Surface segregation on Fe3%Si0.04%VC(100) single crystal surfaces

Surface Science 236 (1990) 29-40 Nosh-Holland

29

Surface segregation on ~~-3~Si-O.O4~V-C~

100) single crystal surfaces

C. Uebing ’ and H. Viefhaus Max-Planck-lnstitui Received

fCr Eisenforschung

GmhH, Postjach 140 260, D-4000 Diirseldorf

2 February 1990; accepted for pub~~tion

I, Fed. Rep. of Germany

2 May 1990

Surface segregation phenomena on (100) oriented single crystal surfaces of the ferritic Fe-3%Si-O.@+%V-C alloy were investigated by AES and LEED. At temperatures below 63S°C vanadium and carbon cosegregation is observed after prolonged heating. At thermodynamic ~ui~b~urn the substrate surface is saturated with the binary surface ~orn~und VC. The tw~~si~~ VC is ~it~aIly arranged on the substrate surface as indicated by LEED inv~tigati~ns. Its structure corresponds to the (IGO) plane of the ~~-dimensional VC with rocksalt structure. Sharp above 635°C the surface compound VC is dissolved into the bulk. At higher temperatures the substrate surface is covered with segregated silicon forming a ~(2 X 2) structure. This surface phase transition is reversible. Because of tbe low concentration and slow diffusion of vanadium, non-equilibrium surface states are formed as intermediates upon segregation of silicon and carbon. Below 500 o C a disordered graphite layer with a characteristical asymmetrical C Auger peak is observed on the substrate surface. Above 500 o C carbon segregation leads to the formation of an ordered ~(2 x 2) structure with a satin C Auger peak being characteristic for carbidic or ato~~liy adsorbed species. At increasing t~~~tur~ silicon segregation takesplace leading to a ~(2 x 2) structure. Between silicon and e;rrbon site competition is effective.

1. lntmductiim

Segregation, the enrichment of solute atoms at interfaces, plays an important role in such processes as heterogeneous catalysis and corrosion. Therefore, the investigation of surface segregation phenomena has received much attention during the last two decades. Until now, segregation phenomena on single crystal surfaces of various binary systems - e.g. Fe-C, Fe-N, Fe-Si, Fe-P, Fe-S, Cr-N, etc. [l-9] - have been investigated applying Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and phot~l~tron s~~os~py (XPS, UPS). Most frequently, ordered structures such as ~(2 X 2) for C, N, Si, P and S on Fe(lO0) and (1 x 1) for 0 on Fe(100) and N on Cr(100) are observed. In some cases .- e.g. C on Fe(100) -

1 Now at: The James Franck Institute, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA. ~39-~Zg~~/~3.5~

the segregation thermodynamics can be described by the Lan~uir-Mc~n equation [2,10]. In multi~m~nent systems M-X,-X,. . . with unlike segregants X,, X,, . . . complex segregation behavior could arise. Depending on the size of the segregants, the surface structure and the chemical interactions between alike segregants, between unlike segregants and between segregants and substrate atoms, com~tition for the available surface sites (site competition) or cosegregation of the different segregants must be expected. Based on a regular solution model, Guttmann proposed theoretical models for the description of such segregation phenomena in multicom~nent alloys [11,12]. According to these models, cosegregation has to be expected in systems with attractive interactions between different solutes. Since these attractions may also cause precipitation, recent investigations of the system Fe-15WCr-30ppmN (100) have been carried out intentionally to distinguish between both processes [13-151, The nitrogen solubility in Fe-lS%Cr-N alloys is a few tenths of a ppm depending strongly on temperature. There-

0 1990 - Elsevier Science Publishers B.V. (gosh-~ioli~nd)

