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Surface oxidation behaviour of amorphous and crystallized Fe75P15C10 and Fe85B15 alloys
Journal of Non-Crystalline Solids 104 (1988) 153-160 North-Holland, Amsterdam
153
SURFACE O X I D A T I O N B E H A V I O U R O F A M O R P H O U S AND C R Y S T A L L I Z E D FeTsPlsClo AND FessBls ALLOYS Jean FUSY i and Pierre PAREJA 2 J CNRS, Laboratoire Maurice Letort associO h l'Universit~ de Nan O, L BP 104, 54600 Villers-les-Nancv, France : UniversitO de Nancy L Laboratoire de Catalyse Hdt~rog~ne, BP 239, 54506 Vandoeuvre Cedex, France
Received 11 November 1987 Revised manuscript received 28 March 1988
In situ oxidation at low pressure of amorphous and crystallized FevsP15C10 and Fe85B15 alloys was carried out in a UHV chamber using AES and XPS. It was observed that the state of crystallinity has no effect on the rate of the oxygen uptake for the two alloys. The determining factor is the actual superficial content of P which favors a higher rate for Fe75P15C10while surface oxidation occurs at the same rate on pure Fe and Fe85B15.
1. Introduction Among the chemical properties of amorphous alloys that have been studied - mainly in the field of corrosion [1] - surface oxidation has received special interest. Early in 1981, Baer and Thomas [2-4] showed the ambiguous role of phosphorus at the alloy-oxide layer. Then, Nigavekar et al. demonstrated the different behaviors of amorphous and crystallized alloys [5,6] and more recently, they published a study of the initial oxidation of Fe40Nia0PlaB 6 [7]. At the same time, we published a similar study [8] using almost the same experimental method on the in situ oxidation at low pressure of amorphous and crystallized Metglas 2826 (Fea0Nia0P16B4), an alloy very close in composition to that used by Nigavekar. In spite of slight differences, both studies can appear as complementary. In fact, the main purpose of this work [8] was to observe any possible difference in the superficial reactivity between the amorphous alloy and the crystallized one obtained by in situ thermal annealing. The chosen test of surface reactivity was the comparison of the rates of oxygen uptake by the two samples. We demonstrated that the difference of reactivity between the two crystalline states could also be explained by the enrichment 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of the surface in phosphorus. The inhibiting effect of this element was clearly demonstrated by the corresponding decrease of the initial rate of the oxygen uptake. In addition, Ni either present in the saturated oxide layer of the annealed or amorphous sample, was shown to undergo no oxidation. Therefore, in order to decide whether the amorphous or crystallized state has a determining influence on the oxygen uptake, we found it interesting to carry out similar experiments with two alloys containing only iron as the metal, one of them containing phosphorus and the other not.
2. Experimental The two samples included in this study were amorphous ribbons of Fe75P15C10 and Fe85B15 both prepared by melt spinning. They were obtained from the Laboratoire de Physique des Solides, Nancy. They were cut into 13.5 x 6.5 m m 2 strips suited to the manipulator and attached on it with gold holders which were used for XPS calibration as well as an internal reference in Auger measurements. The temperature of the manipulator could be monitored from room temperature to 6 3 0 ° C
154
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys
and the sample could be exposed to various gases - specially argon for ion etching and oxygen for in situ oxidation - introduced into the chamber through an adjustable leak valve. Partial pressures were measured with a calibrated ionization gauge. After cleaning with acetone in an ultrasonic cleaner, the ribbons were rinsed with alcohol and dried before being introduced in the UHV-chamber (vacuum= 10 -1° Torr) for Auger and XPS analysis. The argon ion gun was operated at 1 kV, 0-100 mA for cleaning or depth profiling. Auger spectra were recorded in d E / E = constant mode using a 2 V peak to peak modulation voltage while the XPS analysis was performed with an ESCA system using the magnesium K a X-ray radiation.
3. R e s u l t s a n d d i s c u s s i o n
3.1. F e ~ s Q o 3.1.1. Influence of temperature on superficial composition The sample was first cleaned by ion sputtering until its superficial composition as measured by Auger spectroscopy did not change any further. After the ion etching had been stopped, the variation of the Auger signals of each element was recorded at a given temperature. We obseved that phosphorus was the only element the superficial content of which changed significantly. This variation is shown in fig. 1 for different temperatures. It can be noticed that up to 3 4 0 ° C this change was slow and not significant while above this temperature the signal rose rapidly. This phenomenon of phosphorus segregation has already been reported for numberous alloys containing this metalloid [4] and constituted the major difficulty of our previous work [8]. Exactly as in the case of Metglass 2826, it seems that the superficial content of phosphorus rises especially when the temperature at which the recrystallization starts had been reached. This can be observed in fig. 2 where stationary Auger signal amplitudes of the three elements are plotted as a function of the temperature.
