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Diffusion coating of indium on copper
Surface Technology, 15 (1982) 1 - 10
D I F F U S I O N COATING OF INDIUM ON COPPER
N. A. KERMALI and D. R. GABE Department of Materials Engineering and Design, Lough borough University of Technology, Loughborough, Leics. LE11 3 TU (Gt. Britain)
(Received July 13, 1981)
S u mmar y Electrodeposited coatings of indium on copper were subjected to a diffusive heat t r e a t m e n t {from 30 min to 3 h at 120 - 170 °C) typical of that e m p l o y e d in preparing soft metal bearing overlays. A preliminary study of the diffusion layer indicates a metastable two-phased structure. Data are given for the concent r a t i on profile across the diffusion layer and for the consequent or derived diffusion coefficient values.
1. I n t r o d u c t i o n Despite its relative scarcity and cost, the use and applications of indium remain widespread and exploit some of its unique characteristics. Its main use is in germanium-based s e m i c o n d u c t o r devices, but it is also widely applied in the field o f soft metal bearings where, when deposited on a substantial shell substrate, it can offer excellent tribological properties in all lubrication conditions. These properties include strength, hardness, antiseizure characteristics and corrosion resistance, the norm of conventional behaviour usually being a standard P b - S n white metal. Corrosion resistance becomes of importance when b o u n d a r y layer additives of f a t t y acid origin which break down to form significant quantities of water are used in the lubricant. Since Smart [ 1 ] first r epor t e d the unusually strong inhibiting powers of indium its use has been associated in particular with improved corrosion resistance. While it is likely t hat m a n y corrosion data exist in a confidential form, some results have been r e por t ed for indium diffusion coatings p r o d u c e d by heat treating electrodeposits [2] and then testing them in an aircraft engine lubricating oil. 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
The best bearing surfaces are invariably alloys and n o t pure metals, and for bearing stability under general loading conditions the softer alloys are n o t suitable for making the whole shell or collar unit. The technique is therefore to apply a coating overlay system and to diffuse it into the substrate by controlled heat treatment. Although the coating can be applied by various methods including vacuum evaporation [ 3 ] , cathodic sputtering [3 - 5], ion plating [6] and electroless plating [ 7 ] , electrodeposition is the most i m p o r t a n t method. A range of solutions has been developed [ 8 ] , o f which the sulphate formulations are probably the simplest and easiest to use [ 9 ] , but the possibility of electrodepositing an alloy directly should also be considered [ 1 0 ] . The indium coating is then diffused by a standard heat treatment, typically 2 h at 150 °C in an oil bath [2] although some authorities r e c o m m e n d the use of temperatures above the melting point (157 °C) of indium, typically 170 - 180 °C, which minimizes the formation of surface bubbles or voids [11, 1 2 ] . Smith [ 13] has investigated the depth of diffusion of indium coatings into lead, cadmium, copper, silver and certain alloys after prolonged heat treatments (500 h at 150 °C and 200 h at 120 °C), and in all cases it was f o u n d th at the concentration of indium at the surface remained relatively high even after extensive heat treatment, which thus ensured good bearing characteristics. However, good diffusion data are still n o t available so that the p r o d u c t i o n of prime bearings has to be based on empirical standard practice. F or this reason a preliminary a t t e m p t has been made to measure the diffusion coefficients for a typical diffusion couple (indium in copper) to determine w het her a fundamental approach to this problem is feasible.
2. Experimental details
2.1. Electrodeposition Indium was electrodeposited ont o copper sheet I m m thick using a sulphate bath developed previously by Walsh and Gabe [ 9 ] . The bath contained 20 (g In2(SO4)3) dm -3 and 10 (g Na2SO4) dm -~ at a pH ranging from 2.0 to 2.4, and was operated at a current density o f 2 A dm -2 and a temp er atu r e of 20 °C. The cathode current efficiency is known to be very sensitive to pH changes and was f ound to be a bout 64%. This necessitated the use of an insoluble anode; depletion was avoided by using a split I n - P t anode (area ratio of 70:30), and the pH was t he r eby stabilized after initial adjustment with H 2 S O 4 . The copper sheet was prepared as samples of dimensions 100 mm × 50 mm and was pretreated by degreasing in Genklene, pickling in 10% aqueous HC1 for 15 min, rinsing in demineralized water and drying with alcohol. Coatings 18 - 20 p m thick were electrodeposited on each sample.