30

C. Uebing, H. Viefhaw / Surface segregation on Fe-3 %Si-0.04 SV-C

fore, the surface composition could be studied under experimental conditions, which correspond either to the single-phase field (ferrite) or to the two-phase field (ferrite + CrN precipitates) of the corresponding bulk phase diagram. It has been shown that the surface chemistry of that system reflects the transition between both fields, since at temperatures above 600 o C, i.e. within the singlephase field, cosegregation leads to the formation of the two-dimensional surface compound CrN, whereas at lower temperatures, i.e. within the two-phase field, the precipitation of three-dimensional CrN takes plase at the surface caused by heterogeneous nucleation. The present investigations have been carried out in order to study the surface chemistry of ferritic Fe-V-C (100) single crystals under experimental conditions corresponding to both fields of the bulk phase diagram as well. For this purpose a suitable alloy must be chosen, which exhibits the transition between both fields preferably below 700° C. In general, at much higher temperatures the investigation of surface segregation phenomena becomes increasingly difficult, since then the segregation of surface-active impurities like sulphur and phosphorus being present in ferritic alloys in the low ppm range cannot be avoided. Since the solubility product of three-dimensional VC is much lower compared to CrN (c~,,,~~ < 1 ppm in Fe-3%V-C at 600 o C, cC,max= 10 ppm in Fe-O.O4%V-C at 600 ’ C [16], either Fe-V-C alloys with high V contents at impractically low C contents or alloys with low V contents (e.g. 0.04%) at C contents of a few tenths of a ppm must be studied. Unfortunately, at V contents below about 2% the a-y transition is not suppressed and ferritic single crystals cannot be produced applying the Bridgman crystal growth technique [17]. Therefore, another element must be added to the alloy, which effectively stabilizes the ferritic structure but does not form stable carbides. For this purpose a Fe-3%Si-O.O4%V-C alloy has been chosen, since the surface chemistry of Fe-3%Si and Fe-3%Si-C alloys with (100) orientation are known fairly well [7]. In section 4 of this paper the surface chemistry of the Fe-3%Si-O.O4%V-C (100) system is reported in detail. Various phase transitions be-

(100)

tween two-dimensional equilibrium and non-equilibrium surface phases as well as a transition between equilibrium surface phases are observed; the equilibrium phase transition is in accord with a generalized phase rule [18]. For the theoretical treatment of the segregation thermodynamics the basic models of Guttmann’s theory (i.e. site competion and cosegregation [11,12]) are outlined in section 2 together with a combination of both models, which is suitable at least for the qualitative description of the observed surface segregation phenomena.

2. Segregation thermodynamics alloys

in multicomponent

The thermodynamical description of equilibrium segregation in ternary or more complex systems has been given by Guttmann [11,12]. Based on a regular solution model, the theory has been worked out for two basic models, which have originally been designated as regular behavior with competition and regular behavior without competition. Instead of these terms the illustrative expressions site competition and cosegregation are used frequently in recent publications. In the following the basic models are discussed briefly together with a combination of both models, which is used later on for the discussion of the experimental results of this work. It should be emphasized that a simple regular solution model is only a crude approximation for the description of a quaternary alloy such as Fe-3%Si-O.O4%V-C with complex chemical interactions between the solutes. However, it will be shown in section 4, that this approximation describes the observed surface segregation phenomena qualitatively correct. 2. I. Site competition In the following a ternary solution M-X,-X, with two segregating species Xi and X, is considered. Both segregants may be interstitially or substitutionally dissolved. It is assumed that both segregants occupy equivalent surface sites. Therefore, the segregants compete for a limited number of equivalent sites at the surface. In general, attractive or repulsive interactions between the

C. Uebing, H. Viefhaus / Surface segregation on Fe-3%Si-O.O4%V-C

(100)

segregants have to be taken into consideration. Then, the surface coverage of the segregants Xi can be expressed as follows:

i=l,2,

(1)

where x,~ and x” are the mole fractions of the segregants Xi and X, in the bulk and at the surface, respectively. The Gibbs free energy of segregation AGi is determined by different factors. Elastic strain energy is released upon segregation of interstitials or of large substitutional atoms. Additional contributions result from interactions between segregants and unsaturated bonds of substrate atoms and also between different segregants at the surface. Guttmann proposed the following expressions for the Gibbs free energy of segregation: AG, = AG; + (Y(x; - x;),

