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Deposition of composite autocatalytic nickel coatings containing particles
Surface Technology, 13 (1981) 119- 125
119
D E P O S I T I O N OF COMPOSITE AUTOCATALYTIC NICKEL COATINGS CONTAINING PARTICLES
M. S. HUSSAIN and T. E. SUCH Research Laboratory, W. Canning Materials Ltd., P.O. Box 288; Great Hampton Street, Birmingham B18 6AS (Gt. Britain)
(Received October 20, 1980)
Summary A process and apparatus for the a ut oc a tal yt i c (electroless) deposition onto a metal substrate of a nickel coating containing dispersed particles is described. The topics investigated included the effect of heat t r e a t m e n t on the hardness of the composite coating, the time taken for the maximum hardness to be attained.and a comparison of the h a r d n e s s - h e a t t r e a t m e n t temperature curves for standard electroless nickel coatings with those for the composite coatings. The surface morphologies of the deposits were studied with the aid of a scanning electron microscope and a Quantimet 720 image analyser.
1. I n t r o d u c t i o n Electroless nickel coatings often offer a better performance for engineering applications t ha n do electroplated coatings. U nfort unat el y electroless nickel coatings suffer from two major defects. They tend to gall if moved in contact with a n o t h e r electroless-nickel-coated part and they rapidly lose their hardness and wear resistance at temperatures above 200 °C. These are two significant limiting factors to the wider usage of electroless nickel coatings. Therefore it was hoped that inclusions produced by depositing a composite coating of particles occluded in an N i -P alloy matrix would increase the wear resistance and the corrosion protection in a wide range of environments. The matrix material would be an alloy of nickel and phosphorus containing between 7% and 10% P. The electroless nickel plating process has several advantages over the conventional electroplating process in t hat deposits of uniform thickness are obtained even in deep recesses since the deposition does not rely on the external application of an electrical current. Much work has been done on the deposition of electrolytic composite coatings. Until recently relatively
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less w o r k h a d b e e n done w i t h e l e c t r o l e s s c o m p o s i t e c o a t i n g s [1 - 5]. This p a p e r p r o v i d e s a d d i t i o n a l i n f o r m a t i o n on the p l a t i n g of electroless nickel c o m p o s i t e c o a t i n g s , the h a r d n e s s v a l u e s o b t a i n e d from t h e s e c o m p o s i t e s a f t e r a n n e a l i n g at different t e m p e r a t u r e s and the s u r f a c e m o r p h o l o g y of the c o a t i n g s obtained. One of t h e p r o b l e m s t h a t was e n c o u n t e r e d d u r i n g this e x p e r i m e n t a l w o r k was the n e c e s s i t y of o b t a i n i n g q u i c k l y r e f r a c t o r y p a r t i c l e s of u n i f o r m size. T h e p o w d e r s t h a t were r e a d i l y a v a i l a b l e h a d a wide r a n g e of p a r t i c l e sizes, e.g. 3 - 10 pro. T w o t y p e s of p o w d e r s w e r e used in this project, TiO z ( a n a t a s e ) w i t h a l m o s t u n i f o r m p a r t i c l e s in the s u b m i c r o n r a n g e and SiC w h i c h h a d l a r g e r p a r t i c l e s in the r a n g e 3 - 10 pm.