2.2. Diffusion treatment A guillotine was used to cut the plated sheets into pieces of dimensions 7 mm × 6 mm which were then heat treated in an air oven at temperatures of 120, 130, 140 and 170 °C. Samples were removed at 30 min intervals up to a total period of 7 h. When diffusing at 170 °C the samples were initially held at 155 °C for a sufficiently long period to prevent dewetting by liquid indium once the temperature was raised above its melting point (157 °C). To facilitate subsequent examination by microscopy and electron microanalysis a layer of copper 20 pm thick was deposited onto the indium-rich surface using a copper fluoborate solution. Thus treatment adequately prevented separation of any of the coating during specimen polishing. The laminated samples were then m o u n t e d for sectioning in an Araldite coldsetting resin held within a steel ring, a piece of pure indium being included in each set to act as an analytical standard. Curing was carried out in a vacuum chamber to ensure degassed resin mounts. Metallographic polishing was carried out using the techniques described by Carapella and Peretti [14] which emphasize the use of low polishing wheel speeds and a paste composed of 600 grade aluminium in water followed by Linde B powder, green soap and water as the finest polishing paste. Etching was carried out using Vilella's reagent (20 ml aqueous HC1 and 4 g picric acid in 400 ml ethanol).
2.3. Diffusion analysis Concentration gradients throughout the diffusion layers were determined by electron probe microanalysis which is a technique based on the energy spectral analysis of the fluorescent X-rays emitted by the electron-irradiated material. The intensities of the characteristic indium fluorescent X-rays were subjected to certain background correction procedures and compared with the intensity of the X-rays from the pure indium standard thus giving an intensity ratio. Measurements were made using a beam size of 2 t~m in 3 pm steps to develop the indium profile; three such profiles from each sample were averaged into a single profile for computer analysis. Diffusion coefficients were obtained by means of the BoltzmannMatano solution for Fick's second law of diffusion which is based on establishing the Matano interface position such that the concentration X distance integrated area on each side is equal. A computer program was employed [ 15] to calculate sets of diffusion coefficients, the slope of the error curve and the area under the exponential used in the calculation, and for simplicity the Matano interface was set at the 50:50 concentration interface. Such an approach is now standard procedure.
3. Results and discussion
Micrographs of the diffusion layer in profile and cross section were obtained by both optical microscopy (Fig. 1) and scanning electron micros-
Q
Fig. 1. Optical rnicrograph showing a section across the Cu overlay-In-Cu substrate interfaces. (Magnification, 257x .) Fig. 2. Scanning electron micrograph of the Cu overlay-In-Cu substrate interfaces. (Magnification, 1170 x. )
c o p y (Fig. 2) in o r d e r t o try t o define s t r u c t u r e or the presence o f intermetallic alloy phases. No evidence was f o u n d o f either, although the difficulties o f a d e q u a t e l y preparing such a s o f t metal m a y have masked the relevant features. E l e c t r o n p r o b e microanalysis was u n d e r t a k e n in o r d e r t o c o m p a r e e l e c t r o n and X-ray images o f the section and again t o characterize any interm e d i a t e phases. In Fig. 3 the e l e c t r o n m i c r o g r a p h has been used t o d e f i n e the e l e c t r o n beam scan position, with line scans for b o t h c o p p e r and indium; the X-ray micrographs are also shown. T h e i n d i u m scan shows evidence o f multiple peaks, and o t h e r samples s h o w e d this e f f e c t m u c h m o r e m a r k e d l y {Fig. 4). Quantitative analysis o f X-ray intensities, w h e n c o r r e c t e d and subjected t o c o m p u t e r analysis, p r o v i d e d diffusion or c o n c e n t r a t i o n profiles across each section, and the values o f the diffusion coefficients were calculated across this profile. S o m e typical d a t a for t w o h e a t t r e a t m e n t periods at each of t h r e e t e m p e r a t u r e s are given in Figs. 5 - 7. A l t h o u g h the c o n c e n t r a t i o n profile is s m o o t h in each case, the diffusion coefficients calculated show a m a r k e d t w o - p e a k characteristic; this was true for m o s t o f the samples studied, the peaks c o r r e s p o n d i n g n o t t o the d e p t h o f diffusion b u t to conc e n t r a t i o n s o f a b o u t 92% In and a b o u t 20% In. It s h o u l d be n o t e d t h a t the diffusion heat t r e a t m e n t was carried o u t in an air oven with a free indium surface. T h e o x i d e film p r o d u c e d was relatively thin and easily r e m o v e d b e f o r e electroplating the sandwich layer o f c o p p e r
(a)
(c)
(b)
(d)
Fig. 3. Electron probe microanalysis micrographs of the Cu-In-Cu interface: (a) electron image and scan line; (b) line scan for indium and copper intensities; (c) copper X-ray micrograph; (d) indium X-ray micrograph.