(24

AG,=AG,O+a(x~-x;),

(2’4

where AG: and AG: are the Gibbs free energies of segregation in the corresponding binary systems M-X, and M-X,. (Y is the interaction energy between both segregants, For the sake of simplicity, repulsive or attractive interactions between alike segregants are not taken into account. According to the usual sign convention (exothermic enthalpies < 0), the interaction energy cy is positive for repulsive interactions and negative for attractive interactions. Fig. 1 shows calculated values of the surface coverage using the set of equations (1) and (2) for positive values of the interaction energy (Y as indicated. The parameters of the numerical calculation, i.e. the Gibbs free energies of segregation, the interaction energy a! and the solute mole fractions, are given in the figure caption. Depending on the actual parameters, the results of the calculation vary moderately. However, the example given in fig. 1 is related to the experimentally determined surface chemistry of the systems Fe-3%Si40ppmC and Fe-3%Si-P [7,19]. Besides a strongly segregating species at a low concentration, Xi = C, P, these systems contain a weakly segregating

TenperotureT l°Cl Fig. 1. Temperature dependence of surface coverage according to the site competition model (eqs. (1) and (2)). The calculation is based on the following values: AGp = - 80 kJ/mol, x1”= 0.0001; AC,”= -40 kJ/mol, x2”= 0.02. Values for the interaction energy a are indicated (in kJ/moI).

species at a comparatively high concentration, X, = Si ( 1AG~i 1 e 1AG$,p 1, x.$ > x&). According to the calculation, at low temperatures the surface is saturated with the strongly segregating species Xi. At high temperatures X, is dissolved in the bulk and the segregation of X, takes place. With increasing interaction energy the slope of the curves becomes more step-like; such abrupt changes in coverage with temperature approach surface phase tr~sition behavior. 2.2. Cosegregation The theoretical description of cosegregation has been carried out by Guttmann for a regular ternary solid solution M-X,-X,, where X, and X, denote substitutional and interstitial solutes, respectively. It is assumed that both segregants occupy different coordination sites on the surface. Mutual blocking of segregation sites for sterical reasons as well as chemical interactions between alike segregants are not taken into account. Then, the bulk and the surface of the solid can be described in terms of sublattice concentrations, Y, (x = S sub-

C. Uebing, H. Viefhaus / Surface segregation on Fe-3%Si-O.O4%V-C

32

stitutional solutes, x = I interstitial solutes), as defined by eq. (3). The substitutional sublattice is atoms M and n, atoms X,. occupied by nM sublat&e contains Besides XI, the interstitial vacancies V. The ratio of available interstitial to substitutional sites is given by the quotient c/a, where a and c are the mole fractions of substitutional to interstitial sites (a + c = 1); Y, =

nXS

;(Y,m-

1

I

=300.

500

Temperature

700

900

T [‘Cl

Fig. 2. Temperature dependence of surface coverage according to the cosegregation model (eqs. (3)-(5)). The calculation is based on the following values: AC? = - 80 kJ/mol, x: = 0.0001; AG; = -40 kJ/mol, x,” = 0.02. Attractive interaction parameters (P/a, /3/c, a = c = l/2) are indicated (in kJ/mol).

Y:),

AG~=AG~+~~Y~-

Y;).

Attractive interactions between both segregants correspond to negative interaction energies j?. Guttmann noted that the existence of attractive interactions is in contradiction to the applicability of the regular solid solution appro~mation. However, the surface coverage can be expressed by the set of equations:

y,”

9

l_

=1 - Yt@

et,

Xl

In the following the two-dimensional surface phases formed by cosegregation of the constituent components are denoted surface compounds. Upon cosegregation, marked changes in &lectronic and vibrational entropy (excess entropy) arise from the formation of strong chemical bonds at the surface. Neglecting the excess entropy, Guttmann derived with further simplifications the following expressions for the Gibbs free energy of segregation:

Y,”

-

(3)

nM+nXs

AG,=AG,o+

(100)

Y, exp

i

-AG, RT

i

(54

.

