum alloy (AA6061) as the substrate for Ni-P eiectrodeposition.

Materials and methods

Ni-P deposits were obtained on AA6061 substrates by the direct current electrodeposition technique. The plating bath solution consisted of nickel sulfate, nickel chloride, phosphorous acid, phosphoric acid and a wetting agent (sodium lauryl sulfate). The electrolyte was prepared by adding the appropriate amounts of laboratory grade chemicals to de-mineralized water. A nickel plate of 99.99% purity was used as the soluble anode. The cathode (substrate) was a circular disc of AA6061 (A1-97.9%, Mg-1.0%, Si-0.6%, Cu-0.25%, and Cr-0.25%) of 1 inch diameter and 4 mm thickness. Substrate surface cleaning is extremely important prior to plating especially for aluminum since the natural oxide layer is very persistent and when removed it is immediately re-established. Hence, before electrode-position of Ni-P the aluminum substrate was subjected to a series of conventional cleaning procedures such as solvent cleaning, alkaline cleaning, and acid cleaning. This is followed by zincating, stripping of the first zincate layer and re-zincating. Re-zincating is done to obtain a uniform and compact zincate layer on the aluminum substrate that leads to very good adhesion for the subsequent electrodeposited Ni-P coating.

Ni-P alloy deposits were prepared at different plating conditions and the effect of various plating parameters like current density, concentration of phosphorous acid, concentration of phosphoric acid and plating temperature on the P content of the coating as well as the rate of deposition was investigated systematically. The substrate was weighed before and after electroplating by an electronic balance with an accuracy of 0.01 mg and the difference in weight. was used for calculating the rate of deposition in [micro]m [h.sup.-1] assuming a uniform thickness of the coating. The electrolyte concentration and plating conditions used in the present work are consolidated in Table 1. The Ni-P electrodeposits having different P content thus obtained were further investigated to study the effect of P content on the microstructure, surface morphology, and mechanical properties of the deposits. It has been previously reported that the hardness of the as-plated Ni-P coatings can be further increased by heat treating as a result of precipitation of hard nickel phosphide ([Ni.sub.3]P) phase. (7), (12), (17) Hence, Ni-P alloys with some selected compositions were subjected to heat treatment at 400 [degrees]C for 1 h in a hot air oven in an attempt to achieve peak hardness and cooled in air to room temperature. The resulting variation in the microstructure, surface morphology, and mechanical properties after heat treatment was investigated using XRD, SEM, and nanoindentation measurements.
Table 1: The electrolyte concentration and plating
conditions used in the present work

Parameters maintained constant

Nickel sulfate (Ni[S0.sub.4]7[H.sub.2]0)           150 g/L

Nickel chloride (Ni[CI.sub.2]6[H.sub.2]0)           45 g/L

Wetting agent (sodium lauryl sulfate)             0.25 g/L

Parameters varied

Current density                                     5-30 A
                                               d[m.sup.-2]

Concentration of [H.sub.3][P0.sub.3]              0-20 g/L

Concentration of [H.sub.3][P0.sub.4]              0-40 g/L

Temperature                               50-80 [degrees]C


Surface morphology of the deposit was studied by scanning electron microscopy (Leica S 440 I, UK) operated at a voltage of 20 kV and probe current of 200 pA. The filament current was 2.57 A and the detector used was a secondary electron detector. The phosphorus content of the deposit was determined in a semi quantitative way in the same scanning electron microscope which was equipped with energy dispersive spectroscopy facilities (Oxford Instruments, INCA X-Max, UK). X-ray diffraction studies were carried out using a Philips Xspert-Pro instrument (The Netherlands) at 40 kV and 30 mA with Cu K[alpha] radiation [lambda] = 1.5406 A) at a scan rate of 0.08 [degrees] [s.sup.-1] in the range of 30 [degrees] -100 [degrees] and 0.03 [degrees] step size. The Nano Indenter [R] G200 system was used to study the hardness and elastic modulus of the Ni-P deposits obtained at different process parameters and various bath conditions. A diamond Berkovich (pyramidal) tip, with a tip radius of 20 nm was used for indentation studies. Measurements were done using the Continuous Stiffness Measurement (CSM) option which yields hardness and elastic modulus as a continuous function of depth into the sample. This test method uses a displacement-controlled instrumented indentation which employs the use of a constant displacement rate. The specimens were loaded to a constant depth of 2000 nm and the maximum load was held constant for 10 s. A total of 15 indentations were performed on each sample and the average values of hardness and modulus were reported.