2. E x p e r i m e n t a l details T h e basic b a t h used w a s a s t a n d a r d c o m m e r c i a l electroless nickel s o l u t i o n (Nifoss 80, W. C a n n i n g M a t e r i a l s Ltd.) b a s e d on nickel s u l p h a t e a n d s o d i u m h y p o p h o s p h i t e and p r e p a r e d a c c o r d i n g to the s u p p l i e r ' s ins t r u c t i o n s . T h e r e f r a c t o r y p a r t i c l e s (SiC or TiO2) w e r e added to this s o l u t i o n in c o n c e n t r a t i o n s v a r y i n g b e t w e e n 5 a n d 20 g 1-1. The b a t h was o p e r a t e d in the p H r a n g e 4.5 - 4.8 and its p l a t i n g t e m p e r a t u r e r a n g e was 90 - 95 C . T h e a d d i t i o n of v a r i o u s a m o u n t s of SiC and TiO2 was i n v e s t i g a t e d . T h e aim w a s to e s t a b l i s h the o p t i m u m a m o u n t of p o w d e r r e q u i r e d to p r o d u c e a c o m p o s i t e c o a t i n g w i t h the m a x i m u m n u m b e r of included p a r t i c l e s and to a s c e r t a i n w h e t h e r these m a d e the s o l u t i o n u n s t a b l e or inactive. The s o l u t i o n did n o t b e c o m e u n s t a b l e on the addition of p a r t i c l e s n o r did it b e c o m e inactive. W i t h i n the a b o v e limits, the c o n c e n t r a t i o n of p a r t i c l e s did n o t seem to h a v e a significant effect on the t h i c k n e s s a n d d i s p e r s i o n of the codeposits. H o w e v e r , it was e s s e n t i a l t h a t the p a r t i c l e s w e r e well dispersed in t h e s o l u t i o n a n d t h a t the w o r k was r o t a t e d at a r e l a t i v e l y slow speed. It was found to be v i t a l t h a t the w o r k was i m m e r s e d at an a n g l e inside the p l a t i n g s o l u t i o n a n d was n o t j u s t h u n g v e r t i c a l l y (Fig. 1). As usual, the p l a t i n g s o l u t i o n needed to be r e p l e n i s h e d at r e g u l a r i n t e r v a l s . V a r i o u s m e t h o d s of k e e p i n g the p a r t i c l e s in s u s p e n s i o n were tried i n c l u d i n g m a g n e t i c s t i r r i n g a n d t h e use of a p e r i s t a l t i c p u m p to p r o v i d e m o v e m e n t of the p l a t i n g solution. B l o w i n g air (Fig. 1) into a conicalb o t t o m e d vessel p r o d u c e d m o r e c o n s i s t e n t c o a t i n g on all sides of the w o r k b e i n g plated. P a r t i c l e s were t h e n o c c l u d e d on b o t h sides of the plated panels. P a r t i c l e s were i n c l u d e d in the deposit by s e t t l e m e n t due to g r a v i t y a n d s u b s e q u e n t e n t r a p m e n t . V e r y few p a r t i c l e s were i n c o r p o r a t e d on a n y d o w n w a r d - f a c i n g surface. T h e r e f o r e r o t a t i o n a b o u t h o r i z o n t a l axes is e s s e n t i a l to p r o d u c e a u n i f o r m p a r t i c l e d i s t r i b u t i o n on all faces. All the s p e c i m e n s ( m a i n l y mild steel c o m p o n e n t s ) were p r e p a r e d for p l a t i n g u s i n g the following process: (1) s w a b with a n a q u e o u s p a s t e of m a g n e s i u m oxide and t h e n rinse in w a t e r ; (2) i m m e r s e in a c e t o n e ; (3) e l e c t r o l y t i c a l l y cleanse (anodically) in a hot a l k a l i n e c l e a n s e r ; (4) rinse
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[~[]
Electric M o t o r
Plahng ( rubber tube) Plating Solut,on Immersion heater
~mmerslon hea~er
/
Water Tank
Fig. 1. Schematic diagram of the suspension plating apparatus. in running water; (5)dip in 10% H2SO4; (6) rinse in running water; (7) electroless nickel plate; (8) remove from the electroless solution and wash t h o r o u g h ly first in tap water and then in distilled water before drying in hot air. 2.1. Heat treatment A selected number of samples of both the composite (Ni P + T i O 2) coating and standard electroless coatings were heat treated at 200, 400 and 600 °C for 1 h in vacuum and furnace cooled. 2.2. Electron microscopy The distribution of the particles within the deposits was examined with the aid of a scanning electron microscope. Observations were made on the surfaces and on cross sections of the samples. 2.3. Microhardness Microhardness measurements were carried out using a Leitz Miniload hardness tester for small measuring forces. A load of 50 gf was used for measuring the coating hardness. 2.4. Corrosion testing Corrosion testing was carried out using the CASS method according to ISO 3770-1976. The specimens, which had been plated with a TiO2-containing coating 25 ~m thick, were exposed to the corrosive environment for 16 h.