onto the surface to prevent undue damage during polishing. No delamination occurred and the only obvious damage was some slight indium smearing which is noticeable mainly in the X-ray micrographs (see Fig. 4(d)). Free indium remained on the surface after heat treatment in every case, this being an important criterion in establishing "semi-infinite" diffusion conditions. The diffusion profiles obtained showed that the concentration of indium remained high at the surface and then dropped rapidly into the
(a)
(c)
(b)
(d)
Fig. 4. Electron probe microanalysis micrographs of the Cu In-Cu interface: (a) electron image and scan line; (b) line scan for indium and copper intensities; (c) copper X-ray micrographs; (d) indium X-ray micrograph.
c o p p e r s u b s t r a t e , a f e a t u r e n o t e d b y S m i t h [ 13] in his s t u d y of d i f f u s i o n d e p t h s . Because analysis was m a d e at p o i n t s 3 p m a p a r t the c o n c e n t r a t i o n o f i n d i u m necessarily d r o p p e d s u b s t a n t i a l l y f r o m o n e p o i n t to t h e n e x t , a n d the m e t h o d c a n n o t t h e r e f o r e be r e g a r d e d as highly a c c u r a t e ; nevertheless the curves were s m o o t h . Slight d i f f e r e n c e s in t h e t o t a l t h i c k n e s s m a y n o t be real b e c a u s e t h e s a m p l e s were originally e l e c t r o p l a t e d to a ± 5% t o l e r a n c e and n o n - a d j a c e n t s a m p l e s m i g h t well have varied t o this e x t e n t . T h u s d i f f e r e n c e s in the t o t a l t h i c k n e s s in Fig. 6 s h o u l d n o t be c o n s i d e r e d significant. H o w e v e r , t h e m o r e rapid rate o f d i f f u s i o n at 170 °C can be seen very clearly in Fig. 7
6 to
1
I
I
I
I
~
"
(a) 100
8
20
1 (b)
10
Dtstance(pm)
20
Fig. 5. (a) Diffusion coefficient and (b) c o n c e n t r a t i o n profiles across the diffusion layer for samples h e a t treated at 120 °C for 1 h ( x ) and for 4 h (©).
where a nominal 20 pm coating has diffused to over 40 pm. Study of such fine detail is only of real relevance if an a t t e m p t is being made to compare the diffusion layer structure with that of an equilibrium constitutional diagram. It is well known [16, 17] that these structures are rarely comparable except in special cases of thermal equilibration and complete solid solution alloying. The C u - I n constitution diagram shows five intermetallic compounds within the range 44% - 57% In, each essentially of a high temperature type produced by cooling from the liquid state, together with two terminal solid solutions. It seems likely that the two peaks on the diffusion coefficient graphs reflect the presence of two solid solution phases which are attempting to equilibrate. The electron probe traces {Fig. 4) may offer some corroboration but detailed correlation has not been possible. Only four temperatures were employed for the heat treatment in this initial study so any diffusion coefficient values obtained are likely to be inaccurate. Nevertheless, log D was plotted against 1/T using D values
b lo
u
6
g
g~ I
(a) 100
60
c
20
I
I
0
i
1
I
10 Distar~e (pro)
(b)
20
Fig. 6. (a) Diffusion coefficient and (b) concentration profiles across the diffusion layer for samples heat treated at 130 °C for 1.5 h (x) and for 3.5 h (o).
