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Silver coatings obtained using a wire spraying technique
Surface
Technology,
19 (1983)
363 - 371
363
SILVER COATINGS TECHNIQUE
OBTAINED
MOHAMMAD
P. R. KRISHNAMOORTHY
GHOUSE,
USING A WIRE SPRAYING
and B. S. MANN
Materials Science Laboratory, Corporate Research and Development Heavy Electricals Ltd., Vikas Nagar, Hyderabad (India) (Received
February
Division,
Bharat
9, 1983)
Summary Silver coatings were obtained on aluminium alloy (LM 6) using a wire spraying technique and their bond strength and electrical contact resistance were measured using an Elcometer adhesion tester and a microhmmeter respectively. The thickness and the surface roughness of the coatings were measured with an optical microscope and a Taylor Hobson Surtronic 3 instrument respectively; the structure and oxidation of the coatings were examined by means of a scanning electron microscope and a Philips X-ray diffractometer. The results of the present investigation reveal that there is no marked change in the bond strength of the sprayed coatings when bond coatings (Ni,Al/Cu) are applied. The electrical contact resistance of silver coatings obtained without a bond coating seems to be lower than that for silver coatings with a bond coating. Silver coatings obtained using the wire spraying technique were not oxidized.
1. Introduction Silver [ 1, 21 is the whitest metal and has the highest electrical and thermal conductivities. It is relatively corrosion resistant. In recent years, technical and industrial uses for silver, such as photography, electrical contacts, brazing alloys and batteries, have surpassed the aesthetic uses for the metal. Both copper and silver are tarnished by sulphur compounds in the atmosphere but silver is not oxidized and, for this reason, silver is a generally useful electrical contact material. Silver in pure or alloyed form is the most widely used material for medium current make-and-break contacts. It is also used in contacts subjected to occasional sliding such as on rotary switches and, to a limited extent, for low resistance sliding contacts such as slip rings. As aluminium has good electrical and mechanical properties, ready availability and favourable cost, it is now widely used as a conductor material. It has replaced copper in many electrical applications. It has advantages over 0316-4583/83/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
364
c o p p e r since the same mass will c o n d u c t m o r e c u r r e n t for t h e same d r o p in voltage. As a c o n t a c t metal, a l u m i n i u m is generally p o o r because it oxidizes readily. When it is used in c o n t a c t s it s h o u l d be p l a t e d or c o a t e d with c o p p e r or silver. A l t h o u g h silver is generally d e p o s i t e d b y means o f electroplating, this is an u n e c o n o m i c a l and c u m b e r s o m e process, particularly w h e n the c o m p o n e n t s are fairly large. T h u s an alternative t e c h n i q u e k n o w n as the spraying t e c h n i q u e m a y be used f o r depositing silver o n t o large c o m p o n e n t s . In this p a p e r the p r e p a r a t i o n o f silver coatings using a wire spraying techn i q u e and t h e p r o p e r t i e s o f t h e coatings are p r e s e n t e d .
NOZZLE
7
SPRAYED MATERIAL (SILVER) \,
i /
:,
WIRE /
V/#' GASES
.
< ×2
z
,
d"/;< \
FUEL
?
;7 f
/ .~" ;* ,,;+
~
/'/
,~x-~-4q
i
L ASPIRATING
GASES J
BASE M E ~ A L --J(Al- ALLOY )
i
~.0 - 2 5 c m
(a)
f&
t
(
(b) Fig. 1. (a) Schematic diagram of the wire spraying technique; (b) experimental set-up: @ , spray gun with silver w i r e ; @ , oxygen cylinder;@, acetylene cylinder; @ , aluminium alloy sheet; @ , gas flowmeter.
365
2. Experimental details Silver coatings were obtained on aluminium alloy sheets using a wire spraying technique, and the coatings were characterized on the basis of the following parameters: coating thickness; bond strength; surface roughness; electrical contact resistance at various loads (10 - 2000 kgf); X-ray diffraction analysis; microstructure (examined using both optical and scanning electron microscopes). The procedures adopted in these experiments are described in the following paragraphs.
2.1. Experimental procedure of wire spraying Figure 1 shows the experimental set-up for the wire spraying technique in which silver wire 3.17 mm in diameter is fed through an oxyacetylene flame and compressed air in a wire spray gun supplied by the Metallizing Equipment Co., Jodhpur. When the silver wire meets the flame, it melts, and the compressed air atomizes the molten metal which is then sprayed onto the surface of the aluminium alloy (grit blasted) placed in front of the spray gun. The compressed air also helps in feeding the wire at the required rate. An aluminium alloy sheet of dimensions 300 mm X 200 mm X 5 mm was cleaned by grit blasting using 20 mesh (iron grit). The temperature of the surface of the sheet was measured with a contact thermocouple (ironconstantan). The sheet was coated with silver with and without an intermediate coating of nickel aluminide (NiaA1) and copper using the wire spraying technique. To allow the silver-coated sheets to cool slowly, they were covered with ceramic wool. The sheets were cut into sections 60 mm X 40 mm X 5 mm for various studies. The operating conditions for the wire spraying technique are given in Table 1.