Results and discussions

The Ni-P alloy composition depends on the bath composition and plating parameters. By controlling the current density, phosphorous acid concentration, and temperature, bright and adherent Ni-P coatings containing various concentrations of phosphorous were obtained. Coatings with a phosphorous content of up to a maximum of 20% (wt) were obtained.

Appearance

The electrodeposited Ni-P deposits had a fully bright appearance identical to that of electroless Ni-P and appeared to have a relatively low surface roughness. When the bath composition and operating conditions were selected such that pure nickel was deposited without any incorporation of P. the coating had a dull gray appearance. Incorporation of even a small amount of P renders the coating a bright appearance.

Effect of plating parameters

Phosphorous acid concentration

Samples were prepared at a current density of 30 A [dm.sup.-2] 2 at 70 [degrees]C with different concentrations of phosphorous acid (0-20 g/L) while maintaining a high concentration of phosphoric acid in the bath (40 g/L). For a given current density, an increase in the concentration of [H.sub.3][P0.sub.3] in the electrolyte gave rise to an increase in the amount of phosphorous incorporated in the deposit. However, the observed relationship was not linear, as shown in Fig. 1.

[FIGURE 1 OMITTED]

A 100% nickel coating without any phosphorous incorporation was obtained when the plating was carried out in the absence of [H.sub.3][P0.sub.3] indicating that phosphorous acid is indeed the only electrochemically active phosphorous species which acts as the phosphorous source in the present solution. It is clearly evident from the graph that phosphorous content of the Ni-P coating initially rises rapidly with an increase of phosphorous acid concentration in the bath (from 0 to 10 g/L) and eventually reaches a value beyond which no appreciable increase in the phosphorus content was observed. The cathodic current efficiency, on the other hand, decreases with an increase in the [H.sub.3][P0.sub.3] concentration in the bath which is indicated by a decrease in the rate of deposition of the alloy (Fig. 2). This can be explained as a result of the enhancement in the reduction of hydrogen ions at the cathode as more and more [H.sub.3][P0.sub.3] is added in the bath, thereby decreasing the rate of nickel deposition. (28)

[FIGURE 2 OMITTED]

Current density

In order to evaluate the effect of current density on the P content of the coating, samples were prepared at different current densities from 5 to 30 A [dm.sup.-2] at a bath temperature maintained at 70 [degrees]C. The concentrations of phosphorous acid and phosphoric acid in the solution were kept constant at 20 and 40 g/L, respectively. As a general rule, the rate of deposition of Ni-P coatings was found to increase steadily with increasing current density (Fig. 3).

[FIGURE 3 OMITTED]

A general trend of a decreasing phosphorous content with increasing current density is reported in literature by many authors, although a large scatter between the data points of the different authors is noticed. (10) Our experiments indicate that current density has no significant effect on the phosphorous content (Fig. 4). The P content of the coating is found to be between 15 and 16% irrespective of the current density employed.

[FIGURE 4 OMITTED]

In order to investigate this discrepancy in results. experiments were conducted to evaluate the combined effect of current density and concentration of [H.sub.3][P0.sub.3] in the bath on the P content of the deposit. This was done by preparing samples at current densities of 5 and 15 A [dm.sup.-2] with different concentrations of phosphorous acid (0-20 g/L) in the bath. The results were then compared with that obtained at the current density of 30 A [dm.sup.-2]. The bath temperature was maintained at 70[degrees]C and the concentration of phosphoric acid in the bath was kept constant at 40 g/L. The result presented in Fig. 5 indicates that at higher concentration of [H.sub.3][P0.sub.3] in the bath ( [greater than or equal to] 1.5 g/L), the P content in the coating is independent of current density. However, at low phosphorous acid concentration in the bath, the decreasing trend of P content in the coating with increasing current density is clearly evident.

[FIGURE 5 OMITTED]

Hence, it is understood that the dependence of P content of the coating on the variation of current density is actually determined by the amount of [H.sub.3][PO.sub.3] present in the plating bath. In other words, it is the concentration of the phosphorous acid in the plating bath that controls the phosphorous content of the Ni-P coating at a specific current density. It is therefore concluded that the scatter in the results presented by different authors is mainly due to the different bath compositions used. The rate of deposition of the coating is found to increase with an increase in the current density employed and to decrease with an increase in the concentration of phosphorous acid in the plating bath (Fig. 6).