The temperature dependence of the surface coverage according to the set of equations (3) to (5) is shown in fig. 2 for different interaction energies as indicated. In the low temperature range, the simultaneous enrichment of both segregants is observed. As a consequence of attractive interactions, even weakly segregating species can show a remarkable enrichment on the surface. Upon increasing the temperature the degrees of coverage

decrease uniformly. This behavior corresponds to the observed surface chemistry of the system FelS%Cr-30ppmN [13-151. In this system cosegreg&ion of chromium and nitrogen leads to the formation of the binary surface compound CrN at temperatures above 600 o C. According to the calculation, strong attractive interactions cause step-like, simultan~us changes in surface coverage of both segregants; this behavior corresponds to surface phase transitions - the dissolution of two-dimensional surface compounds into the bulk. 2.3. ~ornbina~~on cosegregation

of sirecorn~e~~~~onand

In this section Guttmann’s theory [11,12] is applied to the description of combined site competition and cosegregation processes. A quaternary regular solid solution M-X,,-X,*-X, with two substitutional solutes Xs, and X,, and an interstitially dissolved component X, is considered. Sublattice concentrations, Y, as defined above, are applied. As in the site competition model it is assumed that the segregants Xs7 and X,, compete

C. Uebing, H. Yiefiuus / Surface segregation on Fe-3XSi-O.O4W-C

for equivalent surface sites. Moreover, strong attractive chemical interactions between Xs, and X, are assumed, which lead to the formation of the surface compound Xs,X,. With a, & and /& as interaction energies between Xs, and Xsz, between X, and Xs, and between X, and X,, respectively, the Gibbs free energies of segregation are given by:

41 = AG,q +

33

(io0)

$(YF-

YF)+a(Y$-

Y,B,), PJ)

For the surface coverages combinations of equations (1) and (5) are obtained:

of the set

(74

l_

YP) =&

exp(*).

In fig. 3 the temperature dependence of surface coverage is shown according to eqs. (6) and (7) for the values of the Gibbs free energies of segregation and of the interaction energies given in the figure caption. Fig. 3 demonstrates that the surface compound X,,X, is formed in the low temperature range. Upon increasing the temperature this compound desegregates from the surface more or less abruptly and segregation of X,, takes place. Again this behavior corresponds to a surface phase transition.

d

3. Experimental

TemperatureT 1% Fig. 3. Temperature dependence of surface coverage for the combined site competition and cosegregation model (eqs. (6) and (7)): AC: = -80 kJ/mol, $ = 0.0001; AG& = -20 kJ/moi, x& = 0.02; AG& = - 20 kJ/mol, x& = 0.02. Repulsive interactions between X,, and X, (or=15 kJ/mol) and between X, and Xs, (a/c=15 kJ/mol, a=c=1/2) are assumed. Attractive interactions between X, and Xs, (/II/c) are taken into account as indicated in the figure.

Fe-3%Si-O.O4%V-22ppmC single crystals (all the concentrations given in this paper are in weight percent or wt-ppm) were produced applying the Bridgman crystal growth technique and oriented using a Guinier X-ray diffraction device. The specimens were spark eroded to slices with a thickness of 1.5 mm and cut into appropriate shape (4 X 4 mm’) using a diamond saw. After polishing of the specimens, the investigations were carried out in a Varian UHV chamber equipped with a four-grid LEED optics, a CMAAuger system and a glancing incidence ion gun

C. Uebing, H. V@aus

34

/ Surface segregation on Fe-3 %Si-0.04 % V-C (100)

( EAr+= 3 keV at partial pressures PAr = 5 X lo-“5 x lo-’ mbar and ion currents IA,-= l-10 PA/cm’). Sample temperatures up to 1000 OC were established by indirect resistance heating. The temperature was measured by a pyrometer, which had been calibrated against a thermocouple. The composition of the residual gas atmosphere was monitored using a quadrupole mass spectrometer (UT1 1OOC).