o b t a i n e d at the M a t a n o interface. T h e p l o t is s h o w n in Fig. 8 which, w h e n the Arrhenius e q u a t i o n c o n v e n t i o n D = Do exp( --
Q/RT)
is used, gives an activation energy Q o f 18.3 kcal m o l - : K - : ( 7 6 . 6 8 kJ mo1-1 K - : ) and a f r e q u e n c y f a c t o r Do o f 0.23 cm 2 s -1. T h e activation e n e r g y n o r m a l l y varies substantially with o r i e n t a t i o n and grain size b u t has a value similar t o t h a t for t h e self-diffusion o f i n d i u m (18.7 kcal m o l - : K - z ) and the value o f Do is o f the right o r d e r for a m e t a l o f this t y p e [ 1 8 ] . While d a t a o f this t y p e can r e a s o n a b l y be used t o give estimates o f the d i f f u s i o n layer thickness and the t h e r m a l b e h a v i o u r , the s t r u c t u r e o f the layer is clearly n o t simple and a m o r e detailed s t u d y b y e i t h e r m i c r o b e a m analysis {beam size, less t h a n 1/~m) or an alternative t e c h n i q u e such as Auger s p e c t r o s c o p y is necessary b e f o r e definitive data can be r e p o r t e d .
0 1LO "~ 120 "0 100 u 80 % o 6O; 2
40
20 i
i
i
i
(a) 100
80 60
c 20 r I 10
20
30 Distonce (IJm)
LO
(b) Fig. 7. (a) Diffusion coefficient and (b) concentration profiles across the diffusion layer for samples heat treated at 170 °C for 1 h ( x ) and for 1.5 h (o).
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i
,
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!
1
qO0
c~ o~
-I0 5
-11 0
) ,
l
23
I
I
24
f
l
25
lIT (~10 J)
Fig. 8. Arrhenius p l o t for diffusion c o e f f i c i e n t values at 120, 1 3 0 , 1 4 0 and 1 7 0 °(3.
10 Acknowledgments W e a r e g r a t e f u l t o P r o f e s s o r I. A . M e n z i e s f o r t h e p r o v i s i o n o f l a b o r a t o r y f a c i l i t i e s . W e a r e p a r t i c u l a r l y i n d e b t e d t o Mr. T. C. H o p k i n s f o r a s s i s t a n c e with the electron probe microanalysis and with the computer analysis of the data obtained.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
C. F. Smart, Trans. AIME, Tech. Publ. 900, 1938. J. M. Freund, H. B. Linford and P. W. Schutz, Trans. Electrochem. Soc., 84 (1943) 65. R. B. Belser and W. E. Woolf, J. Inst. Met., 91 (1963) 857. J. E. Castle and H. B. Stevenson, U.S. Patent 3,303,047, 1967, Y. T. Silivonen and D. A. Boyd, Rev. Sci. lnstrum., 31 (1960} 992. D. G. Teer, Trans. Inst. Met. Finish., 54 (1976) 159. N. V. Philips, Dutch Patent 6,606,262, 1967. F. C. Walsh and D. R. Gabe, Surf. Technol., 8 (1979) 87. F. C. Walsh and D. R. Gabe, Surf. Technol., 6 (1978) 425. F. C. Walsh and D. R. Gabe, Surf Technol., 13 (1981) 305. M. Schofield, Metallurgia, 68 (May 1961) 231. J. Albin, Mater. Methods, 27 (1948) 88. A. A. Smith, Jr., Trans. AIME, Tech. Publ. 1640, 1943. S. C. Carapella and E. A. Peretti, in M. T. Ludwick (ed.), Indium, Indium Corporation of America, New York, 1959, p. 122. T. C. Hopkins, personal communication, 1978. D. R. Gabe, Rev. High Temp. Mater., 1 (1972) 157. D. R. Gabe, Iron Steel Inst.-Inst. Met. Conf., Physical Metallurgy o f Surface Coatings, London, 1973, Freund, Tel Aviv, 1972. C. J. Smithells (ed.), Metals Reference Book, Butterworths, London, 4th edn., 1967.