TABLE 1 Operating c o n d i t i o n s for t h e wire spraying t e c h n i q u e Pressure o f c o m p r e s s e d air for grit blasting b e f o r e c o a t i n g w i t h Ag Distance o f j o b f r o m t h e grit blasting gun Surface finish R a a o f AI alloy s h e e t b e f o r e c o a t i n g w i t h Ag (i.e. a f t e r grit blasting) T e m p e r a t u r e o f t h e s u b s t r a t e (A1 alloy s h e e t ) Pressure o f 02 Pressure o f a c e t y l e n e Pressure o f c o m p r e s s e d air Distance o f j o b f r o m t h e gun during c o a t i n g with Ag F e e d rate o f t h e Ag wire Cooling rate o f t h e Ag c o a t i n g s All pressures are gauge pressures. a A r i t h m e t i c m e a n o r c e n t r e - l i n e average value.
5.5 - 6.0 kgf c m - 2 5.0 c m 7 - 8.5 p m 100 °C (danger o f o x i d a t i o n ) 3.0 kgf cm - 2 1.4 kgf em - 2 4.2 kgf c m - 2 10 - 12 c m 2.7 kg h -1 12 °C m i n - 1
366
2.1.1. Advantages of wire spraying T h e a d v a n t a g e s o f t h e wire s p r a y i n g t e c h n i q u e include t h e following. (1) It is e c o n o m i c a l , especially w h e n large c o m p o n e n t s are to be c o a t e d . (2) I t is v e r y a d a p t a b l e . (3) S p r a y i n g can be carried o u t in air. (4) T h e r e are n o p o l l u t i o n hazards.
2.1.2. Disadvantages of wire spraying T h e t w o d i s a d v a n t a g e s o f this t e c h n i q u e are as follows. (1) T h e s u r f a c e o f t h e silver c o a t i n g is r o u g h . (2) T h e b o n d s t r e n g t h is l o w e r t h a n t h a t f o r coatings o b t a i n e d b y electroplating.
2.2. Thickness F o r t h e m e a s u r e m e n t o f t h e t h i c k n e s s o f t h e silver coatings the s a m p l e s were c u t i n t o pieces a p p r o x i m a t e l y 10 m m × 10 m m in size and transverse s e c t i o n s o f t h e pieces w e r e m o u n t e d in cold Araldite. A f t e r t h e resin h a d b e e n c u r e d f o r 24 h at r o o m t e m p e r a t u r e , t h e sections were p o l i s h e d using e m e r y p a p e r a n d t h e n finally l a p p e d o n a S y l v e t c l o t h in t h e p r e s e n c e o f an a l u m i n a slurry. T h e t r a n s v e r s e s e c t i o n s w e r e e x a m i n e d w i t h an o p t i c a l m i c r o s c o p e a n d t h e t h i c k n e s s o f t h e c o a t i n g was m e a s u r e d .
2.3. Bond strength T h e b o n d s t r e n g t h o f t h e silver coatings o b t a i n e d o n an a l u m i n i u m a l l o y s h e e t w i t h a n d w i t h o u t b o n d c o a t i n g s such as Ni3A1 a n d c o p p e r was m e a s u r e d b y using an E l c o m e t e r a d h e s i o n t e s t e r m o d e l 106 supplied b y E l c o m e t e r I n s t r u m e n t s L t d . , M a n c h e s t e r . Circular a l u m i n i u m dollies 20 m m
Fig. 2. Experimental set-up for the measurement of the bond strength of the silver coatings: @ , dolly (fixed with Araldite on the silver coating); @ , load-adjusting s c r e w , @ , silver-coated aluminium alloy sheet;@, wrench spanner.
367 in d i a m e t e r were cleaned b y grit blasting and t h e n degreasing with t r i c h l o r o e t h y l e n e . Special grade Araldite was s m e a r e d o n the base o f the dollies. These dollies were t h e n j o i n e d to t h e silver-coated a l u m i n i u m alloy sheet as s h o w n in Fig. 2. T h e Araldite was c u r e d f o r 72 h at r o o m t e m p e r a t u r e . T h e silver-coated sheet with t h e a l u m i n i u m dollies was fixed t o the t r i p o d assembly o f a pulling device. The dollies were t h e n pulled a w a y f r o m the c o a t e d sheet b y using a spring t e n s i o n system. The load at w h i c h the c o a t i n g b e c a m e d e t a c h e d f r o m t h e substrate was r e c o r d e d .
2.4. Surface (finish) roughness and electrical contact resistance The surface r o u g h n e s s o f t h e silver coatings was m e a s u r e d using a T a y l o r H o b s o n S u r t r o n i c 3 i n s t r u m e n t , and the electrical c o n t a c t resistance o f t h e c o a t i n g was m e a s u r e d with a m i c r o h m m e t e r . T h e e x p e r i m e n t a l p r o c e d u r e a d o p t e d in the p r e s e n t investigation f o r m e a s u r i n g the surface r o u g h n e s s and the electrical c o n t a c t resistance o f the c o a t i n g was t h a t already discussed in detail elsewhere [ 3 ].
2.5. X-ray diffraction analysis A Philips X-ray d i f f r a c t o m e t e r was used to o b t a i n the X-ray d i f f r a c t i o n patterns for a study of the oxidation.
2.6. M icrostruc ture The m i c r o s t r u c t u r e o f t h e silver c o a t i n g was e x a m i n e d with b o t h optical and scanning e l e c t r o n m i c r o s c o p e s .
