CHARACTERIZATION OF OXIDE FILMS ON Fe-Cr-Al ALLOYS AND RF-SPUTTERED FILMS BY SOFT X-RAY SPECTROSCOPY AND MICROANALYSIS

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CHARACTERIZATION OF OXIDE FILMS ON Fe-Cr-Al ALLOYS AND RF-SPUTTERED FILMS BY

SOFT X-RAY SPECTROSCOPY AND MICROANALYSIS

S. Maruno

To cite this version:

S. Maruno. CHARACTERIZATION OF OXIDE FILMS ON Fe-Cr-Al ALLOYS AND RF- SPUTTERED FILMS BY SOFT X-RAY SPECTROSCOPY AND MICROANALYSIS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-653-C2-656. �10.1051/jphyscol:19842152�. �jpa-00223824�

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Colloque C2, suppl6ment a u n02, Tome 45, f6vrier 1984 page C2-653

CHARACTERIZATION OF OXIDE F I L M S ON F e - C r - A l ALLOYS AND RF-SPUTTERED FILMS BY SOFT X-RAY SPECTROSCOPY AND MICROANALYSIS

S . Maruno

MateriaZs Research Laboratory, hragoya I n s t i t u t e of Technology, Nagoya 466, Japan

Resume - I 1 a &ti. montre que l a spectroscopie des rayons X mous e x c i t e s p a r m o n s e t l a microanalyse avec SEM e t STEM, equip& d'un d e t e c t e u r de

rayons X p a r d i s p e r s i o n d ' e n e r g i e sont e f f i c a c e s pour l a c a r a c t e r i s a t i o n e t l ' e t u d e de l a s t r u c t u r e de l a s u r f a c e du f i l m d'oxyde forme s u r l e s a l l i a g e s Fe-Cr-A1 e t s u r l e s f i l m s obtenus par p u l v e r i s a t i o n radio-frequence, chauffes dans l ' a i r pendant des periodes v a r i e e s a des temperatures a l l a n t de 500 a

1250 O C .

Abszract - ^{I t}has been shown t h a t e l e c t r o n - e x c i t e d s o f t X-ray spectroscopy using EPMA and microanalysis w i t h SEM and STEM, equipped w i t h energy d i s p e r s i v e X-ray detector, a r e an e f f e c t i v e measure f o r studying t h e c h a r a c t e r i z a t i o n and surface s t r u c t u r e o f oxide f i l m formed on Fe-Cr-A1 a l l o y s and r f - s p u t t e r e d f i l m s , heated f o r various times a t h i g h temperaturesfrgm5a(LtQ 1250 " C i n a i r -

Fe-Cr-A1 a l l o y s and s i m i l a r a l l o y s c o n t a i n i n g a small q u a n t i t y o f r a r e e a r t h elements a r e w e l l known aS m e l e c t r i c a l h e a t i n g m a t e r i a l and o x i d a t i o n r e s i s t a n c e a t h i g h temperatures [l-31, and a l s o t h e i r t h i n f i l m r e s i s t o r s are o f i n t e r e s t f o r use i n t h i n c i r c u i t [4]. The surface o x i d a t i o n behavior o f Fe-Cr-A1 a l l o y s and r f - s p u t t e r e d f i l m s h e a t - t r e a t e d f o r v a r i o u s times a t h i g h temperatures i n a i r , has been s t u d i e d by EPMA and SEM w i t h energy d i s p e r s i v e X-ray detector(EDX), and i n a d d i t i o n t h e micro- a n a l y s i s o f r f - s p u t t e r e d f i l m s a f t e r o x i d a t i o n was made w i t h a n a l y t i c a l e l e c t r o n microscope. We note, i n special, t h e a p p l i c a b i l i t y o f s o f t X-ray spectroscopy using EPMA t o the c h a r a c t e r i z a t i o n o f oxide f i l m s ( o r scales) on the a l l o y s , predominantly aluminum oxides. A1 Kp emission band spectrum provides u s e f u l i n f o r m a t i o n on chemical bonding s t a t e s i n aluminum oxides [5] and e l e c t r o n s t a t e s o f valence band i n metals c o n t a i n i n g aluminum [6,7]. In t h i s work, t h e chemical s t a t e a n a l y s i s o f o x i d i z e d a l l o y s and r f - s p u t t e r e d f i l m s was made by i n v e s t i g a t i n g the c h a n g e s i n t h e shape and energy p o s i t i o n o f A1 Kp emission band, Fe La,@ and C r La,B l i n e s , and t h e i n t e n s i t y r a t i o o f A1 s a t e l l i t e s Ka3 and Ka4. I t has been found t h a t e l e c t r o n - e x c i t e d s o f t X-ray spectroscopy i s an e f f e c t i v e measure f o r studying the c h a r a c t e r i z a t i o n o f oxide f i l m s formed on t h e a l l o y s i n t h e e a r l i e r stage o f o x i d a t i o n and/or r e l a t i v e l y low temperatures 950 "C o r below.