3. D i s c u s s i o n
Ex amin atio n of the deposits with a scanning electron microscope and a Quantimet 720 image analyser showed t hat there was little variation in the
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p e r c e n t a g e of particles, e i t h e r TiO 2 or SiC, i n c l u d e d in c o a t i n g s deposited n e a r t h e s u r f a c e of the p l a t i n g s o l u t i o n a n d c o a t i n g s deposited n e a r the b o t t o m of the c o n t a i n e r . T h u s the d i s t r i b u t i o n of the s u s p e n d e d p a r t i c l e s in the b a t h m u s t h a v e b e e n r e a s o n a b l y u n i f o r m .
3.1. Composite coating containing Ti02 The s u r f a c e of the c o m p o s i t e c o a t i n g (Fig. 2) was r o u g h e r t h a n t h a t of s t a n d a r d e l e c t r o l e s s n i c k e l c o a t i n g s of the s a m e t h i c k n e s s . The oxide p a r t i c l e s i n c l u d e d were g e n e r a l l y in the s u b m i c r o n range. The c o a t i n g as p l a t e d h a d a dull grey a p p e a r a n c e . A s t u d y of deposits in cross section (Fig. 3) r e v e a l e d an e v e n d i s t r i b u t i o n of the p a r t i c l e s t h r o u g h o u t the deposit. As p r e v i o u s l y m e n t i o n e d , the v o l u m e p e r c e n t a g e of TiO 2 v a r i e d little with the d e p t h of the p a n e l in the p l a t i n g solution. T h e Q u a n t i m e t 720 i m a g e analy s e r r e v e a l e d t h a t the v o l u m e p e r c e n t a g e of p a r t i c l e s in the deposit was b e t w e e n 17% and 1 9 ° . The panels were r o t a t e d slowly d u r i n g p l a t i n g and w e r e p o s i t i o n e d at an a n g l e inside the p l a t i n g solution.
Fig. 2. Electron micrograph of the surface of a composite coating containing TiO 2 particles. Fig. 3. Electron micrograph of a cross section of the coating illustrated in Fig. 2 showing the distribution of the TiO2 particles.
3.2. Composite coatings containing SiC The SiC p a r t i c l e s were l a r g e r t h a n the TiO 2 p a r t i c l e s a n d h a d a d r a m a t i c effect on the s u r f a c e m o r p h o l o g y of the deposit (Fig. 4). A crosss e c t i o n a l view of the deposit (Fig. 5) r e v e a l e d t h a t , a l t h o u g h the v o l u m e p e r c e n t a g e of SiC p a r t i c l e s v a r i e d little w i t h the d e p t h of the s p e c i m e n below t h e s u r f a c e of the p l a t i n g solution, twice as m a n y SiC p a r t i c l e s h a d b e e n i n c o r p o r a t e d in the deposit on the s u r f a c e facing u p w a r d s t h a n on the s u r f a c e facing d o w n w a r d s : 11 - 13 v o l . % c o m p a r e d w i t h 5 - 6 vol.% . The SiC p a r t i c l e s w e r e r e l a t i v e l y e v e n l y d i s t r i b u t e d t h r o u g h o u t the deposit containing the l a r g e r v o l u m e p e r c e n t a g e . In the deposit c o n t a i n i n g 5 - 6 v o l . % t h e r e was a slight i n c r e a s e in the density of SiC p a r t i c l e s n e a r the substratedeposit interface. This could h a v e o c c u r r e d b e c a u s e this p a r t i c u l a r p a n e l was n o t r o t a t e d d u r i n g plating.
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Fig. 4. Electron micrograph of the surface of a composite coating containing SiC particles.
(a)
(b)
Fig. 5. Electron micrographs of cross sections of (a) an upward-facing surface and (b) a downward-facing surface of composite coatings containing SiC particles. 3.3. M i c r o h a r d n e s s m e a s u r e m e n t s
M i c r o h a r d n e s s m e a s u r e m e n t s of as-plated c o a t i n g s i n d i c a t e d t h a t the c o m p o s i t e deposits are h a r d e r t h a n a simple e l e c t r o l e s s N i - P alloy c o a t i n g deposited u n d e r the s a m e conditions. G e n e r a l l y , the h a r d n e s s of as-plated c o m p o s i t e deposits was found to be m u c h h i g h e r t h a n t h a t of s t a n d a r d e l e c t r o l e s s nickel coatings. A f t e r h e a t t r e a t m e n t the h a r d n e s s e s of b o t h the c o m p o s i t e c o a t i n g s a n d the e l e c t r o l e s s nickel c o a t i n g s h o w e d i n c r e a s e s but t h e differences w e r e t h e n less m a r k e d . A f t e r h e a t t r e a t m e n t at 200 C the h a r d n e s s of the c o m p o s i t e i n c r e a s e d only slightly b u t the h a r d n e s s v a l u e s w e r e still h i g h e r t h a n t h a t of a s t a n d a r d e l e c t r o l e s s Ni P alloy coating. At 400 ~C the h a r d n e s s of b o t h t y p e s of c o a t i n g r e a c h e d a m a x i m u m . The h a r d n e s s of the c o m p o s i t e c o a t i n g s was a b o v e 1100 HVs0 and t h a t of the s t a n d a r d e l e c t r o l e s s nickel c o a t i n g was a p p r o x i m a t e l y 1000 HVso (Fig. 6). At
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600 °C b o t h coatings showed a decline in hardness, b u t the composite coatings r e t a i n e d h i g h e r h a r d n e s s values (850 HVs0 c o m p a r e d with 650 HVs0).