3. Results a n d discussion
3.1. Thickness The thicknesses o f the silver coatings o b t a i n e d o n a l u m i n i u m alloy using the wire s p r a y i n g t e c h n i q u e are s h o w n in Table 2. It can be seen t h a t the m i n i m u m thickness o b t a i n e d using the wire spraying t e c h n i q u e is a b o u t TABLE 2 Thicknesses of the silver coatings obtained using the wire spraying technique
Sample number
Coating
Thicknesses (pm) of the following coatings Ni3AI
Cu
Ag
1 2
Ag NiaA1 + Ag
-25 - 35
---
25 - 35 75 - 90
3
Ni3A1 + Cu + Ag
25 - 35
25 - 35
80 - 90
Remarks
Single pass NiaAl , single pass; Ag, double pass NiaA1, single pass; Cu, single pass ; Ag, double pass
368
25 - 35 p m for a single pass. T h e thickness o f the silver coatings for a d o u b l e pass is f o u n d t o be 75 - 90 p m .
3.2. Bond strength T h e b o n d strengths o f t h e silver coatings o b t a i n e d using the wire spraying t e c h n i q u e are s h o w n in Table 3. It can be seen t h a t the b o n d strengths o f the silver coatings o b t a i n e d with and w i t h o u t i n t e r m e d i a t e layers ( b o n d coatings such as Ni3A1 and c o p p e r ) are a l m o s t t h e same, ranging f r o m 80 to 90 kgf cm -2. In c o n t r a s t , s p r a y e d coatings with an Ni3A1 [4] b o n d coating on mild steel substrates s h o w i m p r o v e d b o n d strengths. TABLE 3 Bond strengths of the silver coatings obtained using the wire spraying technique
Sample number
Coating
Bond strength ( k g f c m - 2 )
1 2 3
Ag Ni3A1 + A g Ni3A1 + C u + A g
80 - 90 80 - 90 80 - 90
3.3. Surface roughness T h e surface roughnesses o f the coatings b e f o r e and a f t e r the measurem e n t o f the electrical c o n t a c t resistance with applied loads ranging f r o m 10 t o 2 0 0 0 kgf are given in Table 4. F r o m the table it can be seen t h a t t h e surface roughness is increased a f t e r d e p o s i t i o n o n t o a grit-blasted surface of a l u m i n i u m alloy. It is also n o t e d t h a t t h e r e is no m a r k e d change in the surface roughness o f t h e coating w h e n loads are applied during the measurem e n t o f t h e electrical c o n t a c t resistance. TABLE 4 S u r f a c e r o u g h n e s s e s R a o f t h e c o a t i n g s b e f o r e a n d a f t e r l o a d s a re applied
Sample number
Coating
Surface roughness R a a ( p m ) o f coatings under the following conditions Before deposition (grit-blasted surface)
1 2 3 4
A1 a l l o y ( b a s e ) Ag Ni3AI + Ag Ni3AI + Cu + Ag
7 - 8.5 ----
After deposition
- -
8.5 9.0 - 11.5 11.9 13.0 11.0-14.0 7 . 0
8.5 - 10.0 11 1 2 . 5 11.5-13.5
aArithmetic mean or centre-line average value.
After toads (10 - 2000 kgf) are applied
-
-
-
369
3.4. Electrical c o n t a c t resistance Figure 3 shows the effect o f the load on the electrical c o n t a c t resistance o f aluminium a l l o y - a l u m i n i u m alloy samples (grit-blasted surfaces) and on silver coatings with and w i t h o u t bond coatings (or intermediate layers) such as Ni3A1 and copper. It can be seen t ha t the electrical cont act resistance of aluminium alloy samples (freshly grit blasted) decreases from 280 to 15 p~2 for increasing loads from 10 to 2000 kgf. (Loads were applied using an Instron Universal testing machine.) In contrast, the electrical cont act resistance o f silver coatings w i t h o u t undercoatings (i.e. Ni3A1 and copper) varies from 18 to 2.75 p ~ for the same loads. The electrical cont act resistance of silver coatings with bond coatings, namely Ni3A1 and copper, varies from 21 to 3 p~2; w i t h o u t c oppe r the electrical c on t act resistance of the silver coatings varies from 26 to 3.5 p ~ . It is n o t e d that the electrical c o n t a c t resistance of silver coatings with bond coatings of Ni3A1 is slightly higher than that for the silver coatings obtained w i t hout bond coatings. This is because of the higher electrical resistance of Ni3A1. It is observed that the electrical c o n t a c t resistance decreases with increasing load f or all the coatings. This is expect ed since the c o n t a c t area increases with increasing applied load. The results for the electrical c o n t a c t resistance of the silver coatings obtained using the wire spraying technique are quite comparable with the
300
200
x
100
I
x
x
o
o
#20 ,5 ~ o
c~ 10
o
o
q
~s
J
200
400
600
BOO
1000 LOA0
1200
1400
|
i
t
1600
1800
2000
(kgf)
Fig. 3. Effect of the load on the electrical contact resistance (surface area, 16 cm2): X, aluminium alloy-aluminium alloy (grit-blasted surface); o, (Ni3A1 + Ag)-(Ag + Ni3A1); /A, (Ni3AI + Cu + Ag)-(Ag + Cu + Ni3Al); r3, Ag-Ag.
