Equilibrium surface segragation of carbon on iron (100) faces

Scripta M E T A L L U R G I C A

Vol. 9, pp. 1181-1184, 1975 Printed in the United States

EQUILIBRIUM

SURFACE

OF CARBON ON IRON H.J. Grabke, Max-Planck-Institut

Pergamon

Press,

Inc.

SEGREGATION

(100) FACES

G. Tauber and H. Viefhaus

fHr E i s e n f o r s c h u n g

GMBH,

DUsseldorf,

W.Germany

~Received July 24, 1975) (Revised Sentember 26, 1975) Introduction Chemisorbed

layers of nonmetal

properties

of the metal,

phenomena,

such as recrystallisation,

sticity etc. Therefore,

studies

makes e q u i l i b r i u m are i) that isorbed) there

investigations

quickly,

~ molecules

is sufficient

influence

equilibria

several and many

sintering,

at elevated

pla-

tempera-

studies b~ LEED and AES in an UHV

from the gas phase

is intended,

and possib-

In some other cases the study of surface se~re@ation

the segregation

is e s t a b l i s h e d

Equilibrium

if adsorption

le only in a few cases.

surfaces

surface reaction kinetics,

of c h e m i s o r p t i o n

tures would be of great interest. chamber are difficult,

atoms on metal

like surface energy and surface self diffusion,

using LEED and AES possible.

equilibrium:

atoms(dissolved)

ii) that the desorption

in the gas phase,

of the adsorbate

on can occur without depletion

of solute atoms.

Some studies of the surface (3), in which q u a n t i t a t i v e

species:

can occur only very slowly,

solubility

aces have already been published,

of adsorbed

The preconditions

# atoms(chemisorbed),

in the metal,

segregation

of nonmetal

atoms(chem-

iii) that

so that segregati-

atoms on metal

surf-

on the system Ni-C (1,2) and on the system Nb-O

thermodynamic

data have been obtained

sults reported here are part of studies on surface gen,

sulfur and oxygen on iron single crystals.

ons

are m e a s u r e d

by AES and c h e m i s o r p t i o n

segregation

by AES.

of carbon,

Surface e q u i l i b r i u m

structures

The renitro-

concentrati-

are observed by LEED

(4).

Experimental (100) oriented orienting, different

Fe specimens were prepared

cutting and m e c h a n i c a l carbon c o n c e n t r a t i o n s

equilibration

according

ed carbon concentrations Each doped

to

polishing.

by c a r b u r i z i n g

CH4(g)

plug contact.

It also could be w i t h d r a w n

During this procedure

the pressure

+ 2 H2(g)

By

at 800°C well defin-

in the range O .... 120 w t . p p m C.

sample was m o u n t e d on a small heating

ted in the UHV chamber through an air-lock

by

specimens were doped with

in flowing CH4-H 2 mixtures.

= C(dissolved)

(5-7) were obtained

from iron single crystals

Several

stage.

This assembly was inser-

and joined to the m a n i p u l a t o r

by a

without opening the UHV chamber

(8).

could always be kept below

1181

10 -9 Torr.

1182

SEGREGATION OF CARBON ON IRON

Vol. 9, No. 11

Surface impurities were removed by argon bombardment and heating cycles. The impurity signals could be reduced below detection limit and the experiments were started with a clean surface showing only the Auger electron peaks for iron. It

was very important to have samples which were as low in sulfur content

as possible, otherwise, the sulfur segregation would have suppressed the carbon segregation. The sample was heated to the temperature range in which the solid solution of carbon in a-iron is stable. In this well-known range of temperature and carbon concentration

(9) no precipitation of graphite or cementite is possible.

In some experiments graphite deposits formed on the surface during heating or cooling of the sample. But, as soon as the graphite solubility curve was passed, at higher temperatures the graphite dissolved in the crystal. This could be seen from the LEED pattern

(FIGS. 1 and 2) and also from the shape of the

FIG. 1 LEED pattern of an iron surface with graphite deposits, 72 eV, room temperature

FIG. 2 LEED pattern of an Fe (100) face with chemisorbed carbon, 43 eV, 580°C, 30 ppm C

Auger electron peaks for carbon (FIG. 3). The sequence of events shown in FIG. 3 recorded during heating a sample with 30 ppm C to 450°C shows the changes taking place on the surface during the decomposition of graphite. The lowest curve shows the typical graphite fine structure with a large peak at 270 eV, and the highest curve, obtained after 685 set at 450°C, shows the fine structure which was observed for carbides in several cases (4). Since Fe3C cannot be formed under these conditions the carbon Auger peaks of that curve are attributed to atomic carbon on the iron surface.

Vol.

9, No.

II

S E G R E G A T I O N OF CARBON ON IRON

1183

Results The a d s o r p t i o n of atomic carbon on the (s-Fe)-C crystal was t h o r o u g h l y i n v e s t i g a t e d 5O5

for the Fe(100)

face. The peak height of the

445

carbon peak at 270 eV was m e a s u r e d as a func-

385

tion of the temperature.

325

peak heights

The ratio r of the

for carbon and for iron is plot-

205

ff5

ted in FIG.