The composition o f a l l o y s ( i n wt%) i s 5.6 and 6.7 f o r A l , 23 and 24 f o r C r , small q u a n t i t i e s o f S i (0.3) and T i (0.15) and balance of Fe. Fe-Cr-A1 f i l m s w i t h d i f f e r e n t contents o f A1 were prepared by r f - s p u t t e r i n g (13.56 MHz) segmented t a r g e t s c o n s i s t i n g o f Fe-19.7Cr-4.OA1 and pure A1 i n argon atomosphere o f about 5 ~ 1 0 - ~ t o r r . The measurements o f X-ray emission band spectra were performed w i t h a spectrometer equipped w i t h Johanson type c r y s t a l s PET f o r A1 KB ( r a d i u s o f Rowland c i r c l e R=125 mm), ADP f o r A1 Ka3,a4 (R=200 mm) and KAP f o r Fe La,B (R=125 mm). I n t e n s i t i e s were measured by counting f i v e times each f o r 30 sec under the c o n d i t i o n of a p p l i e d v o l t a g e 15 kV, specimen c u r r e n t 0.7 vA, e l e c t r o n beam diameter about 30 um. Nonconducting specimens were coated w i t h evaporated carbon.

The surface o f a l l o y s a f t e r heating a t various temperatures f o r a p e r i o d o f 3 min i n a i r i s covered w i t h oxide f i l m s as seen i n Fig.1. I n t h e specimens heated i n temper- a t u r e s lower than 950 "C, the oxide products a r e n o t obvious except d o t t e d - p a r t i c l e s of t i t a n i u m d i o x i d e i n which aluminum i s occluded. It was found t h a t X-ray d i f f r a c -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842152

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C2-654 JOURNAL DE PHYSIQUE

9B0°C | loocrc

M

12S0°C 1300"C ]

m

2 v

Fig.l - SEM photographs showing the surface appearance of alloy after oxidation tor 3 min and EDX analysis of the parts of ridge (a) and hollow (b) on alloy at 1250 °C.

tion patterns of the alloys oxidized for 3 min below 1050 °C showed no sign of any aluminum oxides and that a-Al203 only was formed on the alloys oxidized above 1150

°C. The surface of specimens oxidized at 1250 °C or above shows the reticulate struc- ture being composed of convoluted-interlacing a-Al203 (0.3-0.5 ym wide) which appears as ridges. Since the grain growth of this alloy occurs markedly between 1000 and 1100

°C, the formation of oxide on the alloy at around 1050 °C is affected by the orien- tated crystal grain of base metal (as seen in Fig.2). Table 1 shows the results of Al K emission characters. All the Al Kg band positions were determined on the basis of that of pure Al [8]. The Al KB band from the alloys is shifted to lower energy and become sharpen band with narrow width compared with that of pure A l , because the change in the local density of states of 3p electron of Al atom occurs as a result of hybridization of outer p electron of Al with 3d bands of Fe and Cr [7], Moreover, the Al K emission bands undergo changes in shape and energy position which depend on the structural and compositional characters of aluminum oxides formed on the alloys. The Al KB band was shifted to longer wavelength with increasing temperature and its band splitting, resulting from the formation of a-Al203, occurred at above 1050 °C. The band shifting of the alloys oxidized below 950 °C is considered due to the formation of amorphous and/or -y-Al203. Al Ka3 and Ken, satellites are shifted to shorter wavelength (max=0.004 K) and the intensity ratio of Kai, to Ka3 approaches the value of a-Al203. The changes in Al KB band position and intensity ratio (Ka*/Ka3) with increase of temperature are very closely inter-related in each other, and both of the band position and intensity ratio change remarkably between 750 and 950 °C. The rela- tively broadening of the Al KB half width in the alloys oxidized at 750 and 850 °C might be thought to be due to an overlapping of Al KB band from thin oxide layer on the alloys and that from the base metal.