370
results reported by Ghouse and coworkers [3, 5] for silver coatings obtained by electroplating.
3.5. X-ray diffraction analysis Table 5 shows the X-ray diffraction analysis data for silver coatings obtained on aluminium alloy using the wire spraying technique. Silver peaks only are observed in the X-ray diffraction pattern of the silver coating. No silver oxide peaks can be seen. TABLE 5 X-ray diffraction analysis of silver coatings A S T M card n u m b e r [6]
4-783
d value
R e l a t i v e i n t e n s i t y 1/11
(A)
(%)
2.36 2.04 1.44
90 100 70
Phases p r e s e n t
Remarks
Ag only
No oxide peaks are present
3.6. Optical and scanning electron micrographs o f the silver coating Figure 4 shows an optical micrograph (transverse section)and a scanning electron micrograph of the silver coating obtained using the wire spraying technique. Although the surface of the silver coating (sprayed) is uneven
(a)
(b)
Fig. 4. (a) Optical micrograph of the silver coating taken perpendicular to the base: A, aluminium alloy; B, bond coating (Ni3AI); C, silver coating; dark layer, Araldite. (b) Scanning electron micrograph of the silver coating. (Magnification: (a) 100X.)
371 w i t h s o m e surface p o r o s i t y , t h e electrical c o n t a c t r e s i s t a n c e o f t h e c o a t i n g is q u i t e c o m p a r a b l e w i t h t h a t f o r t h e silver c o a t i n g o b t a i n e d b y e l e c t r o p l a t ing [3, 5 ] . T h e r e a s o n f o r this m a y be t h a t t h e t h i c k n e s s o f t h e silver c o a t i n g o b t a i n e d using t h e wire s p r a y i n g t e c h n i q u e is a l m o s t t h r e e t o f o u r t i m e s higher t h a n t h a t o f t h e c o a t i n g o b t a i n e d b y e l e c t r o p l a t i n g . Such surface p o r o s i t y has no e f f e c t o n t h e electrical c o n t a c t resistance o f t h e silver coating. 4. C o n c l u s i o n s (1) A silver c o a t i n g w i t h o u t o x i d a t i o n c a n be o b t a i n e d o n an active m e t a l such as a l u m i n i u m using a n o r m a l wire s p r a y i n g t e c h n i q u e . (2) An i n t e r m e d i a t e b o n d c o a t i n g b e f o r e t h e silver is a p p l i e d d o e s n o t s e e m to be n e c e s s a r y f o r g o o d a d h e s i o n . (3) T h e r e is n o m a r k e d c h a n g e in t h e surface r o u g h n e s s o f t h e c o a t i n g a f t e r loads o f 10 - 2 0 0 0 k g f are applied. (4) N o a p p r e c i a b l e c h a n g e s in t h e electrical c o n t a c t r e s i s t a n c e o f t h e silver c o a t i n g s o c c u r f o r a p p l i e d l o a d s a b o v e 1 0 0 0 kgf. H e n c e , t h e wire s p r a y i n g t e c h n i q u e can be used f o r d e p o s i t i n g silver o n t o large c o m p o n e n t s w h e n e l e c t r o p l a t i n g is d i f f i c u l t a n d u n e c o n o m i c a l . Acknowledgments T h a n k s are d u e to Mr. D. M a h e s w a r R e d d y a n d Mr. H. R. Balu f o r t h e i r assistance d u r i n g t h e s p r a y i n g o f t h e silver coatings a n d to Mr. S. M u k e r j e e a n d Mr. A. B h o o m a i a h o f t h e High V o l t a g e L a b o r a t o r y a n d Mr. K. N a g a c h a n d r a o f t h e I n s u l a t i o n T e c h n o l o g y L a b o r a t o r y f o r t h e i r help d u r i n g the m e a s u r e m e n t o f t h e electrical c o n t a c t r e s i s t a n c e o f t h e silver coatings. T h e a u t h o r s ' t h a n k s are also d u e t o t h e m a n a g e m e n t o f t h e C o r p o r a t e R e s e a r c h a n d D e v e l o p m e n t Division, B h a r a t H e a v y Electricals Ltd., H y d e r a b a d , f o r p e r m i s s i o n to p u b l i s h this p a p e r . References I Metals Handbook, Vol. 1, American Society for Metals, Metals Park, OH, 8th edn.,
1972. 2 Kirk-Othmer (ed.), Encyclopedia o f Chemical Technology, Vol. 18, Wiley-Interscience, New York, 2nd edn., 1969. 3 M. Ghouse, B. S. Mann and D. M. Reddy, Silver coatings on aluminium alloy by wire spraying, electroplating and electroless plating techniques: a comparative study, Project Rep., May 1982 (Bharat Heavy Electricals Ltd., Vikas Nagar, Hyderabad). 4 H. S. Ingham and A. P. Shepard, Metco Flame Spray Handbook, Vol. III, Plasma Flame Process, Metco Inc., Westbury, Long Island, NY, 1965. 5 M. Ghouse, Electrical contact resistance of electroplated silver coatings, submitted to Plat. Surf. Finish.
6 Joint Committee on Powder Diffraction Standards, Selected Powder Diffraction Data: Metals and Alloys Data Book, Vol. 1, International Center for Diffraction Data, Swarthmore, PA, 1978, p. 81.