11o0
-4
1000
~ 9oo tcOmpos~te oohngs
800
=<
\\ 6OO
5OO
200
~00 TEMPERATURE °C
600
Fig. 6. Hardness heat treatment temperature curves for composite coatings and electroless nickel coatings.
Owing to the r e s t r i c t e d t e m p e r a t u r e r a n g e t h a t was available for the h e a t t r e a t m e n t , only the effects of t r e a t m e n t s at 200, 400 and 600 °C were investigated. F o r this r e a s o n the r e l a t i o n of m i c r o h a r d n e s s to h e a t t r e a t m e n t t e m p e r a t u r e is not the usual p a r a b o l i c curve. However, the general p a t t e r n seems to be similar to the parabolic r e l a t i o n s h i p expected from s t a n d a r d electroless nickel coatings but the h a r d n e s s values are always h i g h e r (Fig. 6). The shapes of the graphs show t h a t the b e h a v i o u r of the composite coatings resembles t h a t of electroless nickel in the h a r d e n i n g and softening process a l t h o u g h the a c t u a l h a r d n e s s values are different. The softening of the s t a n d a r d electroless N i - P alloy at t e m p e r a t u r e s above 400 ~'C could be due to the c o a r s e n i n g of the precipitates and to stress relief. It appears t h a t the p r e s e n c e of the particles inhibits the softening t h a t usually occurs with s t a n d a r d electroless nickel coatings at 600 °C. 3.4. Corrosion results
N o r m a l visual and microscope e x a m i n a t i o n s after the CASS test i n d i c a t e d t h a t the c o r r o s i o n r e s i s t a n c e of the composite coatings was no less t h a n t h a t of electroless nickel coatings of the same thickness.
]25
4. Conclusion Electroless nickel plating solutions can be used as a basis for the deposition of composite coatings. The solution did not stop plating or decompose in the presence of suspended particles. As always with electroless nickel deposition the thickness of the composite coating depends on the temperature, the time of plating, the concentrations of nickel and hypophosphite ions present in the plating solution and of course the pH. The amount of suspended particles present in the solution is not very critical: 5 20 g l-1 of particles in the solution can be used but they must be well dispersed in the bath. The work to be plated must be rotated at a fairly slow speed at an angle such th at the particles can fall onto the surface and remain there long enough to become attached to t ha t surface. Thus a composite coating of a uniform thickness containing uniformly dispersed particles will be built up over all the plated sample. Mild steel panels, r e c t a n g u l a r blocks and spherical mild steel components have all been successfully plated by this process, which demonstrates t hat with the correct equipment it should be applicable to most engineering components.
Acknowledgments This work was carried out in the Research L aborat ory of W. Canning Materials Ltd. whom we t h a n k for permission to publish this paper. We also acknowledge the assistance provided by the Departments of Metallurgy and Materials of the Universities of Aston and Nottingham, particularly as regards their provision of facilities for electron microscopy.
References 1 2 3 4 5
W. Metzger, Galvanotechnik, 63'(1972) 722. K. Parker, Plating (East Orange, NJ), 61 (1974) 834. G. Gawrilow and E. Owtscharowa, Metalloberfli~che, 27 (1976) 41. F.N. Hubbell, Trans. Inst. Met. Finish., 56 (1978) 65. H. Honma, N." Ohtake and H. Mitsui, Proc. lOth World Congr. on Metal Finishing, 1980, p. 241.