[FIGURE 6 OMITTED]

Phosphoric acid concentration

The effect of phosphoric acid concentration on the P content of the deposit as well as rate of deposition of the coating was studied by preparing samples at different concentrations of phosphoric acid (from 0 to 40 g/L) in the plating bath. Studies were conducted at two different current densities (15 and 30 A [dm.sup.-2]) and at two different concentrations of [H.sub.3][PO.sub.3] in the bath (10 and 20 g/L) and the temperature of the plating bath was maintained at 70[degrees]C. It has been observed that at a given current density and [H.sub.3][PO.sub.3] concentration, the P content of the coating is independent of the concentration of [H.sub.3][PO.sub.4] in the bath (Fig. 7). This observation is not in agreement with that presented by Narayan and Mungole (18) where they observed an increase in P content of the coating with an increase in [H.sub.3][PO.sub.4] concentration at 80[degrees]C whereas a slight decrease in the P content of the coating with an increase in [H.sub.3][PO.sub.4] concentration at 90[degrees]C. Our observation establishes without doubt that [H.sub.3][PO.sub.4] is not at all acting as a source of phosphorous in the coating. It is the [H.sub.3][PO.sub.3] present in the bath that is solely responsible for the incorporation of P in the coating. However, it is not advisable to operate this bath for a longer time in the absence of [H.sub.3][PO.sub.4] in the plating solution. Phosphorous acid has a tendency to get oxidized to phosphoric acid thereby adversely affecting the co-deposition process. (31) The presence of a slight excess of phosphoric acid in the bath prevents this tendency of oxidation which results in an increased bath life. [H.sub.3][PO.sub.4] also plays an important role in maintaining the pH of the plating solution well within the optimum value (<1.5). Use of excess of [H.sub.3][PO.sub.4] is also not advisable as the rate of deposition of the coating is found to decrease with an increase in the concentration of [H.sub.3][PO.sub.4] at any given current density and [H.sub.3][PO.sub.3] concentration (Fig. 8). Hence, it is important to strike a balance in the concentrations of the two acids to obtain Ni-P coatings of any desired composition in a reasonable time.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Temperature of the plating bath

When the plating bath temperature was increased from 50 to 80 [degrees] C, the amount of phosphorous incorporated in the coating decreased whereas the rate of deposition of the coating increased (Figs. 9, 10). It had been reported earlier that Ni--P coatings exhibit high internal stress when plated at low temperature. (31) Moreover, plating rate and current efficiency both decrease with lower bath temperatures. On the other hand, plating at very high temperature tends to make the process more energy intensive. Hence, bright Ni-P alloy coatings can be produced with a reasonable rate of deposition when the operating temperature of the bath is around 70 [degrees]C.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

Effect of P content on the surface morphology of the coating

The surface morphology of the as-plated Ni-P alloys containing various concentrations of phosphorous were studied using scanning electron microscopy. Figure 11 shows the surface morphology of Ni-P electrodeposits with different P contents. The SEM micrograph of pure nickel deposit without any phosphorous incorporation in the Janice shows coarse nickel grains whose average grain size is in the range of a few micrometers. These deposits also show a relatively large surface roughness and dull appearance. The incorporation of even a very small amount of phosphorous in the nickel lattice refines the nickel grains substantially. As the P content in the deposit increases, a more colony like morphology is obtained where each colony consists of numerous grains with smaller grain size thereby making the coating brighter and smoother in appearance. The surface morphologies indicate that the incorporation of P in the nickel lattice refines the coarse nickel grains ill the micrometer range to the nanometer regime.

[FIGURE 11 OMITTED]

Effect of P content on the microstructure of the coating

The crystal structure of the electrodeposited Ni-P coatings containing various concentrations of phosphorous was studied using XRD. Here we investigated the as-plated structure of electrodeposited Ni-P coatings with up to 15 wt% of P prepared by a careful selection of the main plating parameters, namely. the amount of phosphorous acid in the plating bath and the current density. The XRD results are in good agreement with the data available in literature for electro-deposited Ni--P coatings. It is generally accepted that the crystallographic structure of Ni--P alloys is influenced mainly by the amount of P present in the alloy and it evolves from a crystalline to an amorphous one with increasing P content. (1), (10), (14), (17) However, the composition at which this transition occurs is not well defined.