Technology,
19 (1983)
363 - 371
363
SILVER COATINGS TECHNIQUE
OBTAINED
MOHAMMAD
P. R. KRISHNAMOORTHY
GHOUSE,
USING A WIRE SPRAYING
and B. S. MANN
Materials Science Laboratory, Corporate Research and Development Heavy Electricals Ltd., Vikas Nagar, Hyderabad (India) (Received
February
Division,
Bharat
9, 1983)
Summary Silver coatings were obtained on aluminium alloy (LM 6) using a wire spraying technique and their bond strength and electrical contact resistance were measured using an Elcometer adhesion tester and a microhmmeter respectively. The thickness and the surface roughness of the coatings were measured with an optical microscope and a Taylor Hobson Surtronic 3 instrument respectively; the structure and oxidation of the coatings were examined by means of a scanning electron microscope and a Philips X-ray diffractometer. The results of the present investigation reveal that there is no marked change in the bond strength of the sprayed coatings when bond coatings (Ni,Al/Cu) are applied. The electrical contact resistance of silver coatings obtained without a bond coating seems to be lower than that for silver coatings with a bond coating. Silver coatings obtained using the wire spraying technique were not oxidized.
1. Introduction Silver [ 1, 21 is the whitest metal and has the highest electrical and thermal conductivities. It is relatively corrosion resistant. In recent years, technical and industrial uses for silver, such as photography, electrical contacts, brazing alloys and batteries, have surpassed the aesthetic uses for the metal. Both copper and silver are tarnished by sulphur compounds in the atmosphere but silver is not oxidized and, for this reason, silver is a generally useful electrical contact material. Silver in pure or alloyed form is the most widely used material for medium current make-and-break contacts. It is also used in contacts subjected to occasional sliding such as on rotary switches and, to a limited extent, for low resistance sliding contacts such as slip rings. As aluminium has good electrical and mechanical properties, ready availability and favourable cost, it is now widely used as a conductor material. It has replaced copper in many electrical applications. It has advantages over 0316-4583/83/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
364
c o p p e r since the same mass will c o n d u c t m o r e c u r r e n t for t h e same d r o p in voltage. As a c o n t a c t metal, a l u m i n i u m is generally p o o r because it oxidizes readily. When it is used in c o n t a c t s it s h o u l d be p l a t e d or c o a t e d with c o p p e r or silver. A l t h o u g h silver is generally d e p o s i t e d b y means o f electroplating, this is an u n e c o n o m i c a l and c u m b e r s o m e process, particularly w h e n the c o m p o n e n t s are fairly large. T h u s an alternative t e c h n i q u e k n o w n as the spraying t e c h n i q u e m a y be used f o r depositing silver o n t o large c o m p o n e n t s . In this p a p e r the p r e p a r a t i o n o f silver coatings using a wire spraying techn i q u e and t h e p r o p e r t i e s o f t h e coatings are p r e s e n t e d .
NOZZLE
7
SPRAYED MATERIAL (SILVER) \,
i /
:,
WIRE /
V/#' GASES
.
< ×2
z
,
d"/;< \
FUEL
?
;7 f
/ .~" ;* ,,;+
~
/'/
,~x-~-4q
i
L ASPIRATING
GASES J
BASE M E ~ A L --J(Al- ALLOY )
i
~.0 - 2 5 c m
(a)
f&
t
(
(b) Fig. 1. (a) Schematic diagram of the wire spraying technique; (b) experimental set-up: @ , spray gun with silver w i r e ; @ , oxygen cylinder;@, acetylene cylinder; @ , aluminium alloy sheet; @ , gas flowmeter.
365
2. Experimental details Silver coatings were obtained on aluminium alloy sheets using a wire spraying technique, and the coatings were characterized on the basis of the following parameters: coating thickness; bond strength; surface roughness; electrical contact resistance at various loads (10 - 2000 kgf); X-ray diffraction analysis; microstructure (examined using both optical and scanning electron microscopes). The procedures adopted in these experiments are described in the following paragraphs.
2.1. Experimental procedure of wire spraying Figure 1 shows the experimental set-up for the wire spraying technique in which silver wire 3.17 mm in diameter is fed through an oxyacetylene flame and compressed air in a wire spray gun supplied by the Metallizing Equipment Co., Jodhpur. When the silver wire meets the flame, it melts, and the compressed air atomizes the molten metal which is then sprayed onto the surface of the aluminium alloy (grit blasted) placed in front of the spray gun. The compressed air also helps in feeding the wire at the required rate. An aluminium alloy sheet of dimensions 300 mm X 200 mm X 5 mm was cleaned by grit blasting using 20 mesh (iron grit). The temperature of the surface of the sheet was measured with a contact thermocouple (ironconstantan). The sheet was coated with silver with and without an intermediate coating of nickel aluminide (NiaA1) and copper using the wire spraying technique. To allow the silver-coated sheets to cool slowly, they were covered with ceramic wool. The sheets were cut into sections 60 mm X 40 mm X 5 mm for various studies. The operating conditions for the wire spraying technique are given in Table 1.