D I F F U S I O N COATING OF INDIUM ON COPPER
N. A. KERMALI and D. R. GABE Department of Materials Engineering and Design, Lough borough University of Technology, Loughborough, Leics. LE11 3 TU (Gt. Britain)
(Received July 13, 1981)
S u mmar y Electrodeposited coatings of indium on copper were subjected to a diffusive heat t r e a t m e n t {from 30 min to 3 h at 120 - 170 °C) typical of that e m p l o y e d in preparing soft metal bearing overlays. A preliminary study of the diffusion layer indicates a metastable two-phased structure. Data are given for the concent r a t i on profile across the diffusion layer and for the consequent or derived diffusion coefficient values.
1. I n t r o d u c t i o n Despite its relative scarcity and cost, the use and applications of indium remain widespread and exploit some of its unique characteristics. Its main use is in germanium-based s e m i c o n d u c t o r devices, but it is also widely applied in the field o f soft metal bearings where, when deposited on a substantial shell substrate, it can offer excellent tribological properties in all lubrication conditions. These properties include strength, hardness, antiseizure characteristics and corrosion resistance, the norm of conventional behaviour usually being a standard P b - S n white metal. Corrosion resistance becomes of importance when b o u n d a r y layer additives of f a t t y acid origin which break down to form significant quantities of water are used in the lubricant. Since Smart [ 1 ] first r epor t e d the unusually strong inhibiting powers of indium its use has been associated in particular with improved corrosion resistance. While it is likely t hat m a n y corrosion data exist in a confidential form, some results have been r e por t ed for indium diffusion coatings p r o d u c e d by heat treating electrodeposits [2] and then testing them in an aircraft engine lubricating oil. 0376-4583/82/0000-0000/$02.75
© Elsevier Sequoia/Printed in The Netherlands
The best bearing surfaces are invariably alloys and n o t pure metals, and for bearing stability under general loading conditions the softer alloys are n o t suitable for making the whole shell or collar unit. The technique is therefore to apply a coating overlay system and to diffuse it into the substrate by controlled heat treatment. Although the coating can be applied by various methods including vacuum evaporation [ 3 ] , cathodic sputtering [3 - 5], ion plating [6] and electroless plating [ 7 ] , electrodeposition is the most i m p o r t a n t method. A range of solutions has been developed [ 8 ] , o f which the sulphate formulations are probably the simplest and easiest to use [ 9 ] , but the possibility of electrodepositing an alloy directly should also be considered [ 1 0 ] . The indium coating is then diffused by a standard heat treatment, typically 2 h at 150 °C in an oil bath [2] although some authorities r e c o m m e n d the use of temperatures above the melting point (157 °C) of indium, typically 170 - 180 °C, which minimizes the formation of surface bubbles or voids [11, 1 2 ] . Smith [ 13] has investigated the depth of diffusion of indium coatings into lead, cadmium, copper, silver and certain alloys after prolonged heat treatments (500 h at 150 °C and 200 h at 120 °C), and in all cases it was f o u n d th at the concentration of indium at the surface remained relatively high even after extensive heat treatment, which thus ensured good bearing characteristics. However, good diffusion data are still n o t available so that the p r o d u c t i o n of prime bearings has to be based on empirical standard practice. F or this reason a preliminary a t t e m p t has been made to measure the diffusion coefficients for a typical diffusion couple (indium in copper) to determine w het her a fundamental approach to this problem is feasible.
2. Experimental details
2.1. Electrodeposition Indium was electrodeposited ont o copper sheet I m m thick using a sulphate bath developed previously by Walsh and Gabe [ 9 ] . The bath contained 20 (g In2(SO4)3) dm -3 and 10 (g Na2SO4) dm -~ at a pH ranging from 2.0 to 2.4, and was operated at a current density o f 2 A dm -2 and a temp er atu r e of 20 °C. The cathode current efficiency is known to be very sensitive to pH changes and was f ound to be a bout 64%. This necessitated the use of an insoluble anode; depletion was avoided by using a split I n - P t anode (area ratio of 70:30), and the pH was t he r eby stabilized after initial adjustment with H 2 S O 4 . The copper sheet was prepared as samples of dimensions 100 mm × 50 mm and was pretreated by degreasing in Genklene, pickling in 10% aqueous HC1 for 15 min, rinsing in demineralized water and drying with alcohol. Coatings 18 - 20 p m thick were electrodeposited on each sample.