The electron diffraction pattern of the as-deposited samples, shown in Fig. 12, shows fcc reflections characteristic of nickel and no peaks from Ni-4P or other Ni-P compounds are noticed. Hence, these as-plated Ni-P alloys can be best described as a supersaturated solid solution of phosphorous dissolved in crystalline nickel. For pure Ni, the (200) reflection is much stronger than the (111) reflection. Other characteristic fcc nickel peaks at (220), (311), and (222) are also seen in the diffractogram though with less intensity. When phosphorous is incorporated in the structure, most of the peaks become wider and less intense. The incorporation of phosphorous in the structure has not affected the peak positions. At 4.64 wt% of P. the (200) peak becomes less intense, while the broadened (111) peak becomes more pronounced. This change to a strong (111) and a weaker (222) reflection with increasing phosphorous content and the decrease in peak intensity and peak broadening was reported earlier by other investigators. (14), (16), (28) This broadening of the diffraction peaks as well as a change in the preferred orientation is an indication of a gradual decrease in the grain size as the phosphorous content in the deposit increases.

[FIGURE 12 OMITTED]

As the phosphorous content in the coating is increased to 6.58 wt% of P. only the broadened (111) peak is observed and its intensity further decreases, while other Ni peaks disappear. Ni-P coatings with more than 9.14 wt% of P showed only one broad peak around 2 [theta] = 44 [degrees] (characteristic of Ni (111) peak), indicating an amorphous structure. The sharp peaks in the X-ray diffraction graphs are diffraction lines originating from the aluminum substrate. These substrate peaks are marked "sub" in the diffractograms.

The average grain size of the Ni-P deposit was calculated from the (111) X-ray diffraction peak broadening using the Scherrer equation given by

Crystallite size (average) = K [lambda] /{[B.sub.siruct] cos [theta]}

where K is the crystallite shape factor, somewhat arbitrary value that falls in the range 0.87-1.0 (usually assumed as K = 1), [lambda] is the wavelength of the radiation used, [B.sub.struct] is the structural broadening, which is the difference in integral profile width between a standard ([B.sub.std]) and the sample to be analyzed ([B.sub.obs]), i.e., [B.sub.struct] = ([B.sub.obs]) -([B.sub.std]); [[B.sub.std] is estimated using annealed silica standards]. It has been observed that with the incorporation of phosphorous in the coating, the average grain size of the crystallites decreases from a few micrometers (pure Ni) to 17-20 nm. When the phosphorous content was 9.14 wt% and above, grain size measurements were not possible as the samples were X-ray amorphous.

Influence of plating parameters on the mechanical properties of the deposit

Ni-P alloy obtained by electrolytic deposition has been in prominence due to its high microhardness compared to pure nickel coatings. In general, it has been established that the microhardness of these alloys is highly dependent upon the phosphorous content and the plating conditions. (15), (18) In order to study the effect of plating conditions on the microhardness and elastic modulus of the coating, samples prepared using different concentrations of phosphorous acid (0-20 g/L) in the plating bath at three different current densities (5, 10, and 30 A [dm.sup.-2]) were subjected to nanoindenta-tion studies. Figure 13 shows the microhardness of various deposits as a function of [H.sub.3][P0.sub,3] concentration obtained at different current densities.

[FIGURE 13 OMITTED]

In the absence of phosphorous acid in the plating bath, the coating obtained was that of pure nickel characterized by comparatively low hardness value (2.36-3.73 GPa). The addition of a small amount of phosphorous acid (2 g/L) to the plating bath results in a sharp increase in the deposit hardness (6.73-8.57 GPa). This phenomenon is due to the incorporation of phosphorous in the nickel matrix and the corresponding grain structure refinement. However, deposit microhardness is not found to increase linearly with an increase in the P content of the coating. In fact, a further increase in [H.sub.3][P0.sub,3] concentration (up to 10 g/L) resulted in a slight decrease in the deposit hardness and after which further increase in the phosphorous acid concentration does not result in any appreciable change in the deposit hardness irrespective of the current density employed. The observed trend in hardness indicates that the mechanical properties of Ni-P deposits depend not only on the phosphorous content but also on the deposit microstructure. In general, Ni-P elcctrodeposit with P content in the range of 4-7 wt% of P exhibited superior hardness (7.74-8.57 GPa). Figure 14 shows the modulus of the deposits as a function of [H.sub.3][P0.sub,3] concentration. A steady decrease in the modulus of the electrodeposits is observed when the concentration of [H.sub.3][P0.sub,3] in the plating bath is increased from 0 to 15 g/L at all the current densities. Further increase in [H.sub.3][P0.sub.3] concentration results in no appreciable change in the elastic modulus.