/
t,/
/~/t, /
~
d/< ~
~,/~.~
400 °C
,
370 °C
~
v_-------v-- 340 °C ~-- 270oC
5
10
15
t (ran)
Fig. 1. Fe, P, C - variation of the phosphorus Auger signal with time at different temperatures.
It must also be noted that when the sample was allowed to cool down to 200 ° C after the concentration of the different elements had reached a stationary value at a higher temperature - at 430 ° C for example - this superficial concentration was "quenched". Sputtering of such a "quenched" substrate uncovered an underlayer depleted of phosphorus so that we observed first a decrease of its superficial concentration followed - on continuation of the sputtering - by an increase up to the value relative to the amorphous sample at 200 ° C. This constituted a method by which to obtain at the temperature of 2 0 0 ° C samples of various superficial content in phosphorus for oxidation experiments.
® A
-J
<
o
<
®
o-----~- 8
2~o
3~o
~ 400
( t °C)
Fig. 2. Fe, P, C - variation of the Auger signal of the element with temperature.
J. Fusy, P. Pareja / Surface oxidation behaciour of alloys
155
temperature where the segregation of phosphorus occurred. As in the case of Metglas 2826, we plotted the initial rate of oxygen uptake as a function of the superficial concentration in phosphorus (normalized/Au) at the beginning of the exposure for both the amorphous and in situ annealed sample. So fig. 4 clearly demonstrates that this initial rate depended more on the actual superficial content in phosphorus than on the amorphous or crystallized state of the sample.
kd
a_ z w
x
Fig. 3. Fe, P, C - initial rate of oxygen uptake versus temperature: e, Increasing temperatures; × , after annealing at 420 o C.
3.1.2. Initial rate of oxidation Measurement of the initial rate of oxygen uptake was carried out exactly as previously described for Metglas samples [8]. It consisted in measuring the peak-to-peak amplitude of oxygen and gold Auger signals and in determining the rate of increase with time of the ratio O / A u when the sample was subjected to a pressure of 3.0 × 10 8 Torr of oxygen. The variation with temperature of this ratio considered as the initial rate of oxygen uptake - is shown in fig. 3. It can be observed that it decreased significantly above 300°C, i.e., at the
3.1.3. Oxidation We also followed the change of the Auger signals of the different elements when the sample was subjected at 300 ° C to higher exposures. Figs. 5 and 6 show their variation for an amorphous sample and an annealed one, respectively, (1 L = 10 6 Torr s). It can be noted that the oxygen signal saturated for exposures of about 2000 L in both cases. At this moment, the upper layer consisted only in Fe and O. A determination of the ratio F e / O and its comparison with that measured in the case of pure iron and other samples led to the conclusion that the oxidized layer is Fe203. The formation of this oxide layer hides the signals of P and C completely because of the low escape depth of these elements expecially P. In the case of the amorphous sample, after a change of the steep slope up to the relatively low
. . . 4 . ~ v
i,O
Y
+--
+
6
k z t.d
50
100 Exp(L) o
I
I
I
P AUGER SIGNAL (au)
Fig. 4. Fe, P, C - initial rate of oxygen uptake (values of fig. 3) versus initial phosphorus Auger signal.
Fig. 5. Fe, P, C - oxygen uptake on an amorphous sample at 200 o C. Variation of the Auger signal of the various elements with exposure, o , Fe; e, P; x , C; + , O.
J. Fusy, P. Pareja / Surface oxidation behaviourof alloys
156
v
4-
j
0
350 ° C and the second one around 430 o C, while the entire crystallization is achieved at 470°C. These two stages were observed using resistivity measurements as well as differential enthalpic analysis of Mossbauer spectroscopy. Alloys of less than 20% undergo a phase separation on a microscopic scale.