2.2. Diffusion treatment A guillotine was used to cut the plated sheets into pieces of dimensions 7 mm × 6 mm which were then heat treated in an air oven at temperatures of 120, 130, 140 and 170 °C. Samples were removed at 30 min intervals up to a total period of 7 h. When diffusing at 170 °C the samples were initially held at 155 °C for a sufficiently long period to prevent dewetting by liquid indium once the temperature was raised above its melting point (157 °C). To facilitate subsequent examination by microscopy and electron microanalysis a layer of copper 20 pm thick was deposited onto the indium-rich surface using a copper fluoborate solution. Thus treatment adequately prevented separation of any of the coating during specimen polishing. The laminated samples were then m o u n t e d for sectioning in an Araldite coldsetting resin held within a steel ring, a piece of pure indium being included in each set to act as an analytical standard. Curing was carried out in a vacuum chamber to ensure degassed resin mounts. Metallographic polishing was carried out using the techniques described by Carapella and Peretti [14] which emphasize the use of low polishing wheel speeds and a paste composed of 600 grade aluminium in water followed by Linde B powder, green soap and water as the finest polishing paste. Etching was carried out using Vilella's reagent (20 ml aqueous HC1 and 4 g picric acid in 400 ml ethanol).
2.3. Diffusion analysis Concentration gradients throughout the diffusion layers were determined by electron probe microanalysis which is a technique based on the energy spectral analysis of the fluorescent X-rays emitted by the electron-irradiated material. The intensities of the characteristic indium fluorescent X-rays were subjected to certain background correction procedures and compared with the intensity of the X-rays from the pure indium standard thus giving an intensity ratio. Measurements were made using a beam size of 2 t~m in 3 pm steps to develop the indium profile; three such profiles from each sample were averaged into a single profile for computer analysis. Diffusion coefficients were obtained by means of the BoltzmannMatano solution for Fick's second law of diffusion which is based on establishing the Matano interface position such that the concentration X distance integrated area on each side is equal. A computer program was employed [ 15] to calculate sets of diffusion coefficients, the slope of the error curve and the area under the exponential used in the calculation, and for simplicity the Matano interface was set at the 50:50 concentration interface. Such an approach is now standard procedure.
3. Results and discussion
Micrographs of the diffusion layer in profile and cross section were obtained by both optical microscopy (Fig. 1) and scanning electron micros-
Q
Fig. 1. Optical rnicrograph showing a section across the Cu overlay-In-Cu substrate interfaces. (Magnification, 257x .) Fig. 2. Scanning electron micrograph of the Cu overlay-In-Cu substrate interfaces. (Magnification, 1170 x. )
c o p y (Fig. 2) in o r d e r t o try t o define s t r u c t u r e or the presence o f intermetallic alloy phases. No evidence was f o u n d o f either, although the difficulties o f a d e q u a t e l y preparing such a s o f t metal m a y have masked the relevant features. E l e c t r o n p r o b e microanalysis was u n d e r t a k e n in o r d e r t o c o m p a r e e l e c t r o n and X-ray images o f the section and again t o characterize any interm e d i a t e phases. In Fig. 3 the e l e c t r o n m i c r o g r a p h has been used t o d e f i n e the e l e c t r o n beam scan position, with line scans for b o t h c o p p e r and indium; the X-ray micrographs are also shown. T h e i n d i u m scan shows evidence o f multiple peaks, and o t h e r samples s h o w e d this e f f e c t m u c h m o r e m a r k e d l y {Fig. 4). Quantitative analysis o f X-ray intensities, w h e n c o r r e c t e d and subjected t o c o m p u t e r analysis, p r o v i d e d diffusion or c o n c e n t r a t i o n profiles across each section, and the values o f the diffusion coefficients were calculated across this profile. S o m e typical d a t a for t w o h e a t t r e a t m e n t periods at each of t h r e e t e m p e r a t u r e s are given in Figs. 5 - 7. A l t h o u g h the c o n c e n t r a t i o n profile is s m o o t h in each case, the diffusion coefficients calculated show a m a r k e d t w o - p e a k characteristic; this was true for m o s t o f the samples studied, the peaks c o r r e s p o n d i n g n o t t o the d e p t h o f diffusion b u t to conc e n t r a t i o n s o f a b o u t 92% In and a b o u t 20% In. It s h o u l d be n o t e d t h a t the diffusion heat t r e a t m e n t was carried o u t in an air oven with a free indium surface. T h e o x i d e film p r o d u c e d was relatively thin and easily r e m o v e d b e f o r e electroplating the sandwich layer o f c o p p e r
(a)
(c)
(b)
(d)
Fig. 3. Electron probe microanalysis micrographs of the Cu-In-Cu interface: (a) electron image and scan line; (b) line scan for indium and copper intensities; (c) copper X-ray micrograph; (d) indium X-ray micrograph.