[FIGURE 14 OMITTED]

Effect of heat treatment on the microstruciure of the coating

Figure 15 shows the XRD patterns of electrodeposited Ni-P coatings containing various concentrations of phosphorous subjected to heat treatment at 400 [degrees]C for 1 h in a hot air oven and air-cooled to room temperature. The diffractogram of pure Ni sample remained unchanged after heat treatment. In all the other heat-treated samples, diffraction lines corresponding to both nickel and [Ni.sub.3]P are observed in the diffractogram indicating the equilibrium existence of Ni and [Ni.sub.3] P phases in the deposits. At low phosphorous concentrations (4.64 wt%), the nickel peaks correspond not only to (111) plane, but also to (200), (220), (311), and (222) planes. As the concentration of phosphorous in the coating increases, the only prominent nickel peak in the diffraction pattern is the (1 1 1) peak and the other peaks of nickel start diminishing in intensity and, after 14.69 wt% of P, become almost completely absent in the diffractogram. On the other hand, with increasing concentration of P in the deposit, more [Ni.sub.3] P peaks with increasing intensity start appearing in the diffraction pattern. In fact, at 15.22 wt% of P. the intensity of [Ni.sub.3] P peak outweighs that of Ni (111) peak. Heat treatment of the samples resulted in the precipitation of [Ni.sub.3] Pgrains and the amount of [Ni.sub.3] Pgrains precipitated increased with increase in the concentration of P in the deposit. The average grain size of the deposits increased from 17-20 nm (as-plated samples) to 30-45 nm (heat-treated samples). Another significant observation is that due to the precipitation of [Ni.sub.3]P grains at 400 [degrees ]C, the Ni-P deposits having higher P content which were X-ray amorphous in the as-plated condition lose their amorphous character and become highly crystalline.

[FIGURE 15 OMITTED]

The SEM micrographs of the electrodeposits after heat treatment were, however. not distinctly different from that of the as-plated electrodeposits. The SEM micrographs of Ni-P electrodeposit having a P content of 13.08 wt% are presented in Fig. 16 as an example.

[FIGURE 16 OMITTED]

Effect of heat treatment on mechanical properties

It is observed that Ni-P alloys, especially those with low P content, exhibit superior hardness compared to that of pure nickel coatings. The hardness of these alloys is further increased up to 12 GPa by annealing at 400[degrees] C for 1 11 owing to precipitation of hard [Ni.sub.3] P phase. Figure 17 shows the effect of annealing on the hardness of Ni-P alloys with different P content. It is interesting to note that for pure nickel coating, heat treatment has no effect on the hardness value.

[FIGURE 17 OMITTED]

Modulus of the Ni-P coating is found to increase significantly with heat treatment (Fig. 18). The crystallization of nickel phosphide particles from within the coating causes a volumetric shrinkage within the Ni-P deposits.34 This could increase the internal stress of the deposit resulting in increased modulus of elasticity. For pure nickel coatings, however, there is no such structural transition happening with heat treatment and the slight decrease in modulus value may be attributed to the release of internal stress with heat treatment.

[FIGURE 18 OMITTED]

Conclusions

1. Electrodeposits of Ni-P having desired P content can be obtained by the careful selection of bath composition and plating parameters.

2. For a given current density, with increasing [H.sub.3][P0.sub.3] concentration in the bath, the phosphorous content in the deposit increased while rate of deposition decreased.

3. An increase in the concentration of [H.sub.3][PO.sub.4], however, has no significant effect on the P content of the coating although the rate of deposition decreased continuously.

4. At higher concentration of [H.sub.3][P0.sub.3] in the bath ([greater than or equal to]15 g/L), the P content in the coating is independent of current density whereas at low [H.sub.3][P0.sub.3] concentration in the bath, the P decreases with increasing current density.

5. With an increase in the plating temperature the amount of phosphorous incorporated in the coating decreased whereas the rate of deposition of the coating increased.

6. With increasing P content in the deposit. the structure undergoes transition from crystalline to nanocrystalline and beconic amorphous above 9.14 wt% of P.

7. Ni-P electrodeposits with low P content in the range of 4-7 wt% of P exhibited superior microhardness of 7.74-8.57 GPa and the microhardness of these alloys is increased up to 12 GPa by annealing at 400[degrees]C for 1 h owing to precipitation of hard [Ni.sub.3]P phase.

8. Ni-P deposits haying higher P content which were X-ray amorphous in the as-plated condition lose their amorphous character and become highly crystalline as a result of heat treatment.

9. Heat treatment causes the elastic modulus of the deposit to increase significantly.

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J. Caot. Technol. Res., 9 (6) 785-797, 2012

[c] ACA and OCCA 2012

A. M. Pillai *, A. Rajendra, A. K. Sharma

Thermal Systems Group, ISRO Satellite Centre, Vimanapura Post, Bangalore 560 017, India e-mail: anjum@isae.gov.in

DOI 10.1007/s11998-012-9411-0
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