Table 1. Aluminum K X-ray emission characteristics obtained from pure Al metal, Fe-24.1Cr-6.7Al alloys before and after oxidation for 3 min at various temperatures in air. Data for a- and •Y-Al203 are also listed for reference. _ ^ _ „

Specimen Aluminum Fe-Cr-Al alloys Heat-treated alloys

750 °C 850 °C 950 °C 1050 °C 1150 °C 1250 °C 1350 "C Y-Al20a

a-Al203

Al Ka4/Ka3

0.43 0.62 0.64 0.77 0.95 0.98 1.00 1.01 1.02 1.01 1.02

Al KB position (2/3-1 J max 7.9600 (A)

7.9648 7.9648 7.9722 7.9754 7.9758 7.9764 7.9780 7.9784 7.9776 7.9783

1557.3 (eV) 1556.4 1555.8 1555.0 1554.3 1554.3 1554.1 1553.8 1553.7 1553.9 1553.8

Al KB h a l f 0.0348 (A) 0.0274 0.0304 0.0324 0.0292 0.0328 0.0340 0.0332 0.0344 0.0293 0.0330

width **

6.8 (eV) 5.3 5.9 6.3 5.7 6.4 6.7 6.4 6.7 5.7 6.4

•measurement errors are within 0.001 A (0.2 eV),**Full width at half maximum (FWHM)

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f o r 3 min ( l e f t ) and A 1 K p s p e c t r a from a r e a s (A) and (B) i n photo ( b ) .

When t h e oxidation r a t e of a l l o y s i s not l a r g e compared with t h e growth r a t e of base metal g r a i n s , t h e formation of oxides on a l l o y i s g r e a t l y a f f e c t e d by p r e f e r e n t i a l = o r i e n t a t e d c r y s t a l plane a s seen i n Fig.?. There i s a d i f f e r e n c e between a r e a s (A) and

(B) i n s u r f a c e appearance and oxide morphology. The former (A) has an c o a r s e r and more uneven s u r f a c e . The A1 KB band p o s i t i o n s o f t h e a r e a s (A) and (B) a r e c l o s e l y agreement with t h a t of y-A1?03, i n d i c a t e d a s t h e arrow i n f i g u r e , and then t h e band s p l i t t i n g due t o t h e formation of a-A1203, which appears l i k e a small hump on low energy shoulder, i s d i f i n i t e l y observed f o r t h e a r e a (A). This f a c t suggests t h a t t h e oxidation of t h e a r e a ( A ) proceeds f a s t e r than t h e a r e a (B) i n t h e e a r l y s t a g e of oxidation. Moreover, i t has been found from t h e measurements o f Fe L and Cr L band s p e c t r a t h a t t h e iron oxides on t h e s u r f a c e of a r e a (A), which a r e expected t o be pro- duced p r i o r t o t h e formation of aluminum oxides, a r e b a r e l y d e t e c t a b l e and t h e chromium content within oxide s c a l e i s t o o small t o do t h e s t a t e a n a l y s i s .

SEM observation showed t h a t t h e s u r f a c e s t r u c t u r e o f a l l o y s heated i n t h e temperature range 750-1250 O C and t h e morphology of oxides formed on a l l o y s v a r i e d i n heating time a s follows: flat-like-smooth, where broken i n p a r t with t h e nodules of Ti-oxides i n which A1 i s occluded, -t corn f l a k e s f o r t h e temperatures below 950 ^{O C ,}f i n e o r rough ( c o a r s e r ) a s p e r i t i e s , depending on t h e o r i e n t a t i o n of c r y s t a l g r a i n , -t e l l i p - soidal granules f o r 1050 O C , i n t e r l a c e d network + r e c t a n g u l a r c r y s t a l s f o r 1250 OC.

We could n o t confirm any oxide products by X-ray d i f f r a c t i o n f o r t h e a l l o y s heated within 60 min

A l KO Ernlss~on

bands i n t h e temperature range 750-950 "C.

Fe - ^Cr- ^{A l}( 6 7 ~ 1 ) heated In air

L e

C

L ~ e --a1 0

1 4, ^,^, ⁵ ^,^,^{, ,}^,,-, ^(.") I ⁱ ⁰¹ ^, ^, ^, ^, ⁵⁰^, ^, ^, ^, ^, ¹⁰⁰^,A

7940 7.960 7980 8.000 (A) Time (min)

Fig.3 - A 1 K B s p e c t r a from t h e a l l o y s Fig.4 - R e l a t i o n of A 1 KB band p o s i t i o n and h e a t e d f o r v a r i o u s times a t 850 ^OC. o x i d a t i o n time i n t h e d i f f e r e n t temperatures.