4. Results and discussion Onto (100) oriented surfaces of Fe-3%SiO.O4%V-C single crystals the segregation of vanadium, carbon and silicon is observed at elevated temperatures applying Auger electron spectroscopy (AES). Before heating the sample had been sputter-cleaned at room temperature. In fig. 4 Auger peak height ratios are plotted versus segregation time t at 610°C. Using a logarithmic time scale, two different surface states can be clearly distinguished. Initially, the surface segregation of silicon and carbon takes place. Steady-state S&,/F%,, and C,,,/Fe6s, Auger peak height ratios are obtained after approximately 20 min at 610 “C. The onset of vanadium surface segregation, which is observed after 1 h, is accompanied by the simultaneous increase of the carbon coverage, whereas the silicon coverage decreases to the co~esponding bulk concentration. At thermodynamic equilibrium, which is achieved after about

.-El 2 z

Table 1 Diffusion coefficients of C, Si and V in a-Fe at 600°C Element

D (cm’/s)

cB (cm-‘)

c”fi

C Si V

6x10-s

9x10’8

2 x 10’5

(cmv2 s-‘12)

Ref. [211

3x10-‘5

5x102’

3x10’4

1221

1 x10-‘5

4x10”

1 x lo’?

[231

200 h, the surface is covered with vanadium and carbon. This complex segregation behavior can be understood if the bulk concentrations and the bulk diffusion coefficients of the solutes are taken into consideration. In table 1 diffusion coefficients of V, C and Si in a-Fe are compiled. Assuming that the bulk diffusion of the solutes is the rate-limiting step of surface segregation, an approximate solution of the diffusion equation given by Crank [20] can be applied. cfp = 2+/~)

(8)

where c’ [cmi2 ] and cB [cmm3] are the surface and bulk concentrations of the solutes, D is the bulk diffusion coefficient and t is the segregation time. The products c”&? for the interstitially dissolved carbon and the substitutionally dissolved silicon are more than two orders of magnitude larger than the corresponding value for vanadium. Therefore, it may be concluded that the comparatively fast bulk diffusion of carbon and silicon leads to the initial formation of non-equilibrium surface phases. Later on these non-equilibrium surface phases are replaced by equilibrium surface phases containing vanadium as shown in fig. 4. In the following sections, the experimental results concerning equilibrium and non-equilibrium surface phases are presented separately. 4.1. Equilibrium surface phases

he

t [mini

Fig. 4. Time dependence of surface coverage on Fe-3SSiO.O4%V-C (100). Auger peak height ratios are used to characterize the surface en~c~ent of the various segregants.

As discussed before, after prolonged heating at 610” C cosegregation of vanadium and carbon is observed onto the (100) oriented substrate surface. At thermodynamic equilibrium, the surface is saturated with segregated vanadium and carbon (fig. 5a). The comparison of the CKL2.3L2,3 peak shape with published reference spectra [l] indicates that the bonding state of the segregated

C. Uebing. H. Viefiaus / Surface segregation on Fe-3 Mi-O.O4%V-C

1

L

250

500

I

750

[kin [evl Fig. 5. Auger spectra corresponding to: (a) the vanadium carbide surface phase on Fe-3%Si-O.O4XV-C (100) at 610 o C, (b) the silicon saturated substrate surface at 660 o C.

35

(100)

carbon corresponds to carbidic or atomically adsorbed carbon species; graphite is not observed at thermodynamic equilibrium. Therefore, it is concluded that the cosegregation of vanadium and carbon leads to the formation of a vanadium carbide surface phase. The LEED pattern of the vanadium carbide saturated surface reveals a well-ordered (1 X 1) structure with low background intensity (fig. 6a). Additional overstructure spots are not detectable. Hence, the symmetry of the vanadium and carbon sublattices, respectively, and the interatomic distances within both sublattices correspond to the (100) substrate surface. The observed (1 X 1) LEED pattern of the VC surface compound is similar to the corresponding pattern of the surface compounds CrN and CrC formed at elevated temperatures on (100) oriented surfaces of Fe15%Cr-30ppmN and Fe-15%Cr-20ppmC single crystals, respectively [13-151. Therefore, it is concluded that the surface compounds VC, CrN and CrC are isomorphous. The most probable arrangement of the adatoms can be derived from the (100) plane of the three-

Fe-3%Si-O.O4%V-C

VC surface compound

Fe, V

b Fig. 6. (a) (1 x 1) LEED pattern of the (100) oriented Fe-3%Si-O.O4%V-C surface saturated with segregated vanadium and carbon. (b) Schematic representation of the structural relationship between the (100) substrate surface and the vanadium carbide in real space. Lattice vectors of both structures are indicated.