4-
0
O--
Exp x l 0
-3 ( L )
Fig. 6. Fe, P, C - oxygen uptake on a crystallized sample at
200 ° C. Variation of the Auger signal of the various elements with exposure. ©, Fe; •, P; x, C; +, O.
exposure of about 100 L, the curves giving the Fe and O content show a variation of lower slope. Note that 100 L is the exposure for which P and C vanish. On the contrary, the increase in the O signal was slower in the case of the annealed sample in exactly the same way that the decreases of Fe and P were slower. But one must notice that in this case the P content was relatively high when the experiment started and this is probably the reason for the slower rate of oxygen uptake. In fact, the same oxide layer was formed but at different rates according to the initial concentration of the surface in phosphorus. This suggests that the diffusion of Fe toward the surface in order to build the upper layer of Fe203 is inhibited by P. Once formed, the growth of this oxide layer does not proceed significantly any more. This is not inconsistent with the fact that on Metglass the average thickness of the layer ( - 35 A) does not depend significantly on the superficial composition, the state of the surface or even more on the partial pressure of oxygen used.
3.2.1. Influence of the temperature on superficial composition The sample was first cleaned by ion etching at a temperature of 200 ° C until the Auger signals of oxygen and sulfur were absent from the spectrum. The Ar sputtering was then stopped and the cleanliness of the sample was considered as achieved when the superficial composition was stable with time. This stable value was measured before proceeding in exactly the same way at a higher temperature. It is to be noted that it was impossible to completely remove carbon from this sample. We think that this element was an impurity of the bulk, probably introduced along with iron. Fig. 7 shows the variation of Auger signals of Fe, B and C as a function of temperature between 200 ° C and 600 ° C. The peak heights are normalized relative to the internal reference provided by the gold holder of the sample. A continuous decrease of the B signal was observed between 200 ° C and 360 ° C which is the
i
~/)
8
--
v
o
o °0
0
@
0 0
3.2. Fe85B/5 x
The crystallization of this amorphous alloy was studied by Tete et al. [9]. For alloys of low content in B like the one used in this study, crystallization occurs in two stages: the first one takes place at
200
300
' ~ - ' ~
400
x
×
500 (~ oc) ~
Fig. 7. Fe, B - variation of the Auger signal of the elements with temperature. © D, increasing temperature; • II, after annealing at 560 ° C.
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys temperature where the crystallization starts. Above 4 5 0 ° C - i.e., the end of crystallization - the B peak increased again up to its initial value restored at 600 ° C. By decreasing temperatures this value remains stable and was measured at 3 5 0 ° C and 200 o C. The Fe signal did not change markedly and could be considered as constant within the experimental accuracy, which is very poor at the highest temperatures. The content in carbon remained almost constant. 3.2.2. Initial rate of oxidation The measurement of the initial rate of oxygen uptake was carried out in the following way: the sample was first prepared at the temperature of the experiment according to the previous test of cleanliness and stability with the time of the Auger spectrum. A low pressure of oxygen (3 × 10 -s Torr) was then admitted in the U H V chamber and the variation of the Auger signal of oxygen was recorded for a few minutes. As previously mentioned, the peak height was normalized relative to the peak height of gold under the same conditions. It can be seen in fig. 8 that the initial rate of oxygen uptake of the F e - B alloy did not depend very much on the temperature and almost kept within the experimental accuracy.
o
d hA
"
o
o o
2;0
o
~ 300
•
o
4~o
o
5~o
~oc) e>
Fig. 8. Fe, B - initial rate of oxygen untake versus temperature. ©, increasing temperature; O, after annealing at 560 o C; zx,pure Fe.
157
_ + /
tO t~ hA tO ,<
~
1
~10 2
e 4
~"
0B 6
--O-Exp x iO-3 (L_)
Fig. 9. Fe, B - oxygen uptake on an amorphous sample at 200 o C. Variation of the Auger signal of the various elements with exposure, o, Fe; +, O; rn B; • I~, B oxide.
This result is in good agreement with the work of Blakely [10] and is consistent with other experiments carried out separately on a pure Fe ribbon and that are not reported here for the sake of simplicity. However, let us emphasize the fact that the initial rate of oxygen uptake had the same value for both ribbons of the F e - B alloy - amorphous and annealed - and for pure Fe submitted to the same experiment. 3.2.3. Oxidation In these experiments we followed the variation of the Auger spectrum for two samples of the Fe85B]5 alloy, one amorphous and the other annealed under vacuum at 560°C. The oxidation was carried out at 2 0 0 ° C with an exposure to oxygen as large as 10 000 L. This variation of the different signals in the case of the amorphous sample is shown in fig. 9. It can be inferred from the Auger spectra that the B peak corresponding to the B metal disappeared for very low exposures and were replaced by two other peaks shifted to lower energy. There is a little ambiguity concerning the position of the first satellite, which is very close to the position of t h e sulfur peak. However, taking into account the care we had taken when cleaning the sample and the parallel variation of the two quantities, it seems most likely to assign both peaks to B oxide.
