Plating of nickel-phosphorus multilayer alloys: current pulse effects on the microstructural and mechanical properties

Surface and Coatings Technology, 45 (1991) 161 170

Plating of nickel-phosphorus effects on the microstructural

multilayer alloys: current and mechanical properties

161

pulse

P. Pouderoux, I. Chassaing, J. P. Bonino and A. Rousset Laboratoire de Chimie des Matdriaux Inorganiques, URA CNRS 13111 Universitd Paul Sabatier, 118 Route de Narbonne, F-31062 Toulouse Cedex (France)

Abstract Nickel phosphorus multilayer coatings have been elaborated by pulse plating. We show in this paper the strong dependence of the phosphorus content on the pulse-plating parameters, in particular the cathodic pulse time. We show that multilayers appear when the reactions are controlled by a diffusion phenomenon. This can be attenuated when the applied pulse time corresponds to the faradaic range of the potential time curve. We show also that microstructural heterogeneity can be induced by pH variations. Thermal stability and mechanical properties are not directly affected by the current pulses.

1. Introduction Nickel-phosphorus alloys can be prepared by some methods usually used for amorphous metal elaboration. Among these, electrodeposition is the easiest to use and allows one to produce, by electroforming, shaped mechanical pieces with raised thickness [1]. In electrodeposition by direct current the composition of the coatings and the microstructure are controlled by the chemical composition of the electrolytic bath and by the current density [2 4]. Some authors have shown that nickel phosphorus coatings are constituted of multilayers induced by the phosphorus content evolution through the deposit thickness [5, 6]. The variations of the phosphorus content are due to the change of the pH in the diffusion layer at the cathode-electrolyte interface [7]. Some recent works in pulse plating have proved that it is possible, via the electrical parameters, to control the physicochemical properties of metal coatings [8]. The pulse-plating technique is based on the use of discontinuous current (Fig. 1). Each period is constituted of a cathodic pulse followed or not by a reverse pulse and by a relaxation time during which the current is zero. For an average current I m the pulse parameters are linked by the relations

gm--(doncTonc~-donaTona)F F = 1 / ( T o n c + Ton a -~- Toff ) Elsevier Sequoia/Printed in The Netherlands

162 J (A/dmZ) Jon(

® Jl

...............

:[ (,-) w

Tone

Toff

Jonc

........

Ji

~. (")

® i

Jce*a

.....

Eiectroi4fe

(~

NiSO 4, 7H20

: 210

g/l

: 0.75

M

NiCl 2, 6H20

:

60

g/l

: 0.25

M

Na2SO 4

:

50

g/l

: 0.35

M

H3PO 4

:

50

g/l

: 0.51

M

H3PO3

: 10-30 g / l

T ° = 80 °C

pH = 2

: 0.i-.2 M

-

Jm = I0 A/dm 2

Fig. 1. Schematic representation of pulse waveforms (A, pulse; B, pulse reverse) and composition of the nickel phosphorus electrolyte (C). We show in this p a p e r the effect of pulse p l a t i n g on the chemical composition, band s t r u c t u r e and m i c r o s t r u c t u r a l h e t e r o g e n e i t y of n i c k e l p h o s p h o r u s alloys. After a t h e r m o s t r u c t u r a l analysis of these m e t a s t a b l e alloys, this study is c o m p l e t e d by a m e c h a n i c a l c h a r a c t e r i z a t i o n , i.e. h a r d n e s s and u l t i m a t e tensile s t r e n g t h m e a s u r e m e n t s .

163

2. Experimental procedure The n i c k e l - p h o s p h o r u s coatings were p r e p a r e d from a p h o s p h o r u s acid b a t h (Fig. 1). The plating e x p e r i m e n t s were c o n d u c t e d in a glass c o n t a i n e r of c a p a c i t y 1.5 1. The plating solutions were s t i r r e d with a m a g n e t i c a l l y driven P T F E - c o a t e d stirring bar. T h e a n o d e was a large nickel sheet (28 cm 2) of high purity. The electrolysis time was 90 min u n d e r a c u r r e n t density of 10 A dm -2. The e l e c t r o d e p o s i t i o n was made on copper s u b s t r a t e s (28 cm 2) after e t c h i n g in nitric acid solution. E a c h c o a t i n g was t h e n r e c u p e r a t e d by a selective dissolution of c o p p e r in t r i c h l o r o a c e t i c acid and a m m o n i a c a l solution. The chemical compositions were d e t e r m i n e d by atomic a b s o r p t i o n spectrometry. The m i c r o s t r u c t u r a l analyses were made by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y ( T E M ) while the band s t r u c t u r e was observed, after cross-sectional e t c h i n g (sample t h i c k n e s s 100 pm), by s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM). The t h e r m a l stability studies were done with a differential m i c r o c a l o r i m e t e r . For the m e c h a n i c a l c h a r a c t e r i z a t i o n the sample thickness was fixed at 200 p m in o r d e r to limit the effects of surface defects.