onto the surface to prevent undue damage during polishing. No delamination occurred and the only obvious damage was some slight indium smearing which is noticeable mainly in the X-ray micrographs (see Fig. 4(d)). Free indium remained on the surface after heat treatment in every case, this being an important criterion in establishing "semi-infinite" diffusion conditions. The diffusion profiles obtained showed that the concentration of indium remained high at the surface and then dropped rapidly into the
(a)
(c)
(b)
(d)
Fig. 4. Electron probe microanalysis micrographs of the Cu In-Cu interface: (a) electron image and scan line; (b) line scan for indium and copper intensities; (c) copper X-ray micrographs; (d) indium X-ray micrograph.
c o p p e r s u b s t r a t e , a f e a t u r e n o t e d b y S m i t h [ 13] in his s t u d y of d i f f u s i o n d e p t h s . Because analysis was m a d e at p o i n t s 3 p m a p a r t the c o n c e n t r a t i o n o f i n d i u m necessarily d r o p p e d s u b s t a n t i a l l y f r o m o n e p o i n t to t h e n e x t , a n d the m e t h o d c a n n o t t h e r e f o r e be r e g a r d e d as highly a c c u r a t e ; nevertheless the curves were s m o o t h . Slight d i f f e r e n c e s in t h e t o t a l t h i c k n e s s m a y n o t be real b e c a u s e t h e s a m p l e s were originally e l e c t r o p l a t e d to a ± 5% t o l e r a n c e and n o n - a d j a c e n t s a m p l e s m i g h t well have varied t o this e x t e n t . T h u s d i f f e r e n c e s in the t o t a l t h i c k n e s s in Fig. 6 s h o u l d n o t be c o n s i d e r e d significant. H o w e v e r , t h e m o r e rapid rate o f d i f f u s i o n at 170 °C can be seen very clearly in Fig. 7
6 to
1
I
I
I
I
~
"
(a) 100
8
20
1 (b)
10
Dtstance(pm)
20
Fig. 5. (a) Diffusion coefficient and (b) c o n c e n t r a t i o n profiles across the diffusion layer for samples h e a t treated at 120 °C for 1 h ( x ) and for 4 h (©).
where a nominal 20 pm coating has diffused to over 40 pm. Study of such fine detail is only of real relevance if an a t t e m p t is being made to compare the diffusion layer structure with that of an equilibrium constitutional diagram. It is well known [16, 17] that these structures are rarely comparable except in special cases of thermal equilibration and complete solid solution alloying. The C u - I n constitution diagram shows five intermetallic compounds within the range 44% - 57% In, each essentially of a high temperature type produced by cooling from the liquid state, together with two terminal solid solutions. It seems likely that the two peaks on the diffusion coefficient graphs reflect the presence of two solid solution phases which are attempting to equilibrate. The electron probe traces {Fig. 4) may offer some corroboration but detailed correlation has not been possible. Only four temperatures were employed for the heat treatment in this initial study so any diffusion coefficient values obtained are likely to be inaccurate. Nevertheless, log D was plotted against 1/T using D values
b lo
u
6
g
g~ I
(a) 100
60
c
20
I
I
0
i
1
I
10 Distar~e (pro)
(b)
20
Fig. 6. (a) Diffusion coefficient and (b) concentration profiles across the diffusion layer for samples heat treated at 130 °C for 1.5 h (x) and for 3.5 h (o).