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C2-656 JOURNAL DE PHYSIQUE

The change i n A1 K emission band spectra from the a l l o y s heated i n t h e r e l a t i v e l y low temperatures occurs as a f u n c t i o n o f r e a c t i o n p e r i o d . The r e s u l t s o f A1 Kp spectra are shown i n Fig.3. I n t e n s i t y i s normalized a t the peak value. The KB band s h i f t t o lower energy d i r e c t i o n i n d i c a t e s t h a t t h e aluminum oxide i s , s u r e l y formed on t h e a l l o y w i t h i n c r e a s i n g o x i d a t i o n time. The change i n Kak/Ka3 i n t e n s i t y r a t i o was a l s o observed and i t served t o evaluate t h e e x t e n t t o which t h e a l l o y w a s o x i d i z e d . The A1 KB band s p l i t t i n g appears s l i g h t l y a t 10 minand becomes c l e a r l y a t 40 min o r above.

I t can be seen t h a t t h e composition and/or s t r u c t u r e changes o c c u r r i n g i n t h e oxide scale a t 850 O C r e s u l t i n t h e formation o f y-A1203 and subsequently t h e transforma- t i o n from y-A12O3 t o a-Alp03, h.c.p. s t r u c t u r e having 6-coordinated A1 atoms. The changes i n the A1 KB band p o s i t i o n are shown as a f u n c t i o n o f o x i d a t i o n time i n Fig.4 Below 850 OC these show a s t r o n g dependence on t h e temperature. The thickness o f oxide f i l m formed on t h e a l l o y s heated a t 750 ^{O C}may n o t be s u f f i c i e n t t o i n v e s t i g a t e the p h y s i c a l and chemical changes o c c u r r i n g i n t h e oxide f i l m , b u t we can see t h e o x i d a t i o n behavior o f a l l o y o r t h e development o f oxide accompanying t h e increase o f o x i d a t i o n time. The composition o f r f - s p u t t e r e d f i l m i s n e a r l y same f o r Fe and Cr as compared w i t h t h a t o f t h e t a r g e t a l l o y s , b u t t h e amount o f A1 i n t h e f i l m s decrease s l i g h t l y . The composition o f r f - s p u t t e r e d f i l m f o r Fe-19.7Cr-4.OA1 t a r g e t a l l o y was Fe-19.4Cr-2.9A1 and t h e f i m has f i n e mat s t r u c t u r e c o n s i s t i n g o f r o d - l i k e c r y s t a l - l i t e s w i t h 250x 1000-1700 A i n s i z e as seen i n Fig.5-(a), and changes t o the complex oxide s t r u c t u r e a f t e r h e a t i n g f o r 20 min a t 500 ^{O C}(b), where alumina p a r t i c l e s o f 200 a i n s i z e are dispersed and chromium segregated s t r e a k i l y . Since t h e a f f i n i t y o f A1 f o r oxygen i s much g r e a t e r than t h a t o f Fe o r C r , aluminum oxides a r e preferen- t i a l l y formed i n t h e f i l m . I n t h e A1 KB band spectra from t h e o x i d i z e d Fe-18.7Cr=

6.2A1 f i l m s (Fig.6), t h e band s h i f t appears d i f i n i t e l y a t 750 O C and t h e band s p l i t - t i n g caused by t h e formation o f a-A1203 occurs a t 850 ^{O C .} Through t h e measurements o f A1 KB band spectra from the r f - s p u t t e r e d f i l m s before and a f t e r o x i d a t i o n , i t was suggested t h a t t h e change i n band p o s i t i o n and shape of t h e A1 KB emission spectra from t h e o x i d i z e d r f - s p u t t e r e d f i l m s occurs a t a low temperature o f 100-150 O C than t h a t o f t h e b u l k a l l o y s .

n Al Kg from

Fe-Cr- 6.2AI (sp) f ilms heatea lor 20 mi"

950-c

850.C

Fig.5 - FE-SEN (a) and STEM (b) photographs ¹⁵⁶⁵ ¹⁵⁶⁰ ¹⁵⁵⁵ ^15% ¹⁵⁴⁵ ¹⁵⁴⁰^(eV) of t h e r f - s p u t t e r e d Fe-19.4Cr-2.9A1 f i l m be- i . " , ~ ' . " '

7.95 8.00 e.bs c i )

f o r e ( a ) and a f t e r ( b ) h e a t i n g f o r 2 0 min a t

500 "C. ( c ) i n d i c a t e s t h e r e s u l t s of EDX Fig.6 - A 1 KB s p e c t r a from sput- a n a l y s i s i n t h e a r e a s ( A ) and (B) i n photo (b). t e r e d f i l m s h e a t e d f o r 20 min.

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