TABLE 1 Operating c o n d i t i o n s for t h e wire spraying t e c h n i q u e Pressure o f c o m p r e s s e d air for grit blasting b e f o r e c o a t i n g w i t h Ag Distance o f j o b f r o m t h e grit blasting gun Surface finish R a a o f AI alloy s h e e t b e f o r e c o a t i n g w i t h Ag (i.e. a f t e r grit blasting) T e m p e r a t u r e o f t h e s u b s t r a t e (A1 alloy s h e e t ) Pressure o f 02 Pressure o f a c e t y l e n e Pressure o f c o m p r e s s e d air Distance o f j o b f r o m t h e gun during c o a t i n g with Ag F e e d rate o f t h e Ag wire Cooling rate o f t h e Ag c o a t i n g s All pressures are gauge pressures. a A r i t h m e t i c m e a n o r c e n t r e - l i n e average value.
5.5 - 6.0 kgf c m - 2 5.0 c m 7 - 8.5 p m 100 °C (danger o f o x i d a t i o n ) 3.0 kgf cm - 2 1.4 kgf em - 2 4.2 kgf c m - 2 10 - 12 c m 2.7 kg h -1 12 °C m i n - 1
366
2.1.1. Advantages of wire spraying T h e a d v a n t a g e s o f t h e wire s p r a y i n g t e c h n i q u e include t h e following. (1) It is e c o n o m i c a l , especially w h e n large c o m p o n e n t s are to be c o a t e d . (2) I t is v e r y a d a p t a b l e . (3) S p r a y i n g can be carried o u t in air. (4) T h e r e are n o p o l l u t i o n hazards.
2.1.2. Disadvantages of wire spraying T h e t w o d i s a d v a n t a g e s o f this t e c h n i q u e are as follows. (1) T h e s u r f a c e o f t h e silver c o a t i n g is r o u g h . (2) T h e b o n d s t r e n g t h is l o w e r t h a n t h a t f o r coatings o b t a i n e d b y electroplating.
2.2. Thickness F o r t h e m e a s u r e m e n t o f t h e t h i c k n e s s o f t h e silver coatings the s a m p l e s were c u t i n t o pieces a p p r o x i m a t e l y 10 m m × 10 m m in size and transverse s e c t i o n s o f t h e pieces w e r e m o u n t e d in cold Araldite. A f t e r t h e resin h a d b e e n c u r e d f o r 24 h at r o o m t e m p e r a t u r e , t h e sections were p o l i s h e d using e m e r y p a p e r a n d t h e n finally l a p p e d o n a S y l v e t c l o t h in t h e p r e s e n c e o f an a l u m i n a slurry. T h e t r a n s v e r s e s e c t i o n s w e r e e x a m i n e d w i t h an o p t i c a l m i c r o s c o p e a n d t h e t h i c k n e s s o f t h e c o a t i n g was m e a s u r e d .
2.3. Bond strength T h e b o n d s t r e n g t h o f t h e silver coatings o b t a i n e d o n an a l u m i n i u m a l l o y s h e e t w i t h a n d w i t h o u t b o n d c o a t i n g s such as Ni3A1 a n d c o p p e r was m e a s u r e d b y using an E l c o m e t e r a d h e s i o n t e s t e r m o d e l 106 supplied b y E l c o m e t e r I n s t r u m e n t s L t d . , M a n c h e s t e r . Circular a l u m i n i u m dollies 20 m m
Fig. 2. Experimental set-up for the measurement of the bond strength of the silver coatings: @ , dolly (fixed with Araldite on the silver coating); @ , load-adjusting s c r e w , @ , silver-coated aluminium alloy sheet;@, wrench spanner.
367 in d i a m e t e r were cleaned b y grit blasting and t h e n degreasing with t r i c h l o r o e t h y l e n e . Special grade Araldite was s m e a r e d o n the base o f the dollies. These dollies were t h e n j o i n e d to t h e silver-coated a l u m i n i u m alloy sheet as s h o w n in Fig. 2. T h e Araldite was c u r e d f o r 72 h at r o o m t e m p e r a t u r e . T h e silver-coated sheet with t h e a l u m i n i u m dollies was fixed t o the t r i p o d assembly o f a pulling device. The dollies were t h e n pulled a w a y f r o m the c o a t e d sheet b y using a spring t e n s i o n system. The load at w h i c h the c o a t i n g b e c a m e d e t a c h e d f r o m t h e substrate was r e c o r d e d .
2.4. Surface (finish) roughness and electrical contact resistance The surface r o u g h n e s s o f t h e silver coatings was m e a s u r e d using a T a y l o r H o b s o n S u r t r o n i c 3 i n s t r u m e n t , and the electrical c o n t a c t resistance o f t h e c o a t i n g was m e a s u r e d with a m i c r o h m m e t e r . T h e e x p e r i m e n t a l p r o c e d u r e a d o p t e d in the p r e s e n t investigation f o r m e a s u r i n g the surface r o u g h n e s s and the electrical c o n t a c t resistance o f the c o a t i n g was t h a t already discussed in detail elsewhere [ 3 ].
2.5. X-ray diffraction analysis A Philips X-ray d i f f r a c t o m e t e r was used to o b t a i n the X-ray d i f f r a c t i o n patterns for a study of the oxidation.
2.6. M icrostruc ture The m i c r o s t r u c t u r e o f t h e silver c o a t i n g was e x a m i n e d with b o t h optical and scanning e l e c t r o n m i c r o s c o p e s .