119
D E P O S I T I O N OF COMPOSITE AUTOCATALYTIC NICKEL COATINGS CONTAINING PARTICLES
M. S. HUSSAIN and T. E. SUCH Research Laboratory, W. Canning Materials Ltd., P.O. Box 288; Great Hampton Street, Birmingham B18 6AS (Gt. Britain)
(Received October 20, 1980)
Summary A process and apparatus for the a ut oc a tal yt i c (electroless) deposition onto a metal substrate of a nickel coating containing dispersed particles is described. The topics investigated included the effect of heat t r e a t m e n t on the hardness of the composite coating, the time taken for the maximum hardness to be attained.and a comparison of the h a r d n e s s - h e a t t r e a t m e n t temperature curves for standard electroless nickel coatings with those for the composite coatings. The surface morphologies of the deposits were studied with the aid of a scanning electron microscope and a Quantimet 720 image analyser.
1. I n t r o d u c t i o n Electroless nickel coatings often offer a better performance for engineering applications t ha n do electroplated coatings. U nfort unat el y electroless nickel coatings suffer from two major defects. They tend to gall if moved in contact with a n o t h e r electroless-nickel-coated part and they rapidly lose their hardness and wear resistance at temperatures above 200 °C. These are two significant limiting factors to the wider usage of electroless nickel coatings. Therefore it was hoped that inclusions produced by depositing a composite coating of particles occluded in an N i -P alloy matrix would increase the wear resistance and the corrosion protection in a wide range of environments. The matrix material would be an alloy of nickel and phosphorus containing between 7% and 10% P. The electroless nickel plating process has several advantages over the conventional electroplating process in t hat deposits of uniform thickness are obtained even in deep recesses since the deposition does not rely on the external application of an electrical current. Much work has been done on the deposition of electrolytic composite coatings. Until recently relatively
0376-4883/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
120
less w o r k h a d b e e n done w i t h e l e c t r o l e s s c o m p o s i t e c o a t i n g s [1 - 5]. This p a p e r p r o v i d e s a d d i t i o n a l i n f o r m a t i o n on the p l a t i n g of electroless nickel c o m p o s i t e c o a t i n g s , the h a r d n e s s v a l u e s o b t a i n e d from t h e s e c o m p o s i t e s a f t e r a n n e a l i n g at different t e m p e r a t u r e s and the s u r f a c e m o r p h o l o g y of the c o a t i n g s obtained. One of t h e p r o b l e m s t h a t was e n c o u n t e r e d d u r i n g this e x p e r i m e n t a l w o r k was the n e c e s s i t y of o b t a i n i n g q u i c k l y r e f r a c t o r y p a r t i c l e s of u n i f o r m size. T h e p o w d e r s t h a t were r e a d i l y a v a i l a b l e h a d a wide r a n g e of p a r t i c l e sizes, e.g. 3 - 10 pro. T w o t y p e s of p o w d e r s w e r e used in this project, TiO z ( a n a t a s e ) w i t h a l m o s t u n i f o r m p a r t i c l e s in the s u b m i c r o n r a n g e and SiC w h i c h h a d l a r g e r p a r t i c l e s in the r a n g e 3 - 10 pm.