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys
158
N(E) cts/s +
J
f
300
0~0~
200
.o--
m-m
m--
2
EXPxl0-3
4
Fig. 10. Fe, B - oxygen u p t a k e on a crystalline s a m p l e at 200 o C. V a r i a t i o n of the A u g e r signal of the various elements with exposure. ©, Fe; + , 0; D, B; • V1 B oxide.
The two signals exhibit a slight m a x i m u m between around 550-750 ° C and then decrease regularly with increasing exposure. Fig. 10 relates to the sample previously annealed at 560 o C in order to achieve its crystallization. In that case, the peak assigned to the B metal
N(F)
I
cls/s
2O0
N
100
.
200
,
.-
195
190
,
..
•
Eb(e.v)
Fig. 11. Fe, B - oxygen u p t a k e on a n a m o r p h o u s s a m p l e at 200 o C. XPS spectra of B 1 s at different exposures.
100
.
".
200
'
195
-
"'.'"'." " i
190
Eb(e.v)
Fig, 12. Fe, B - o x y g e n u p t a k e on a crystallized s a m p l e at 200 ° C. XPS spectra of B 1 s at different exposures.
decreased rapidly but without totally disappearing even for the largest exposures. The two statellites corresponding to B oxide were still the main species and followed a parallel variation: an increase up to saturation obtained for an exposure of about 2000 L. In both cases, the Fe signals decreased while that of B increased. It could be observed that this decrease is steeper in the case of the amorphous sample and one can even imagine a slight minim u m corresponding to the B maximum. Let us recall that such a phenomenon - a m a x i m u m of B and minimum of Fe - had been observed in our previous work [8] on Metglas. Figs. 11 and 12 show the XPS spectra of B ls before and during the oxidation of amorphous and annealed samples. It must be noted that even on a clean amorphous sample, a little part of the B was oxidized, as demonstrated by the presence of a second feature shifted by 4.0 eV. When exposed to oxygen, the XPS line centered at 188 eV disappeared progressively. For the crystallized sample the second peak was not observable on the XPS spectrum of the
J. Fusy, P. Pareja /Surface oxidation behaviour of alloys N(E) cts/s
2x104
f
.,. "
730
'
,'./
720
N(E)
I cts/s
xo 'L' , , , . ,
159
i
I i 'i
..
"
710
700
Eb(e.v)
730
Fig. 13. Fe, B - oxygen uptake on an amorphous sample at 200 o C. XPS spectra of Fe 2p at different exposures.
720
710
700
Eb[ev)
Fig. 15. Pure Fe - XPS spectra of Fe 2p at different oxygen exposures.
clean sample. On exposure to oxygen, the peak at 188 eV disappeared as another appeared, shifted by 4.1 eV. Figs. 13 and 14 show the change of the Fe 2p~/2 and Fe 2p3/2 spectra on exposure to oxygen of the two amorphous and crystallized samples. Fig. 15 is given for comparison and relates to the experiments mentioned above and carried out on pure Fe. In each case, a shift of 3.55 eV to higher energy was observed, allowing characterization of Fe III [11].
NIE) cts/s
3xi04
2xi04
~
N
I×104
730
7½0
710
'E b(e.v)
Fig. 14. Fe, B - oxygen uptake on a crystallized sample at 200 ° C. XPS spectra of Fe 2p at different exposures.
4. Conclusion
Let us recall the aim of this work. In a previous study carried out on Metglas, we had observed that the oxygen uptake occurred at a higher rate on the surface of the corresponding crystallized alloy obtained by in situ annealing. But a careful examination of this phenomenon led us to establish a correlation between the actual initial content of phosphorus of the surface and its ability to fix oxygen.