o b t a i n e d at the M a t a n o interface. T h e p l o t is s h o w n in Fig. 8 which, w h e n the Arrhenius e q u a t i o n c o n v e n t i o n D = Do exp( --
Q/RT)
is used, gives an activation energy Q o f 18.3 kcal m o l - : K - : ( 7 6 . 6 8 kJ mo1-1 K - : ) and a f r e q u e n c y f a c t o r Do o f 0.23 cm 2 s -1. T h e activation e n e r g y n o r m a l l y varies substantially with o r i e n t a t i o n and grain size b u t has a value similar t o t h a t for t h e self-diffusion o f i n d i u m (18.7 kcal m o l - : K - z ) and the value o f Do is o f the right o r d e r for a m e t a l o f this t y p e [ 1 8 ] . While d a t a o f this t y p e can r e a s o n a b l y be used t o give estimates o f the d i f f u s i o n layer thickness and the t h e r m a l b e h a v i o u r , the s t r u c t u r e o f the layer is clearly n o t simple and a m o r e detailed s t u d y b y e i t h e r m i c r o b e a m analysis {beam size, less t h a n 1/~m) or an alternative t e c h n i q u e such as Auger s p e c t r o s c o p y is necessary b e f o r e definitive data can be r e p o r t e d .
0 1LO "~ 120 "0 100 u 80 % o 6O; 2
40
20 i
i
i
i
(a) 100
80 60
c 20 r I 10
20
30 Distonce (IJm)
LO
(b) Fig. 7. (a) Diffusion coefficient and (b) concentration profiles across the diffusion layer for samples heat treated at 170 °C for 1 h ( x ) and for 1.5 h (o).
i
I
i
,
i
!
1
qO0
c~ o~
-I0 5
-11 0
) ,
l
23
I
I
24
f
l
25
lIT (~10 J)
Fig. 8. Arrhenius p l o t for diffusion c o e f f i c i e n t values at 120, 1 3 0 , 1 4 0 and 1 7 0 °(3.
10 Acknowledgments W e a r e g r a t e f u l t o P r o f e s s o r I. A . M e n z i e s f o r t h e p r o v i s i o n o f l a b o r a t o r y f a c i l i t i e s . W e a r e p a r t i c u l a r l y i n d e b t e d t o Mr. T. C. H o p k i n s f o r a s s i s t a n c e with the electron probe microanalysis and with the computer analysis of the data obtained.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
C. F. Smart, Trans. AIME, Tech. Publ. 900, 1938. J. M. Freund, H. B. Linford and P. W. Schutz, Trans. Electrochem. Soc., 84 (1943) 65. R. B. Belser and W. E. Woolf, J. Inst. Met., 91 (1963) 857. J. E. Castle and H. B. Stevenson, U.S. Patent 3,303,047, 1967, Y. T. Silivonen and D. A. Boyd, Rev. Sci. lnstrum., 31 (1960} 992. D. G. Teer, Trans. Inst. Met. Finish., 54 (1976) 159. N. V. Philips, Dutch Patent 6,606,262, 1967. F. C. Walsh and D. R. Gabe, Surf. Technol., 8 (1979) 87. F. C. Walsh and D. R. Gabe, Surf. Technol., 6 (1978) 425. F. C. Walsh and D. R. Gabe, Surf Technol., 13 (1981) 305. M. Schofield, Metallurgia, 68 (May 1961) 231. J. Albin, Mater. Methods, 27 (1948) 88. A. A. Smith, Jr., Trans. AIME, Tech. Publ. 1640, 1943. S. C. Carapella and E. A. Peretti, in M. T. Ludwick (ed.), Indium, Indium Corporation of America, New York, 1959, p. 122. T. C. Hopkins, personal communication, 1978. D. R. Gabe, Rev. High Temp. Mater., 1 (1972) 157. D. R. Gabe, Iron Steel Inst.-Inst. Met. Conf., Physical Metallurgy o f Surface Coatings, London, 1973, Freund, Tel Aviv, 1972. C. J. Smithells (ed.), Metals Reference Book, Butterworths, London, 4th edn., 1967.