3. Results a n d discussion
3.1. Thickness The thicknesses o f the silver coatings o b t a i n e d o n a l u m i n i u m alloy using the wire s p r a y i n g t e c h n i q u e are s h o w n in Table 2. It can be seen t h a t the m i n i m u m thickness o b t a i n e d using the wire spraying t e c h n i q u e is a b o u t TABLE 2 Thicknesses of the silver coatings obtained using the wire spraying technique
Sample number
Coating
Thicknesses (pm) of the following coatings Ni3AI
Cu
Ag
1 2
Ag NiaA1 + Ag
-25 - 35
---
25 - 35 75 - 90
3
Ni3A1 + Cu + Ag
25 - 35
25 - 35
80 - 90
Remarks
Single pass NiaAl , single pass; Ag, double pass NiaA1, single pass; Cu, single pass ; Ag, double pass
368
25 - 35 p m for a single pass. T h e thickness o f the silver coatings for a d o u b l e pass is f o u n d t o be 75 - 90 p m .
3.2. Bond strength T h e b o n d strengths o f t h e silver coatings o b t a i n e d using the wire spraying t e c h n i q u e are s h o w n in Table 3. It can be seen t h a t the b o n d strengths o f the silver coatings o b t a i n e d with and w i t h o u t i n t e r m e d i a t e layers ( b o n d coatings such as Ni3A1 and c o p p e r ) are a l m o s t t h e same, ranging f r o m 80 to 90 kgf cm -2. In c o n t r a s t , s p r a y e d coatings with an Ni3A1 [4] b o n d coating on mild steel substrates s h o w i m p r o v e d b o n d strengths. TABLE 3 Bond strengths of the silver coatings obtained using the wire spraying technique
Sample number
Coating
Bond strength ( k g f c m - 2 )
1 2 3
Ag Ni3A1 + A g Ni3A1 + C u + A g
80 - 90 80 - 90 80 - 90
3.3. Surface roughness T h e surface roughnesses o f the coatings b e f o r e and a f t e r the measurem e n t o f the electrical c o n t a c t resistance with applied loads ranging f r o m 10 t o 2 0 0 0 kgf are given in Table 4. F r o m the table it can be seen t h a t t h e surface roughness is increased a f t e r d e p o s i t i o n o n t o a grit-blasted surface of a l u m i n i u m alloy. It is also n o t e d t h a t t h e r e is no m a r k e d change in the surface roughness o f t h e coating w h e n loads are applied during the measurem e n t o f t h e electrical c o n t a c t resistance. TABLE 4 S u r f a c e r o u g h n e s s e s R a o f t h e c o a t i n g s b e f o r e a n d a f t e r l o a d s a re applied
Sample number
Coating
Surface roughness R a a ( p m ) o f coatings under the following conditions Before deposition (grit-blasted surface)
1 2 3 4
A1 a l l o y ( b a s e ) Ag Ni3AI + Ag Ni3AI + Cu + Ag
7 - 8.5 ----
After deposition
- -
8.5 9.0 - 11.5 11.9 13.0 11.0-14.0 7 . 0
8.5 - 10.0 11 1 2 . 5 11.5-13.5
aArithmetic mean or centre-line average value.
After toads (10 - 2000 kgf) are applied
-
-
-
369
3.4. Electrical c o n t a c t resistance Figure 3 shows the effect o f the load on the electrical c o n t a c t resistance o f aluminium a l l o y - a l u m i n i u m alloy samples (grit-blasted surfaces) and on silver coatings with and w i t h o u t bond coatings (or intermediate layers) such as Ni3A1 and copper. It can be seen t ha t the electrical cont act resistance of aluminium alloy samples (freshly grit blasted) decreases from 280 to 15 p~2 for increasing loads from 10 to 2000 kgf. (Loads were applied using an Instron Universal testing machine.) In contrast, the electrical cont act resistance o f silver coatings w i t h o u t undercoatings (i.e. Ni3A1 and copper) varies from 18 to 2.75 p ~ for the same loads. The electrical cont act resistance of silver coatings with bond coatings, namely Ni3A1 and copper, varies from 21 to 3 p~2; w i t h o u t c oppe r the electrical c on t act resistance of the silver coatings varies from 26 to 3.5 p ~ . It is n o t e d that the electrical c o n t a c t resistance of silver coatings with bond coatings of Ni3A1 is slightly higher than that for the silver coatings obtained w i t hout bond coatings. This is because of the higher electrical resistance of Ni3A1. It is observed that the electrical c o n t a c t resistance decreases with increasing load f or all the coatings. This is expect ed since the c o n t a c t area increases with increasing applied load. The results for the electrical c o n t a c t resistance of the silver coatings obtained using the wire spraying technique are quite comparable with the
300
200
x
100
I
x
x
o
o
#20 ,5 ~ o
c~ 10
o
o
q
~s
J
200
400
600
BOO
1000 LOA0
1200
1400
|
i
t
1600
1800
2000
(kgf)
Fig. 3. Effect of the load on the electrical contact resistance (surface area, 16 cm2): X, aluminium alloy-aluminium alloy (grit-blasted surface); o, (Ni3A1 + Ag)-(Ag + Ni3A1); /A, (Ni3AI + Cu + Ag)-(Ag + Cu + Ni3Al); r3, Ag-Ag.