3. Pulse p a r a m e t e r effects on chemical c o m p o s i t i o n Some a u t h o r s [4, 9] h a v e r e p o r t e d t h a t the p h o s p h o r u s c o n t e n t is a f u n c t i o n of the c u r r e n t density. L a s h m o r e and W e i n r o t h [3] h a v e also s h o w n -E(m/V)e.c.s. 4.000

1 ms

"G~ ao,o

:

100

zoo

^/d, 2

80

A/dm 2

ms

I

i

F

i

Jonc

000 i Jonc

=

----

[

.,

I

\

\ \ 2000

...

"x

~o.~

""~"~2"d

\, \, Jonc

z--O

:~

20

A/dnl 2 .--

i

~ n c ( ms) 0

0.5

1.0

1.5

10

50

100

Fig. 2. Evolution of the characteristic ranges of the potential response curves as a function of the current density of the cathodic pulses.

164

Jonc (A/dm 2 ) 100 -zo 90

A/cha 2 . . . . . .

6O

A/din 2 -

!i?

4.0 -!.6 30

A/~

2. . . . .

0.01 %

of

0.'1 C

,1~0--

T~nc

1'0 (ms),

1"000 ,

I

I

I

I

I

I

I I

I I I

P 16

I

L

"

! ~,

l"Z

o c = 30

A/r~m 2

JjoI = 60 A/Om2 = 9O A/din2

0.01

0.1

1.0 Tonc

10 [ ms )

100

1000

Fig. 3. Influence of t h e on-time (Tone) and the c u r r e n t density of t h e cathodic pulses (Jonc) in relation to the c h a r a c t e r i s t i c range of t h e p o t e n t i a l response curves (I, II, III).

that the phosphorus content is not significantly affected by the off-time. We report in this paper the dependence of the composition of the coatings on the on-time (Tone). We can see from Fig. 2 the potential response for different pulse current densities Jone and a constant current density Jm = 10 A dm -2. From these curves it is possible to define three characteristic electrochemical ranges for the interface reactions (Fig. 3(a)). The first (I) corresponds to the capacitive charge (charging time re) of the diffusion double layer and the first reaction of charge transfer. This latter phenomenon, inducing a low faradic current, is responsible for the high phosphorus content in the deposit (Fig. 3(b)). Moreover, in Fig. 2 we can see that in this high frequency range the off-time is not sufficient for the double-layer discharge; consequently the potential response is attenuated and little affected by the pulses. In this case it can be assimilated to a constant potential response under direct current. In the second range (II), corresponding to the faradic plateau, a balance is established between the charge transport and the mass transport, inducing a constant potential response whatever the on-time (Tone) used. We can observe in Fig. 3(b) that the chemical composition is not affected by the on-time. When the on-time increases, a third range (III) appears from a diffusion time ~a. As under direct current, the reactions are governed by species diffusion in the double layer. A fraction of the cathodic current is used for

165

the H ÷ reduction, inducing an increase of the interface pH [7] and a decrease of the phosphorus content in the deposits (Fig. 3(b)).

4. P u l s e

effects

on multilayers

The cross-section of nickel-phosphorus coatings elaborated by pulse and direct current was observed by SEM. The micrographies of the samples, characteristic of the three electrochemical ranges previously discussed, are reported in Fig. 4. Multilayers are observed in coatings obtained (a) by direct current (b) by pulse plating in the capacitive range and (c) in the diffusion range. In the first electrochemical range (I) the phenomena occurring at high frequency electrodeposition can be compared with thos~ appearing under direct current at lower current density. At larger on-times (range III) the reactions are governed as under direct current by a diffusion phenomenon. The diffusion rate induced by these two particular pulse-plating conditions is responsible for the appearance of multilayers. In the faradic range (II) corresponding to an equilibrium rate between the mass and charge transport, multilayers could not be observed or were hardly noticeable (Fig. 4(c)).

a

c

1 pm

b

d

Fig. 4. Etched cross-sections of N i - P alloys elaborated by (a) direct current, (b) pulse current, 2000 Hz, (c) pulse current, 10 Hz and (d) pulse current, 0.5 Hz.