160
3". Fusy, P. Pareja /Surface oxidation behaviour of alloys
W e have then r e p r o d u c e d the same m e a s u r e m e n t s on two different F e - b a s e d alloys, one having P in its f o r m u l a a n d the o t h e r not. In the case of FevsP15C10, we have o b s e r v e d exactly as for M e t g l a s - that the rate of o x y g e n u p t a k e d e p e n d s m o r e on the superficial c o n t e n t in p h o s p h o r u s at the b e g i n n i n g of the o x i d a t i o n t h a n on the crystalline state. Moreover, because of the strong segregation of this element, this key factor is very difficult to control. In the case of the second alloy - FessB15 which contains n o p h o s p h o r u s - we o b s e r v e d a very small change in the initial rate of oxygen u p t a k e according to the temperature. This change was of the o r d e r of m a g n i t u d e of the e x p e r i m e n t a l accuracy, although the superficial c o n t e n t in m e t a l l o i d u n d e r w e n t a very large change, p a s s i n g t h r o u g h a m a r k e d m i n i m u m in the range of t e m p e r a t u r e where crystallization occurred. T h e i n d e p e n d e n c e of the rate of oxygen u p t a k e with t e m p e r a t u r e is otherwise k n o w n for p o l y c r y s t a l l i n e F e as well as for single crystals of the same metal. W e can also note that we m e a s u r e d exactly the s a m e rate for the a m o r p h o u s a n d a n n e a l e d s a m p l e s a n d for p u r e F e in the same range of temperature. Finally, we have n o t o b s e r v e d with a n y o f the three alloys used in these studies - M e t g l a s [8], Fe75P15C10 (this work) - a n y difference in the
superficial reactivity t o w a r d oxygen which could n o t be e x p l a i n e d b y a c h a n g e in the surface c o m p o s i t i o n d u e to the segregation of the p h o s p h o r u s which acts as a n i n h i b i t o r of the reaction.
References [1] R.B. Diegle, N.R. Sorensen, T. Turu and M.R. Lataniesen, Treatise on Material Science and Technology, Vol. 23, ed. J.C. Scully (Academic Press, New York, 1983). [2] M.T. Thomas and D.R. Baer, Proc. 4th Int. Conf. Rapidly Quenched Metals. Vol. II (Japan. Inst. of Metals, Sendal', 1981). [3] D.R. Baer and M.T. Thomas, J. Vac. Sci. Technol. 18 (1981) 722. [4] D.R. Baer, D.A. Petersen, L.R. Pederson and M.T. Thomas, J. Vac. Sci. Technol. 20 (1982) 957. [5] P.P. Karve, S.K. Kulkarni and A.S. Nigavekar, Sol. St. Commun. 49 (1984) 719. [6] P.P. Karve, M.G. Thube, S.K. Kulkarni and A.S. Nigavekar, Sol. St. Commun. 50 (1984) 1027. [7] M.G. Thube, C.V. Dharmadhikari, G.A. Dixit, S.K. Kulkarni and A.S. Nigavekar, Surface Sci. 182 (1987) 439. [8] J. Fusy and P. Pareja, J. Non-Cryst. Solids 89 (1987) 131. [9] K. Dehghan, J.M. Dubois, G. Le Caer and C. Tete, J. Non-Cryst. Solids. 65 (1984) 87. [10] S.E. Greco, J.P. Roux and J.M. Blakely, Surface Sci. 55 (1982) 203. [11] C.R. Brundle, E. Silverman and R.J. Madix, J. Vac. Sci. Technol. 16 (1979) 474.
153
SURFACE O X I D A T I O N B E H A V I O U R O F A M O R P H O U S AND C R Y S T A L L I Z E D FeTsPlsClo AND FessBls ALLOYS Jean FUSY i and Pierre PAREJA 2 J CNRS, Laboratoire Maurice Letort associO h l'Universit~ de Nan O, L BP 104, 54600 Villers-les-Nancv, France : UniversitO de Nancy L Laboratoire de Catalyse Hdt~rog~ne, BP 239, 54506 Vandoeuvre Cedex, France
Received 11 November 1987 Revised manuscript received 28 March 1988
In situ oxidation at low pressure of amorphous and crystallized FevsP15C10 and Fe85B15 alloys was carried out in a UHV chamber using AES and XPS. It was observed that the state of crystallinity has no effect on the rate of the oxygen uptake for the two alloys. The determining factor is the actual superficial content of P which favors a higher rate for Fe75P15C10while surface oxidation occurs at the same rate on pure Fe and Fe85B15.