370
results reported by Ghouse and coworkers [3, 5] for silver coatings obtained by electroplating.
3.5. X-ray diffraction analysis Table 5 shows the X-ray diffraction analysis data for silver coatings obtained on aluminium alloy using the wire spraying technique. Silver peaks only are observed in the X-ray diffraction pattern of the silver coating. No silver oxide peaks can be seen. TABLE 5 X-ray diffraction analysis of silver coatings A S T M card n u m b e r [6]
4-783
d value
R e l a t i v e i n t e n s i t y 1/11
(A)
(%)
2.36 2.04 1.44
90 100 70
Phases p r e s e n t
Remarks
Ag only
No oxide peaks are present
3.6. Optical and scanning electron micrographs o f the silver coating Figure 4 shows an optical micrograph (transverse section)and a scanning electron micrograph of the silver coating obtained using the wire spraying technique. Although the surface of the silver coating (sprayed) is uneven
(a)
(b)
Fig. 4. (a) Optical micrograph of the silver coating taken perpendicular to the base: A, aluminium alloy; B, bond coating (Ni3AI); C, silver coating; dark layer, Araldite. (b) Scanning electron micrograph of the silver coating. (Magnification: (a) 100X.)
371 w i t h s o m e surface p o r o s i t y , t h e electrical c o n t a c t r e s i s t a n c e o f t h e c o a t i n g is q u i t e c o m p a r a b l e w i t h t h a t f o r t h e silver c o a t i n g o b t a i n e d b y e l e c t r o p l a t ing [3, 5 ] . T h e r e a s o n f o r this m a y be t h a t t h e t h i c k n e s s o f t h e silver c o a t i n g o b t a i n e d using t h e wire s p r a y i n g t e c h n i q u e is a l m o s t t h r e e t o f o u r t i m e s higher t h a n t h a t o f t h e c o a t i n g o b t a i n e d b y e l e c t r o p l a t i n g . Such surface p o r o s i t y has no e f f e c t o n t h e electrical c o n t a c t resistance o f t h e silver coating. 4. C o n c l u s i o n s (1) A silver c o a t i n g w i t h o u t o x i d a t i o n c a n be o b t a i n e d o n an active m e t a l such as a l u m i n i u m using a n o r m a l wire s p r a y i n g t e c h n i q u e . (2) An i n t e r m e d i a t e b o n d c o a t i n g b e f o r e t h e silver is a p p l i e d d o e s n o t s e e m to be n e c e s s a r y f o r g o o d a d h e s i o n . (3) T h e r e is n o m a r k e d c h a n g e in t h e surface r o u g h n e s s o f t h e c o a t i n g a f t e r loads o f 10 - 2 0 0 0 k g f are applied. (4) N o a p p r e c i a b l e c h a n g e s in t h e electrical c o n t a c t r e s i s t a n c e o f t h e silver c o a t i n g s o c c u r f o r a p p l i e d l o a d s a b o v e 1 0 0 0 kgf. H e n c e , t h e wire s p r a y i n g t e c h n i q u e can be used f o r d e p o s i t i n g silver o n t o large c o m p o n e n t s w h e n e l e c t r o p l a t i n g is d i f f i c u l t a n d u n e c o n o m i c a l . Acknowledgments T h a n k s are d u e to Mr. D. M a h e s w a r R e d d y a n d Mr. H. R. Balu f o r t h e i r assistance d u r i n g t h e s p r a y i n g o f t h e silver coatings a n d to Mr. S. M u k e r j e e a n d Mr. A. B h o o m a i a h o f t h e High V o l t a g e L a b o r a t o r y a n d Mr. K. N a g a c h a n d r a o f t h e I n s u l a t i o n T e c h n o l o g y L a b o r a t o r y f o r t h e i r help d u r i n g the m e a s u r e m e n t o f t h e electrical c o n t a c t r e s i s t a n c e o f t h e silver coatings. T h e a u t h o r s ' t h a n k s are also d u e t o t h e m a n a g e m e n t o f t h e C o r p o r a t e R e s e a r c h a n d D e v e l o p m e n t Division, B h a r a t H e a v y Electricals Ltd., H y d e r a b a d , f o r p e r m i s s i o n to p u b l i s h this p a p e r . References I Metals Handbook, Vol. 1, American Society for Metals, Metals Park, OH, 8th edn.,
1972. 2 Kirk-Othmer (ed.), Encyclopedia o f Chemical Technology, Vol. 18, Wiley-Interscience, New York, 2nd edn., 1969. 3 M. Ghouse, B. S. Mann and D. M. Reddy, Silver coatings on aluminium alloy by wire spraying, electroplating and electroless plating techniques: a comparative study, Project Rep., May 1982 (Bharat Heavy Electricals Ltd., Vikas Nagar, Hyderabad). 4 H. S. Ingham and A. P. Shepard, Metco Flame Spray Handbook, Vol. III, Plasma Flame Process, Metco Inc., Westbury, Long Island, NY, 1965. 5 M. Ghouse, Electrical contact resistance of electroplated silver coatings, submitted to Plat. Surf. Finish.
6 Joint Committee on Powder Diffraction Standards, Selected Powder Diffraction Data: Metals and Alloys Data Book, Vol. 1, International Center for Diffraction Data, Swarthmore, PA, 1978, p. 81.