166 These results fit perfectly with those reported by Nee and Weil [7], who link the multilayers obtained by direct current to a pH variation in the diffusion layer. We can add that by pulse plating, when the on-time used corresponds to the faradaic plateau, the interface phenomenon is in a pseudosteady state between mass and charge transport. The interfacial pH variations are reduced by the absence of diffusional range and the heterogeneities in chemical composition of nickel-phosphorus are limited.

5. P u l s e effects o n m i c r o s t r u c t u r e

We have seen, as have other authors [2, 10] by X-ray diffraction, that the deposit microstructure depends on the phosphorus content whatever the plating technique used, i.e. direct or pulse current. The crystallite size decreases when the phosphorus content increases to 16 at.%. Beyond this value the alloys are amorphous (Fig. 3(a)). We have observed in the particular condition of pulse reverse, when the on-time is longer than the time of the double-layer discharge, that a large part of the anodic current is used for the dissolution of the deposit and the H + formation. The pH decreases in the anodic region and induces an increase of the phosphorus content above 25 at.%. When the phosphorus content is higher than the stoichiometric composition of Ni3P (Fig. 5(b), 27 at.% P), the formation of a crystallized complex phase Ni x Py with a high metalloid content, not easily attributable to Ni~P2 or Ni7P3, and the simultaneous formation of an amorphous phase appear. We have verified that such results are also obtained under direct current when the pH is below 1.4 [11]. To summarize these results, we can say that the microstructure of nickel-phosphorus electrodeposits depends on the phosphorus content but is not particularly affected by the current pulses.

6. T h e r m a l s t a b i l i t y

The differential thermoanalysis curves of three alloys obtained by pulse plating with different phosphorus c o n t e n t s - - ( a ) Ni93P 7, (b) NisoP20 and (c) Ni73P27--are plotted in Fig. 5. Ni73P27 is elaborated by pulse reverse under the conditions previously mentioned. For the pulse-plated Ni93P7 and NisoP20 coatings the transformation temperatures are the same as those observed for the N i - P alloys obtained by direct current. The first transformation at 320 °C in the NisoP20 amorphous alloy is the precipitation of complex phases constituted of nickel-phosphorus in needle-shaped form (Fig. 5(a)) and others (NisP2, NiTP2, Ni2P) [2, 12] in a lower ratio. The second transformation at 400 °C corresponds to coalescence with the formation of stable phases of nickel and Ni~P [2,12] and grain growth. The microcrystallized alloys, as electrodeposited, are constituted of a

167 7.0

LHeot Flow

(roW)

4..0

1,0

-2.0 C -5"020

Tern peroture 120

220

320

420

(o C ) 520

Fig. 5. Differential t h e r m o a n a l y s i s curves of electrodeposited N i - P alloys: (a) Ni93P~, pulse (10 Hz); (b) NisoP20, pulse (2000 Hz); (c) Ni73P27, pulse reverse (pH = 2, Ton a = 8 ms) or direct c u r r e n t plated (pH = 1.4).

phosphorus-supersaturated solution of nickel microcrystallized [13]. The first transformation does not appear as in the case of an amorphous alloy; only the Ni3P precipitation appears at 400 °C (Fig. 5(b)) followed by grain growth of nickel and Ni3P [2]. For the biphase alloy Ni73P27 (Fig. 5(c)), two exothermic transformations appear at lower temperature. In Fig. 6, TEM observations of (A) an amorphous alloy and (B) a biphase alloy after an annealing are shown. For these two alloys we can see a similar crystallization of amorphous phases, characteristic of the eutectic phase, but this time appearing at lower temperature for the biphase alloy. This shows that the coexistence of a crystallized phase with an amorphous one modifies the kinetics of crystallization of the latter.