55

4, l o g a r i t h m i c a l l y vs. the reci-

procal t e m p e r a t u r e

I/T. At low t e m p e r a t u r e s

the lines a p p r o a c h the l i m i t i n g value r =0.6. s This value p r e s u m a b l y c o r r e s p o n d s to a saturation of the Fe(IOO) a chemisorption

~uJg~¢" MIctro¢l peak5 for CClrbOn on Fe (100), $omple with 30ppm C, di~o~ution of g r ~ N t e ot ~50" C , Olomic ~Lqorbed corbon iS

surface by f o r m a t i o n of

r,maimng on me surface

The degree of c o v e r a g e was c a l c u l a t e d

""

FIG. 3. Change of the A u g e r e l e c t r o n peaks for carbon, when graphite transforms to c h e m i s o r b e d carbon

a c c o r d i n g to @ I r/rs . If the c h e m i s o r p t i o n of carbon on Fe(IOO) isotherm

i,

structure.

follows the L a n g m u i r - M c L e a n

(2), a plot of in O/(I-8)

yield s t r a i g h t lines,

FIG.

vs I/T should

5, from w h i c h the e n t h a l p y and excess entropy of se-

g r e g a t i o n can be o b t a i n e d a c c o r d i n g to @ ~Hse~r in I - @ = RT

+

AS xs se~r R

xC + In -~-

In this e q u a t i o n x C is the m o l e fraction of d i s s o l v e d carbon and b is the number of i n t e r s t i t a l octahedral

sites per metal atom, w h i c h may be o c c u p i e d by carbon atoms

sites in bcc ~-Fe: b=3).

(for

The value of the s e g r e g a t i o n e n t h a l p y is

AHseg r = -20 ± I k c a l / m o l e C, the change of the excess e n t r o p y upon s e g r e g a t i o n

850

800

750

700

650

600

550 °C

3

06 0.5

•<

z/

0.3

/3o

l ~.. 02

c 01 00g

/

0.08 i

0~7

I

• 0,9

i

I

0~5

1,0

,

1.05

IJO

i

1.15

1.20

I03/T FIG. 4. L o g a r i t h m i c plot of the ratio carbon peak (270 e V ) / i r o n peak (650 eV)

vs

Io3/T

[K-I]

-2

0.9

lO

I]

1.2 13 103/T

FIG. 5. Plot of in @/ (1-@) v s . 1 0 3 / T

1184

SEGREGATION

OF CARBON ON IRON

Vol.

9, No.

ii

is AS xs = O ± 1 cal/K mole C. A c c e p t i n q the value of the relative partial ensegr thalpy of carbon in e-iron p r o p o s e d by Dunn and McLellan (10), 23.34 kcal/mole C at 1000 K, a c h e m i s o r p t i o n C(graphite)

enthalpy

= C(chemisorbed)

is o b t a i n e d

for the reaction

~H = -3 ± I kcal/mole C. Discussion

The small change of the excess entropy upon segregation the carbon c h e m i s o r p t i o n onal entropy octahedral

on Fe(IOO)

is insignificant.

interstital

the surface.

In that site there is plenty of space for the car-

diminished,

iron atoms is lowered by one,

Thus,

the p o s s i b i l i t y

of chemical

the c o o r d i n a t i o n

interaction

but on the other hand a c o n s i d e r a b l e

force for the surface

According

to a c a l i b r a t i o n

des the saturation

value r

p(2x2)

adsorption

te a c(2x2)

interaction

This surface c o n c e n t r a t i o n

in domains

energy

segregation.

(4) with some Fe-C alloys and

structure whereas

structure

of chemical

is li-

site. The dif-

(Fe,Cr)-carbi-

of the Auger peak height ratio corresponds

s tio C/Fe = 0.2 on the surface.

Z

of iron and carbon is

amount of elastic energy

release of elastic energy minus decrease

is the driving

number

from Z = 6 in the bulk to Z = 5 in

berated w h e n the carbon atom pops up from the bulk to the surface ference,

that

in vibrati-

Most p r o b a b l y the carbon atoms are chemisorbed in .I,I. - on Fe(IOO) in the site [~ ~; in

If that site is occupied, upon segregation

of n e i g h b o u r i n g

indicates

and that the change

sites at the surface,

the center of the unit mesh. bon atom.

is localized

to a ra-

is in agreement with a

the LEED studies on the same samples

(FIG. 2). Further

investigation

indica-

is being per-

formed to find out if the LEED p a t t e r n can be explained by the formation of special domain

structures. Acknowledgement

This research was supported schungsvereinigungen

through the A r b e i t s g e m e i n s c h a f t

(AIF) by the B u n d e s m i n i s t e r i u m

Industrieller

fur Wirtschaft,

For-

Federal

Republic Germany. References I.

J.C.

Shelton,

2.

L.C.

Isett,

3.

A. Joshi,

4.

H.J. Grabke,

5.

R.P.

Smith,

E. SchHrmann, H.J.

8.

G. Tauber,

9.

J.C.

Grabke,

Th. Schmidt, E. Martin, Trans.

Blakely,

Surface Sci.

Scripta Met.

Chem.

H. Viefhaus,

Swartz,

J.M.

W. Paulitschke, J. A~er.

7.

Dunn,

Patil,

M. Strongin,

6.

10. W.W.

H.R.

J.M. Blakely,

Surface 47, 645

8, 413

G. Tauber,

Soc.

AIME 245,

(1974).

(1974).

H. Viefhaus,

to be published.

GieBerei-Forsch.

EisenhHttenwes.

44, 837

to be published.

R.B. McLellan,

43, 493

68, 1163(1946).

F. Tillmann,

Arch.

Sci.

(1975).

1083

(1969).

Met. Trans.

2, 1079

(1971).

19, 35

(1972).

(1967).