Applied Surface Science 33/34 (1988) 1001-1008 1001 North-Holland, Amsterdam

C O R R O S I O N O F C o - N i AND C o - C r S P U T T E R E D F I L M S IN H I G H H U M I D I T Y A T M O S P H E R E S

Y. H A Y A S H I , K. H Y O D O and H. H A G I

Department of Iron & Steel Metallurgy, Faculty of Engineering Kyushu University, Fukuoka 812, Japan

Received 23 August 1987; accepted for publication 22 October 1987

Corrosion properties of Co-20Ni and Co-20Cr sputtered films have been studied in high humidity atmospheres at room temperature. Corrosion took place in the form of localized corrosion, and resulted in the formation of spotty corrosion products on the surface. The corrosion products are cobalt oxide for both Co-Ni and Co-Cr alloys. The corrosion is of electrochemical nature occurring in the adsorbed water layer caused by the preferential dissolution and oxide formation of cobalt. The rate of corrosion was estimated by measurement of the change in electrical resistance and the observation of the growth of corrosion products.

1. Introduction

Thin films of cobalt base alloys have been receiving much at tent ion as magnetic recording media [1]. Sputter deposited C o - N i and C o - C r films do not have adequate resistance to atmospheric corrosion, so that the alloy films are usually designed to be used with protective overcoat layers. Unders tand ing the corrosion properties of uncoated alloy films themselves, however, is profitable for developing recording media with high resistance against corro- sion.

Corrosion properties of cobalt and cobalt alloy films have been studied in various controlled atmospheres [2,3]. Fujinaga [4] observed localized corrosion spots on C o - N i sputtered films exposed to high humidi ty atmospheres. The composi t ion of the atmosphere has a great influence on corrosion. However, indoors, and at ambient temperature, corrosion of an electrochemical nature is the essential feature, since capillary condensat ion of water vapor takes place at a concave meniscus even at low humidi ty [5]. Detailed studies on the form of corrosion, format ion of corrosion products and rate of corrosion in humid atmospheres are required for unders tanding the corrosion mechanism and also for developing reliable recording media.

Corrosion properties of C o - 2 0 N i and C o - 2 0 C r sputtered films in high humidity atmospheres have been studied by observat ion and analysis of

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1002 Y. Hayashi et aL / Corrosion of Co Ni and Co Cr sputtered films

corrosion products on the surface. The main interest was in clarifying the effect of the alloying element and film structure on the localized nature of the corrosion. The rate of corrosion was studied by observing the change ot surface coverage of the corrosion products and also by measurement of the change in the electrical resistance of the films during corrosion.

2. Experimental

Specimen films were deposited on glass substrates by RF sputtering at various Ar gas pressures using Co 20Ni and Co-20Cr alloy targets, which were made by vacuum melting of the respective electrolytically pure metals. Film thickness was 0.5 ttm and substrate temperature was 423 K. For TEM observation a thin sputtered film was formed on a micromesh grid.

Specimens were exposed to high humidity atmospheres, 80% to 100% RH at 293 K, to accelerate corrosion. For each exposure time, the specimen surface was observed using an optical microscope. Corrosion took place in the form of localized corrosion, and small spotty corrosion products were formed. The corrosion products were examined by EPMA, ESCA, and TEM. The progress of the corrosion was monitored by measuring the change in the electrical resistance of the specimen films. The resistance was measured by a "four- terminal" method with specimens 200 mm long with cross section of 0.4 mm 2. The rate of corrosion was also estimated from the rate of surface coverage of the corrosion products.

3. Results and discussion

3. i. Observation of surface appearance

Film specimens were exposed to high humidity atmospheres at room temperature. Films of Co-Ni alloy showed quick tarnishing at 80% RH. But C o - C r alloy showed no change in surface appearance even after 30 days at a humidity below 90%. The corrosion is electrochemical in nature, and a water layer adsorbed on the surface plays an essential role. The corrosion occurred in the form of localized corrosion, and spotty corrosion products were ob- served with an optical microscope. The appearance of corrosion products is a little different for specimens sputter-deposited at different Ar gas pressures. Optical micrographs of the surface appearance of corroded Co-Ni and C o - C r films are shown in fig. 1. Films are sputter deposited at various Ar gas pressures, and corrosion conditions are 1.7 Ms (20 days) at 80% RH for Co Ni films and 1.7 Ms at 100% RH for Co Cr films.