7. Mechanical properties We report in Fig. 7 the mechanical characteristics, (a) hardness and (b) tensile strength, of N i - P alloys with different phosphorus contents elaborated by pulse and direct current. The Vickers hardness evolution with phosphorus content is the same whatever the current type used. The higher values obtained from 3 to 9 at.% P correspond to the microcrystallized alloys. We can see, for the samples elaborated by high frequency pulse plating, t h a t the Vickers hardness has an identical variation with the phosphorus content to that under direct current. The higher values are obtained at lower frequency. In Fig. 7 we can see that the tensile strength does not depend on the current ~ype. Beyond 3 at.% P the alloys show no plastic deformation; rupture occurs before the end of the elastic deformation.

168

@

L

®

I

I

/

\

I

0.2 ~m

320 C

250 °C

5 0 0 °C

500 C

Fig. 6. Transmission electron microscopy of electrodeposited (A) NisoP20 and (B) Ni73P27 alloys: (a) as electrodeposited; (b), (c) after annealing.

169

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DIRRCT CURRENT

z~00~ 2

0 0

0

~ 6

'

/ 12

o

ot 18

P 2~

(a) 1600

1200 1

~

frengfh (MPo)

,

•

DIRECT

~

PULSE

CURRENT F

•

PULSE

F

-

7

Hz

PULSZ

F

-

0.5

Hz

2000

Hz

800

~OC

w

v

v v

%

0

6

12

18

of P 2&

(h) Fig. 7. (a) Microhardness and (b) tensile strength as a function of the phosphorus content of N i - P alloys elaborated by direct and pulse current.

8. C o n c l u s i o n s The correlation established between the chemical composition, the potential and the galvanostatic pulses has allowed us to define three characteristic electrochemical ranges on which the phosphorus content depends. Multilayers appearing in N i - P electrodeposited by direct current can be attenuated by the use of current pulses. In fact, in the faradic range, corresponding to an equilibrium between the charge and mass transport rate phenomena, the interfacial pH variations are reduced by the absence of diffusional range. These results are in perfect agreement with those reported by Nee and Weil [7], who attribute the multilayers to pH variations. The microstructure is not directly affected by the pulse parameters but rather by the phosphorus content. Only under pulse reverse have we noticed a microstructural heterogeneity which seems to originate from a pH decrease

170 a t l o n g r e v e r s e p u l s e t i m e (Tona). T h e s i m u l t a n e o u s p r e s e n c e o f a m o r p h o u s and crystallized phases of Ni~P 2 has an impact on the kinetics of crystallization of the amorphous phase. T h e e v o l u t i o n o f t h e m e c h a n i c a l p r o p e r t i e s is n o t s i g n i f i c a n t l y m o d i f i e d by the current pulses. Only for the Vickers hardness could we observe that higher values are obtained at lower frequency pulses. T h e s e r e s u l t s s h o w t h a t , b y p u l s e p l a t i n g , i t is p o s s i b l e t o h o m o g e n i z e t h e composition of Ni-P alloys. In the near future, thanks to this technique, we can hope to elaborate multilayers with a controlled thickness and to study more precisely the evolution of the physicochemical properties.

References 1 2 3 4 5 6 7 8 9 10 11 12 13

J. P. Bonino, A. Rousset, C. Rossignol and Y. Blottiere, Mater. Tech., 1 (1990) 25. C. Rossignol, Th~se, Toulouse III, 1988. D. S. Lashmore and J. F. Weinroth, Plating Surf. Finish., 8 (1982) 72. A. Brenner, P. Couch and E. Williams, J. Res. NBS, 44 (1950) 109. M. Ratzker, D. S. Lashmore and K. W. Pratt, Plating Surf. Finish., 9 (1986) 74. K. Shimizu, G. E. Thompson and G. C. Wood, Phil. Mag. Lett., 61 (2) (1990) 43. C. C. Nee and R. Weil, Surf. Technol., 25 (1985) 7. M. J. Avila, in F. Leaman and J. C1. Puippe (eds.), Theory and Practice of Pulse Plating, AESF, Orlando, FL, 1986, p. 189. E. Toth-Kadar, I. Bakonyi, A. Solyom, J. Hering, G. Konczos and F. Pavlyak, Surf. Coat. Technol., 31 (1987) 31. G. McMahon and U. Erb, J. Mater. Sci. Lett., 8 (1989) 865. P. Pouderoux, Th~se, Toulouse III, 1991. E. Vafaei-Makhsoos, J. Appl. Phys., 51 (1980) 6366. E. Vafaei-Makhsoos, E. L. Thomas and L. E. Toth, Metall. Trans. A, 9 (1978) 1449.