2. E x p e r i m e n t a l details T h e basic b a t h used w a s a s t a n d a r d c o m m e r c i a l electroless nickel s o l u t i o n (Nifoss 80, W. C a n n i n g M a t e r i a l s Ltd.) b a s e d on nickel s u l p h a t e a n d s o d i u m h y p o p h o s p h i t e and p r e p a r e d a c c o r d i n g to the s u p p l i e r ' s ins t r u c t i o n s . T h e r e f r a c t o r y p a r t i c l e s (SiC or TiO2) w e r e added to this s o l u t i o n in c o n c e n t r a t i o n s v a r y i n g b e t w e e n 5 a n d 20 g 1-1. The b a t h was o p e r a t e d in the p H r a n g e 4.5 - 4.8 and its p l a t i n g t e m p e r a t u r e r a n g e was 90 - 95 C . T h e a d d i t i o n of v a r i o u s a m o u n t s of SiC and TiO2 was i n v e s t i g a t e d . T h e aim w a s to e s t a b l i s h the o p t i m u m a m o u n t of p o w d e r r e q u i r e d to p r o d u c e a c o m p o s i t e c o a t i n g w i t h the m a x i m u m n u m b e r of included p a r t i c l e s and to a s c e r t a i n w h e t h e r these m a d e the s o l u t i o n u n s t a b l e or inactive. The s o l u t i o n did n o t b e c o m e u n s t a b l e on the addition of p a r t i c l e s n o r did it b e c o m e inactive. W i t h i n the a b o v e limits, the c o n c e n t r a t i o n of p a r t i c l e s did n o t seem to h a v e a significant effect on the t h i c k n e s s a n d d i s p e r s i o n of the codeposits. H o w e v e r , it was e s s e n t i a l t h a t the p a r t i c l e s w e r e well dispersed in t h e s o l u t i o n a n d t h a t the w o r k was r o t a t e d at a r e l a t i v e l y slow speed. It was found to be v i t a l t h a t the w o r k was i m m e r s e d at an a n g l e inside the p l a t i n g s o l u t i o n a n d was n o t j u s t h u n g v e r t i c a l l y (Fig. 1). As usual, the p l a t i n g s o l u t i o n needed to be r e p l e n i s h e d at r e g u l a r i n t e r v a l s . V a r i o u s m e t h o d s of k e e p i n g the p a r t i c l e s in s u s p e n s i o n were tried i n c l u d i n g m a g n e t i c s t i r r i n g a n d t h e use of a p e r i s t a l t i c p u m p to p r o v i d e m o v e m e n t of the p l a t i n g solution. B l o w i n g air (Fig. 1) into a conicalb o t t o m e d vessel p r o d u c e d m o r e c o n s i s t e n t c o a t i n g on all sides of the w o r k b e i n g plated. P a r t i c l e s were t h e n o c c l u d e d on b o t h sides of the plated panels. P a r t i c l e s were i n c l u d e d in the deposit by s e t t l e m e n t due to g r a v i t y a n d s u b s e q u e n t e n t r a p m e n t . V e r y few p a r t i c l e s were i n c o r p o r a t e d on a n y d o w n w a r d - f a c i n g surface. T h e r e f o r e r o t a t i o n a b o u t h o r i z o n t a l axes is e s s e n t i a l to p r o d u c e a u n i f o r m p a r t i c l e d i s t r i b u t i o n on all faces. All the s p e c i m e n s ( m a i n l y mild steel c o m p o n e n t s ) were p r e p a r e d for p l a t i n g u s i n g the following process: (1) s w a b with a n a q u e o u s p a s t e of m a g n e s i u m oxide and t h e n rinse in w a t e r ; (2) i m m e r s e in a c e t o n e ; (3) e l e c t r o l y t i c a l l y cleanse (anodically) in a hot a l k a l i n e c l e a n s e r ; (4) rinse
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[~[]
Electric M o t o r
Plahng ( rubber tube) Plating Solut,on Immersion heater
~mmerslon hea~er
/
Water Tank
Fig. 1. Schematic diagram of the suspension plating apparatus. in running water; (5)dip in 10% H2SO4; (6) rinse in running water; (7) electroless nickel plate; (8) remove from the electroless solution and wash t h o r o u g h ly first in tap water and then in distilled water before drying in hot air. 2.1. Heat treatment A selected number of samples of both the composite (Ni P + T i O 2) coating and standard electroless coatings were heat treated at 200, 400 and 600 °C for 1 h in vacuum and furnace cooled. 2.2. Electron microscopy The distribution of the particles within the deposits was examined with the aid of a scanning electron microscope. Observations were made on the surfaces and on cross sections of the samples. 2.3. Microhardness Microhardness measurements were carried out using a Leitz Miniload hardness tester for small measuring forces. A load of 50 gf was used for measuring the coating hardness. 2.4. Corrosion testing Corrosion testing was carried out using the CASS method according to ISO 3770-1976. The specimens, which had been plated with a TiO2-containing coating 25 ~m thick, were exposed to the corrosive environment for 16 h.
3. D i s c u s s i o n
Ex amin atio n of the deposits with a scanning electron microscope and a Quantimet 720 image analyser showed t hat there was little variation in the
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p e r c e n t a g e of particles, e i t h e r TiO 2 or SiC, i n c l u d e d in c o a t i n g s deposited n e a r t h e s u r f a c e of the p l a t i n g s o l u t i o n a n d c o a t i n g s deposited n e a r the b o t t o m of the c o n t a i n e r . T h u s the d i s t r i b u t i o n of the s u s p e n d e d p a r t i c l e s in the b a t h m u s t h a v e b e e n r e a s o n a b l y u n i f o r m .