Films deposited at higher Ar gas pressures showed coarse-grained and columnar structures. For pressures lower than 5.3 Pa the fine structure of the

Y. Hayashi et al. / Corrosion of Co-Ni and Co-Cr sputtered films 1003

Co - Ni ( RH 80%, 1.7 Ms )

Co-Cr ( RH i00%, 1.7Ms )

1.2 Pa 5.3 Pa~ ~ 50 Pa ~91 i~ ~

~ o . ~ t ,

• + * * 0 ~ % D , - +..: .0 I I ' O , - ' • '*,Q ~ ~+

, I + ~ . , , * ~ O 120/Jml

Fig:q. O-~tie~micrograph~ of the surface appearance of corroded C o - N i an-fL-~-Cr riims, l-iims are sputtered at various Ar gas pressures. Corrosion conditions: 1.7 Ms at 80% RH for C o - N i

films, and 1.7 Ms at 100% RH for C o - C r films.

Co- Ni ( RH 80%, 2.6 Ms )

Co - Cr

+i !

~o ~co "~S Ni

( RH 100%, 1.7 MS )

Fig. 2. Analyses of corrosion products of C o - N i and C o - C r alloy films with an EPMA. The image is formed by absorbed electrons.

1004 Y. Hayashi et al. / Corrosion of Co - N i and Co - Cr sput tered film.s

f i lms was unresolved. Fi lm structure has an effect on the appearance of the corrosion spots. A dent or a meet ing point of grain boundaries on the surface or a crevice between dust and surface seems to become a preferred corrosion site.

3.2. Identification of corrosion products

The corrosion products were analyzed with an EPMA. The corrosion products could be observed in an absorbed electron image, but not in a

o~ c

, '1 l A,o,Z,L " 80% 26 Ms ~

g

8~o a~5 8&o Binding Energy / eV

775

• (]s depo

"" 80°/° 26 Ms

780 785 Binding Energy /eV

Cr

as depo.

~ z

~'xlO0 % 1.7Ms

1 i

57,3 575 580 Binding Energy / eV

Co

ds depo.

o) z

i i 775 780 785

Binding Energy ~ eV

Fig. 3. ESCA analyses of C o - N i and C o - C r films before and a f t e r c o r r o s i o n . 2p3/2 peaks of e a c h

e l e m e n t a r e s h o w n .

E Hayashi et al. / Corrosion of Co- Ni and Co-Cr sputteredfilms 1005

secondary electron image. The image and characteristic X-ray signals are shown in fig. 2. Oxygen concentrates on the corrosion products. Sulfur may also accumulate on the spots, but this is not clear. The corrosion products are mainly composed of oxides.

The results of ESCA analyses are shown in fig. 3; energy spectra of Co 2p3/2, Ni 2p3/2 and Cr 2p3/2 in the films before and after corrosion are shown. Nickel and chromium do not show any change, but cobalt shows a change to an oxide state after corrosion.

Since spotty and thin layered corrosion products did not give sufficient intensities for the usual X-ray diffraction analysis, they were examined with an electron microscope. As shown in fig. 4, corrosion products are identified as

Fig. 4. Electron diffraction patterns of C o - N i and C o - C r films before and after corrosion. Corrosion products are Co304 for both C o - N i and C o - C r films. Micrographs show corrosion

products on the films.