36

C. Uebing, H. Viefhaus / Surface segregation on Fe-3 %Si-0.04

% V-C (100)

dimensional vanadium carbide VC (VC3D) with rocksalt structure (fig. 6b, right side). The structural relationship between substrate (fig. 6b, left side) and VC surface phase (VCZD) is given by eq. (9):

0

1

2

3

L

Ar’ ion dose [10”

The lattice mismatch of both structures 6 as defined in eqs. (10) and (11) is about - 2.4% (a,,, = A [25]), which is 2.86 A [24], a,(-~> = 4.13-4.17 rather low. Therefore, it is concluded that the two-dimensional VC surface phase is epitaxially arranged on the (100) substrate surface [26-291. Assuming ideal order of the surface phase, the atomic density of a single VC compound layer can be calculated from the lattice constant of the bee substrate, it is Fv = Fc = 1.22 x 1015 cm-2. The thickness of the surface phase was estimated from Ar+ depth profiles recorded at room temperature. In fig. 7 Auger peak height ratios of V,,,/F%,, and C,,,/F%,, are given as a function of the Ar+ ion dose per unit area. With increasing ion dose a monotonous decrease of the V/Fe and C/Fe peak height ratios is observed. After a dose of about 6 X 1015 cm’ the measured surface concentrations of the segregants are below the Auger detection limit. Neglecting preferential sputtering, the number n of sputtered surface atoms per unit area can be expressed in the following way: n = j,St,

(12)

where jp is the Ar+ current density, t the sputter time and S the mean sputter yield of vanadium and carbon. Since precise data for the sputter yield of V and C at EAr+= 3 keV are not available, S has been arbitrarily taken as unity, although the mean sputter yield of the isomorphous surface compound TiC on Fe-6%Al-O.S%Ti-C (100) has been determined by depth profiling as about 0.1 [30]. However, taking Fv = Fc = 1.22 x 1015 cm-* as evaluated from the (1 x 1) LEED pattern, it follows that the thickness of the vanadium carbide surface phase is approximately 2 atomic layers, or even smaller if S < 1. Within the range of detec-

5

6

1

cm-‘]

Fig. 7. Ar + ion depth profile of the VC surface compound on Fe-3&Si-0.04%VPC (100) ( EAr+= 3 keV).

tion by AES three-dimensional precipitates of vanadium carbide could not be detected after removal of the surface phase. Even at temperatures as low as 500°C i.e. within the stability range of VC bulk precipitates, surface precipitation does not take place. From Auger spectra, LEED pattern and Ar+ depth profiles it can be derived that the vanadium carbide surface phase is a two-dimensional surface compound. The ideal stoichiometry of the surface compound is VC. However, surface defects will be present. Upon increasing the temperature above 635°C the vanadium carbide is dissolved into the bulk and silicon segregation takes place. The Auger spectrum fig. 5b, which is recorded at silicon saturation for a sample temperature of 660” C, reveals a small carbon Auger peak besides a marked silicon peak at 92 eV. The same surface coverage is obtained on sputter-cleaned single crystals, which have been annealed directly to 660 o c. The LEED pattern of the silicon-saturated substrate surface reveals a c(2 X 2) structure (fig. 8a). This pattern has been observed previously by de Rugy and Viefhaus for silicon on Fe-3%Si (100) [7]. As for C,, N, S and P and P on (100) a-Fe surfaces, the most probable coordination sites of the segregants are the four-fold hollow positions, which allow high coordination between segregants and substrate atoms. The c(2 X 2) Si structure is shown schematically in fig. 8b. At saturation only one half of the available four-fold hollow posi-

C. Uebing, H. Yiejhuus / Surface segregation on Fe-3%Si-O.O4%V-C (100)

b Fig. 8. (a) c(2 x 2) LEED pattern of the (100) oriented Fe-3%S~-~.~%V-C repr~enta~on

surface saturated with segregating silicon. (b) Schematic of the ~$2 X 2) Si structure in real space.