1. Introduction Among the chemical properties of amorphous alloys that have been studied - mainly in the field of corrosion [1] - surface oxidation has received special interest. Early in 1981, Baer and Thomas [2-4] showed the ambiguous role of phosphorus at the alloy-oxide layer. Then, Nigavekar et al. demonstrated the different behaviors of amorphous and crystallized alloys [5,6] and more recently, they published a study of the initial oxidation of Fe40Nia0PlaB 6 [7]. At the same time, we published a similar study [8] using almost the same experimental method on the in situ oxidation at low pressure of amorphous and crystallized Metglas 2826 (Fea0Nia0P16B4), an alloy very close in composition to that used by Nigavekar. In spite of slight differences, both studies can appear as complementary. In fact, the main purpose of this work [8] was to observe any possible difference in the superficial reactivity between the amorphous alloy and the crystallized one obtained by in situ thermal annealing. The chosen test of surface reactivity was the comparison of the rates of oxygen uptake by the two samples. We demonstrated that the difference of reactivity between the two crystalline states could also be explained by the enrichment 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
of the surface in phosphorus. The inhibiting effect of this element was clearly demonstrated by the corresponding decrease of the initial rate of the oxygen uptake. In addition, Ni either present in the saturated oxide layer of the annealed or amorphous sample, was shown to undergo no oxidation. Therefore, in order to decide whether the amorphous or crystallized state has a determining influence on the oxygen uptake, we found it interesting to carry out similar experiments with two alloys containing only iron as the metal, one of them containing phosphorus and the other not.
2. Experimental The two samples included in this study were amorphous ribbons of Fe75P15C10 and Fe85B15 both prepared by melt spinning. They were obtained from the Laboratoire de Physique des Solides, Nancy. They were cut into 13.5 x 6.5 m m 2 strips suited to the manipulator and attached on it with gold holders which were used for XPS calibration as well as an internal reference in Auger measurements. The temperature of the manipulator could be monitored from room temperature to 6 3 0 ° C
154
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys
and the sample could be exposed to various gases - specially argon for ion etching and oxygen for in situ oxidation - introduced into the chamber through an adjustable leak valve. Partial pressures were measured with a calibrated ionization gauge. After cleaning with acetone in an ultrasonic cleaner, the ribbons were rinsed with alcohol and dried before being introduced in the UHV-chamber (vacuum= 10 -1° Torr) for Auger and XPS analysis. The argon ion gun was operated at 1 kV, 0-100 mA for cleaning or depth profiling. Auger spectra were recorded in d E / E = constant mode using a 2 V peak to peak modulation voltage while the XPS analysis was performed with an ESCA system using the magnesium K a X-ray radiation.
3. R e s u l t s a n d d i s c u s s i o n
3.1. F e ~ s Q o 3.1.1. Influence of temperature on superficial composition The sample was first cleaned by ion sputtering until its superficial composition as measured by Auger spectroscopy did not change any further. After the ion etching had been stopped, the variation of the Auger signals of each element was recorded at a given temperature. We obseved that phosphorus was the only element the superficial content of which changed significantly. This variation is shown in fig. 1 for different temperatures. It can be noticed that up to 3 4 0 ° C this change was slow and not significant while above this temperature the signal rose rapidly. This phenomenon of phosphorus segregation has already been reported for numberous alloys containing this metalloid [4] and constituted the major difficulty of our previous work [8]. Exactly as in the case of Metglass 2826, it seems that the superficial content of phosphorus rises especially when the temperature at which the recrystallization starts had been reached. This can be observed in fig. 2 where stationary Auger signal amplitudes of the three elements are plotted as a function of the temperature.
/
t,/
/~/t, /
~
d/< ~
~,/~.~
400 °C
,
370 °C
~
v_-------v-- 340 °C ~-- 270oC
5
10
15
t (ran)
Fig. 1. Fe, P, C - variation of the phosphorus Auger signal with time at different temperatures.
It must also be noted that when the sample was allowed to cool down to 200 ° C after the concentration of the different elements had reached a stationary value at a higher temperature - at 430 ° C for example - this superficial concentration was "quenched". Sputtering of such a "quenched" substrate uncovered an underlayer depleted of phosphorus so that we observed first a decrease of its superficial concentration followed - on continuation of the sputtering - by an increase up to the value relative to the amorphous sample at 200 ° C. This constituted a method by which to obtain at the temperature of 2 0 0 ° C samples of various superficial content in phosphorus for oxidation experiments.
® A
-J
<
o
<
®
o-----~- 8
2~o
3~o
~ 400
( t °C)
Fig. 2. Fe, P, C - variation of the Auger signal of the element with temperature.