3.1. Composite coating containing Ti02 The s u r f a c e of the c o m p o s i t e c o a t i n g (Fig. 2) was r o u g h e r t h a n t h a t of s t a n d a r d e l e c t r o l e s s n i c k e l c o a t i n g s of the s a m e t h i c k n e s s . The oxide p a r t i c l e s i n c l u d e d were g e n e r a l l y in the s u b m i c r o n range. The c o a t i n g as p l a t e d h a d a dull grey a p p e a r a n c e . A s t u d y of deposits in cross section (Fig. 3) r e v e a l e d an e v e n d i s t r i b u t i o n of the p a r t i c l e s t h r o u g h o u t the deposit. As p r e v i o u s l y m e n t i o n e d , the v o l u m e p e r c e n t a g e of TiO 2 v a r i e d little with the d e p t h of the p a n e l in the p l a t i n g solution. T h e Q u a n t i m e t 720 i m a g e analy s e r r e v e a l e d t h a t the v o l u m e p e r c e n t a g e of p a r t i c l e s in the deposit was b e t w e e n 17% and 1 9 ° . The panels were r o t a t e d slowly d u r i n g p l a t i n g and w e r e p o s i t i o n e d at an a n g l e inside the p l a t i n g solution.
Fig. 2. Electron micrograph of the surface of a composite coating containing TiO 2 particles. Fig. 3. Electron micrograph of a cross section of the coating illustrated in Fig. 2 showing the distribution of the TiO2 particles.
3.2. Composite coatings containing SiC The SiC p a r t i c l e s were l a r g e r t h a n the TiO 2 p a r t i c l e s a n d h a d a d r a m a t i c effect on the s u r f a c e m o r p h o l o g y of the deposit (Fig. 4). A crosss e c t i o n a l view of the deposit (Fig. 5) r e v e a l e d t h a t , a l t h o u g h the v o l u m e p e r c e n t a g e of SiC p a r t i c l e s v a r i e d little w i t h the d e p t h of the s p e c i m e n below t h e s u r f a c e of the p l a t i n g solution, twice as m a n y SiC p a r t i c l e s h a d b e e n i n c o r p o r a t e d in the deposit on the s u r f a c e facing u p w a r d s t h a n on the s u r f a c e facing d o w n w a r d s : 11 - 13 v o l . % c o m p a r e d w i t h 5 - 6 vol.% . The SiC p a r t i c l e s w e r e r e l a t i v e l y e v e n l y d i s t r i b u t e d t h r o u g h o u t the deposit containing the l a r g e r v o l u m e p e r c e n t a g e . In the deposit c o n t a i n i n g 5 - 6 v o l . % t h e r e was a slight i n c r e a s e in the density of SiC p a r t i c l e s n e a r the substratedeposit interface. This could h a v e o c c u r r e d b e c a u s e this p a r t i c u l a r p a n e l was n o t r o t a t e d d u r i n g plating.
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Fig. 4. Electron micrograph of the surface of a composite coating containing SiC particles.
(a)
(b)
Fig. 5. Electron micrographs of cross sections of (a) an upward-facing surface and (b) a downward-facing surface of composite coatings containing SiC particles. 3.3. M i c r o h a r d n e s s m e a s u r e m e n t s
M i c r o h a r d n e s s m e a s u r e m e n t s of as-plated c o a t i n g s i n d i c a t e d t h a t the c o m p o s i t e deposits are h a r d e r t h a n a simple e l e c t r o l e s s N i - P alloy c o a t i n g deposited u n d e r the s a m e conditions. G e n e r a l l y , the h a r d n e s s of as-plated c o m p o s i t e deposits was found to be m u c h h i g h e r t h a n t h a t of s t a n d a r d e l e c t r o l e s s nickel coatings. A f t e r h e a t t r e a t m e n t the h a r d n e s s e s of b o t h the c o m p o s i t e c o a t i n g s a n d the e l e c t r o l e s s nickel c o a t i n g s h o w e d i n c r e a s e s but t h e differences w e r e t h e n less m a r k e d . A f t e r h e a t t r e a t m e n t at 200 C the h a r d n e s s of the c o m p o s i t e i n c r e a s e d only slightly b u t the h a r d n e s s v a l u e s w e r e still h i g h e r t h a n t h a t of a s t a n d a r d e l e c t r o l e s s Ni P alloy coating. At 400 ~C the h a r d n e s s of b o t h t y p e s of c o a t i n g r e a c h e d a m a x i m u m . The h a r d n e s s of the c o m p o s i t e c o a t i n g s was a b o v e 1100 HVs0 and t h a t of the s t a n d a r d e l e c t r o l e s s nickel c o a t i n g was a p p r o x i m a t e l y 1000 HVso (Fig. 6). At
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600 °C b o t h coatings showed a decline in hardness, b u t the composite coatings r e t a i n e d h i g h e r h a r d n e s s values (850 HVs0 c o m p a r e d with 650 HVs0).