tions is occupied, the maximum silicon coverage @,, is 0.5. The temperature dependence of the eq~~b~urn surface coverage is shown in fig. 9; Auger peak height ratios are used to characterize the surface composition. Vanadium carbide is stable as a surface compound up to 435 o C. With increasing temperatures the surface coverage changes abruptly. V~adium is completely dissolved into the bulk at temperatures above 635”C, whereas small amounts of carbon are still present at the surface up to 750°C. The reverse changes in surface coverage are observed upon quenching to temperatures below 635O C. Thus, the surface phase transition is reversible. The temperature dependence of surface coverage as shown in fig. 9 indicates the qualitative correspondence with the model of combined site competition and cosegregation (fig. 3), outlined above (X, = C, X, = V, Xs2 = Si). Because of the absence of precise Auger calibration factors for the surface compound VC on Fe-3%Si-O.O4%V-C alloys, exact quantification of the surface coverage is not possible. Therefore, the observed surface phase transition could not be evdtuated quantitatively using eqs. (6) and (7).

Fig. 9. Temperature dependence of equilibrium surface coverage on Fe-3%S~-O.~%V-C (100). Auger peak height ratios are used to characterize the surface enrichment of the segregants.

3x

C.

Uehing, H. Viefhaus / Surface segregamn

At equilibrium the number of coexisting bulk and surface phases can be calculated applying a generalized phase rule 1181. The variance W of the solid is given by W=CSS,+2-P3o-P2o,

on Fe-3%Si-O.O4%V-C

(100)

r--' dNiE1 dE

(13)

where C is the number of components, Pj” and PZD are the number of bulk and surface phases and S, denotes the total number of different surfaces. In the following the generalized phase rule is applied to the phase equilibrium between the ferritic substrate, the VC surface compound and Si segregated on Fe-3WSi-0.04%V-C (100). Taking S, = I, C = 4, P3D = 1 and P2D = 2, the variance W equals 4. In this case the intensive quantities of the system including the temperature can be written as function e.g. of the total pressure and the silicon, vanadium and carbon bulk concentrations. Thus, for a given bulk composition and total pressure, the coexistence of both surface phases on the ferritic substrate is possible only at a distinct temperature, as observed.

250

SO0

7%

Ekln kV1 Fig. 10. Auger spectra corresponding to non-equilibrium surface phases: (a) the graphite surface phase at 460’ C, (b) segregated carbon at 560 o C.

As stated before, rapid carbon and silicon diffusion facilitates the formation of non-equilibrium surface phases after short annealing times. As can be seen from fig. 4, steady-state surface coverages are obtained before the onset of vanadium surface segregation and subsequent formation of the VC surface compound at temperatures below 635’C. Therefore, it may be assumed that the observed surface phases are metastable. In the following, only these non-equilib~um steady-states are described and discussed. The Auger spectra shown in figs. 1Oa and 10b are recorded at 460 and 560°C, respectively. The fine structure of the CKL,,,L,,, peak in fig. 10a indicates the presence of graphite on the substrate surface at low temperatures. Graphite as surface phase is stable up to 500° C. Above 500* C the chemical nature of the segregated carbon corresponds to atomically adsorbed carbon species [l]. The temperature dependence of steady-state surface coverage is plotted in fig. 11 in the temperature range from 500 to 900 a C. The surface

coverage is calculated from the measured Auger peak height ratios using the calibration factors for segregated Si and C on (100) oriented Fe-3%Si-C single crystals, which have been given by de Rugy

TemperatureT PC1 Fig. 11. Temperature dependence of surface temperature range from 500 to 900” C. The calculated according to the eqs. (1) and kJ/moI, AGZ, = -- 34 kJ/mol, a = 14.3

coverage in the solid curves are (2) (A@ = -76 kJ/moi).