1006 Y. Hayashi et al. / Corrosion o f Co - Ni and Co - Cr sput tered f ihns

~. :o

o

t~

Co-Ni ( RH80%, 298 I() Co-Cr ( RHI00%, 298 K)

41 +

0 Ms

U

112Ms(T3dceJs) 2.GO M~.(~,:la~)

Time ;

, , o f _ + -,~ /

o

/ , o

+ ' 3 2 0 ¢ ~ )

~ 1 + % 1 +..ooO' qlPo"' _11 .,L~'-,',,~-~.-q , , - ' . - ' + ' . + +

,~I,+ + • .'W t I: +. % + .'+'+%,, ' " " e - . ' " : : " ~'dlm~ qU+~-II I qW p I + + + I + 3 I

.#,:. , , ~ + o - .+ ¢ - dl~ q k ' v I',,,,II +.30Ms (15doys) +.56Ms (t 8doys)

Time / dQy 5 10 15 2 0 2.5 30 i i i i p

~ 0 - 0 - - 0 , 0

o / °

0 °I 3S ++ 3O

T +o +

T~rne I Ms ~ Time t MS

Fig. 5. Change in surface coverage of the corromon products during exposure of the specimens to high humidity atmospheres.

Co304 for both Co Ni and C o - C r films. Though the analyses were done in vacuum, this result agrees with the in situ study of anodic corrosion films by Melendres and Xu [6], which proved the corrosion product of Co to be Co304.

Corrosion in high humidity atmospheres took place in the form of localized corrosion, which was caused by the preferential dissolution of cobalt and its oxide formation.

3. 3. Progress of corrosion

Progress of corrosion was studied by measurement of the change of surface coverage of the corrosion products as observed in an optical microscope. The results are shown in fig. 5. After a certain period of exposure, corrosion spots appear and increase their surface coverage to reach saturation. The change in electrical resistance of the films during the corrosion is shown in fig. 6. The resistance change does not show saturation, so that the corrosion proceeds to thicken the corrosion products after saturation of the surface coverage. As the relation between resistance change and the amount of corrosion product present is not known, a detailed discussion is not possible. But the corrosion

Y. Hayashi et aL / Corrosion of Co-Ni and Co-Cr sputtered films 1007

0.04

0.03

d

0.02 n., 0.01

004

C o - Ni humi / d ~ /////////

.sj., / I, / "/ / ./" B0" /%1. . - - - /

~ -~--~'- --r . . . . "" ' - - , 10 ~ 104 10 s 10 6

T i m e / S i i i i

Co - Cr

0.03 i~ humidity

0.02

: j 9o. \

I - - - 7 - - ~ r - : - z - - ~ - ~ 2 Z ' - - " " ~ ~ I

0 10 3 10 4 I0 s 10 6 Time/ s

Fig. 6. Change in electrical resistance of Co-Ni and Co-Cr films during corrosion.

can be said to proceed at least in two stages: anodic dissolution of cobalt and growth of cobalt oxide. The growth of the oxide is caused by the transporta- tion of metallic ions through the oxide layer. Preferential penetration of cobalt ions may be the reason for the formation of the cobalt oxide corrosion products.

4. Conclusion

Corrosion of Co-Ni and Co-Cr sputtered films takes place in the form of localized corrosion in high humidity atmospheres. The corrosion is electro-

1008 K Hm,ashi et al. / Corrosion of C o - Ni and Co ('r sputtered.films

chemical in nature and an adsorbed water layer plays an impor tan t role. The

corrosion proceeds with the preferent ial dissolut ion and oxide fo rmat ion of

cobal t for both alloys. The micros t ruc ture of sputter films has a great effect on

the distr ibut ion of localized corrosion sites.

Acknowledgments

The authors would like to thank Mr.T. Fuj inaga of Sumi tomo Metal

Industries for valuable discussions. They also thank the H.V. EM Lab. and the

Center of Adv. Instr. Anal. of Kyushu Univ. for EM and ESCA analyses.

References

[1] For example, S. lwasaki and K. Ouchi, IEEE Trans. Magnetics MAG-14 (1978) 849. [2] D.W. Rice, P.B.P. Phipps and R. Tremoureux, J. Electrochem. Soc. 126 (1979) 1459. [3] R.R. Dubin, K.D. Winn, L.P. Davis and R.A. Cutler, J. Appl. Phys. 53 (1982) 2579. [4] T. Fujinaga, private communication. [5] N.D. Tomashov, Theory of Corrosion and Protection of Metals (MacMillan, New York, 1967)

p. 370. [6] C.A. Melendres and S. Xu, J. Electrochem. Soc. 131 (1984) 2239.