11o0
-4
1000
~ 9oo tcOmpos~te oohngs
800
=<
\\ 6OO
5OO
200
~00 TEMPERATURE °C
600
Fig. 6. Hardness heat treatment temperature curves for composite coatings and electroless nickel coatings.
Owing to the r e s t r i c t e d t e m p e r a t u r e r a n g e t h a t was available for the h e a t t r e a t m e n t , only the effects of t r e a t m e n t s at 200, 400 and 600 °C were investigated. F o r this r e a s o n the r e l a t i o n of m i c r o h a r d n e s s to h e a t t r e a t m e n t t e m p e r a t u r e is not the usual p a r a b o l i c curve. However, the general p a t t e r n seems to be similar to the parabolic r e l a t i o n s h i p expected from s t a n d a r d electroless nickel coatings but the h a r d n e s s values are always h i g h e r (Fig. 6). The shapes of the graphs show t h a t the b e h a v i o u r of the composite coatings resembles t h a t of electroless nickel in the h a r d e n i n g and softening process a l t h o u g h the a c t u a l h a r d n e s s values are different. The softening of the s t a n d a r d electroless N i - P alloy at t e m p e r a t u r e s above 400 ~'C could be due to the c o a r s e n i n g of the precipitates and to stress relief. It appears t h a t the p r e s e n c e of the particles inhibits the softening t h a t usually occurs with s t a n d a r d electroless nickel coatings at 600 °C. 3.4. Corrosion results
N o r m a l visual and microscope e x a m i n a t i o n s after the CASS test i n d i c a t e d t h a t the c o r r o s i o n r e s i s t a n c e of the composite coatings was no less t h a n t h a t of electroless nickel coatings of the same thickness.
]25
4. Conclusion Electroless nickel plating solutions can be used as a basis for the deposition of composite coatings. The solution did not stop plating or decompose in the presence of suspended particles. As always with electroless nickel deposition the thickness of the composite coating depends on the temperature, the time of plating, the concentrations of nickel and hypophosphite ions present in the plating solution and of course the pH. The amount of suspended particles present in the solution is not very critical: 5 20 g l-1 of particles in the solution can be used but they must be well dispersed in the bath. The work to be plated must be rotated at a fairly slow speed at an angle such th at the particles can fall onto the surface and remain there long enough to become attached to t ha t surface. Thus a composite coating of a uniform thickness containing uniformly dispersed particles will be built up over all the plated sample. Mild steel panels, r e c t a n g u l a r blocks and spherical mild steel components have all been successfully plated by this process, which demonstrates t hat with the correct equipment it should be applicable to most engineering components.
Acknowledgments This work was carried out in the Research L aborat ory of W. Canning Materials Ltd. whom we t h a n k for permission to publish this paper. We also acknowledge the assistance provided by the Departments of Metallurgy and Materials of the Universities of Aston and Nottingham, particularly as regards their provision of facilities for electron microscopy.
References 1 2 3 4 5
W. Metzger, Galvanotechnik, 63'(1972) 722. K. Parker, Plating (East Orange, NJ), 61 (1974) 834. G. Gawrilow and E. Owtscharowa, Metalloberfli~che, 27 (1976) 41. F.N. Hubbell, Trans. Inst. Met. Finish., 56 (1978) 65. H. Honma, N." Ohtake and H. Mitsui, Proc. lOth World Congr. on Metal Finishing, 1980, p. 241.