C. Uebing, H. Viefiaus / Surface segregation on Fe-3%Si-O.O4%V-C

and Viefhaus [7]. Between carbon and silicon site competition is effective; with increasing temperature the carbon coverage decreases and silicon segregation takes place. Above 635” C the observed Si and C coverages correspond to the thermodynamic equilibrium, as described before. The surface coverages 0, and esi are evaluated using eqs. (1) and (2). The best fit between experimentally determined values and calculated curves as shown in fig. 11 is obtained for AG: = -76 kJ/mol, AGii = - 34 kJ/mol and LY= 14.3 kJ/mol. These values differ moderately from those which have been determined for the ternary system Fe-3%Si-C (100) (AG: = -102.7 + 16.80, kJ/mol, AG$ = -48 + 0.016T kJ/mol, cx= 25 kJ/mol [7,31]). The LEED pattern of the graphite covered surface shows a drastically increased background intensity besides intensive substrate spots (fig. 12). The c(2 x 2) overstructure spots, which are visible in fig. 12 at a low intensity, are due to small amounts of segregated carbon as described below. In contrast to the surface compound VC, the graphite layer is not epitaxially arranged on the substrate. As in the case of graphite on Fe-C (100) [32,33], the graphite is disordered and statistically oriented on Fe-3%Si-O.O4%V-C (100). At saturation with segregated carbon above 500” C,

\

~(2x21

39

(100)

Si

segregotlon

structure

Fig. 13. Schematic representation of a simplified time-temperature-surface phase transition diagram in order to illustrate the complexity of non-equilibrium segregation phenomena observed on Fe-3%Si-0.044&V-C (100).

the single crystal surface exhibits a c(2 X 2) LEED pattern similar to the pattern shown for segregated silicon in fig. 8a. The structure of the segregated carbon layer corresponds to the c(2 X 2) Si structure (fig. 8b). These complex non-equilibrium segregation phenomena are illustrated schematically in a simplified time-temperature-surface phase transition diagram (fig. 13). It should be noted that the transitions between graphite layer, c(2 x 2) C and c(2 x 2) Si structure observed upon varying the temperature are reversible: incArT graphitelayer de;

c(2 x 2) c,

04)

c(2 x 2) c zs;

c(2 X 2) Si.

(15)

However, surface phase transitions which involve the VC surface compound, are not reversible: c(2 x 2) c “2 graphite layer “2

VC surface camp, VC surface camp.

(16) (17)

5. Conclusions

Fig. 12. LEED

pattern of the graphite surface 3%X-O.O4%V-C (100).

phase

on Fe-

The temperature and time dependence of surface segregation phenomena on the (100) surface of Fe-3%Si-O.O4%V-C has been investigated applying AES and LEED. Thermodynamic equilibrium is obtained only after prolonged heating because of the low vanadium bulk concentration. At equilibrium at

40

C. Uebing, H. Viejhnus / Surface segregation on Fe-3%Si-O.O4%V-C

temperatures below 635 o C the substrate surface is covered with the binary surface compound VC. From the (1 X 1) LEED pattern it can be concluded that the two-dimensional surface compound is epitaxially arranged on the substrate surface. Its structure corresponds to the (100) plane of the three-dimensional carbide VC with rocksalt structure. The thickness of the surface compound has been estimated from Art depth profiles as about 2 atomic layers. Even within the stability range of VC bulk precipitates at low temperatures, surface precipitation does not take place. At temperatures above 635 o C the surface compound VC desegregates into the bulk and the formation of a silicon c(2 X 2) structure takes place. This reversible surface phase transition has been interpreted qualitatively applying an extension of Guttmann’s theory of surface segregation in multicomponent alloys. Because of the low concentration of the substitutionally dissolved vanadium, the formation of metastable non-equilibrium surface phases by rapid diffusion of silicon and carbon is possible in the system investigated. Below 500” C a disordered graphite layer is observed on the substrate surface. With increasing the temperature site competition between segregating carbon and silicon both elements forming c(2 X 2) structures - has been observed. Between silicon and carbon repulsive interactions are effective on the (100) oriented substrate surface.

Acknowledgements The authors are indebted to Professor Dr. H.J. Grabke, Dr. C.A. Long and Dr. J.C. Nava Paz for many critical discussions and helpful comments. This work has been financially supported by the Deutsche Forschungsgemeinschaft.

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