J. Fusy, P. Pareja / Surface oxidation behaciour of alloys
155
temperature where the segregation of phosphorus occurred. As in the case of Metglas 2826, we plotted the initial rate of oxygen uptake as a function of the superficial concentration in phosphorus (normalized/Au) at the beginning of the exposure for both the amorphous and in situ annealed sample. So fig. 4 clearly demonstrates that this initial rate depended more on the actual superficial content in phosphorus than on the amorphous or crystallized state of the sample.
kd
a_ z w
x
Fig. 3. Fe, P, C - initial rate of oxygen uptake versus temperature: e, Increasing temperatures; × , after annealing at 420 o C.
3.1.2. Initial rate of oxidation Measurement of the initial rate of oxygen uptake was carried out exactly as previously described for Metglas samples [8]. It consisted in measuring the peak-to-peak amplitude of oxygen and gold Auger signals and in determining the rate of increase with time of the ratio O / A u when the sample was subjected to a pressure of 3.0 × 10 8 Torr of oxygen. The variation with temperature of this ratio considered as the initial rate of oxygen uptake - is shown in fig. 3. It can be observed that it decreased significantly above 300°C, i.e., at the
3.1.3. Oxidation We also followed the change of the Auger signals of the different elements when the sample was subjected at 300 ° C to higher exposures. Figs. 5 and 6 show their variation for an amorphous sample and an annealed one, respectively, (1 L = 10 6 Torr s). It can be noted that the oxygen signal saturated for exposures of about 2000 L in both cases. At this moment, the upper layer consisted only in Fe and O. A determination of the ratio F e / O and its comparison with that measured in the case of pure iron and other samples led to the conclusion that the oxidized layer is Fe203. The formation of this oxide layer hides the signals of P and C completely because of the low escape depth of these elements expecially P. In the case of the amorphous sample, after a change of the steep slope up to the relatively low
. . . 4 . ~ v
i,O
Y
+--
+
6
k z t.d
50
100 Exp(L) o
I
I
I
P AUGER SIGNAL (au)
Fig. 4. Fe, P, C - initial rate of oxygen uptake (values of fig. 3) versus initial phosphorus Auger signal.
Fig. 5. Fe, P, C - oxygen uptake on an amorphous sample at 200 o C. Variation of the Auger signal of the various elements with exposure, o , Fe; e, P; x , C; + , O.
J. Fusy, P. Pareja / Surface oxidation behaviourof alloys
156
v
4-
j
0
350 ° C and the second one around 430 o C, while the entire crystallization is achieved at 470°C. These two stages were observed using resistivity measurements as well as differential enthalpic analysis of Mossbauer spectroscopy. Alloys of less than 20% undergo a phase separation on a microscopic scale.
4-
0
O--
Exp x l 0
-3 ( L )
Fig. 6. Fe, P, C - oxygen uptake on a crystallized sample at
200 ° C. Variation of the Auger signal of the various elements with exposure. ©, Fe; •, P; x, C; +, O.
exposure of about 100 L, the curves giving the Fe and O content show a variation of lower slope. Note that 100 L is the exposure for which P and C vanish. On the contrary, the increase in the O signal was slower in the case of the annealed sample in exactly the same way that the decreases of Fe and P were slower. But one must notice that in this case the P content was relatively high when the experiment started and this is probably the reason for the slower rate of oxygen uptake. In fact, the same oxide layer was formed but at different rates according to the initial concentration of the surface in phosphorus. This suggests that the diffusion of Fe toward the surface in order to build the upper layer of Fe203 is inhibited by P. Once formed, the growth of this oxide layer does not proceed significantly any more. This is not inconsistent with the fact that on Metglass the average thickness of the layer ( - 35 A) does not depend significantly on the superficial composition, the state of the surface or even more on the partial pressure of oxygen used.
3.2.1. Influence of the temperature on superficial composition The sample was first cleaned by ion etching at a temperature of 200 ° C until the Auger signals of oxygen and sulfur were absent from the spectrum. The Ar sputtering was then stopped and the cleanliness of the sample was considered as achieved when the superficial composition was stable with time. This stable value was measured before proceeding in exactly the same way at a higher temperature. It is to be noted that it was impossible to completely remove carbon from this sample. We think that this element was an impurity of the bulk, probably introduced along with iron. Fig. 7 shows the variation of Auger signals of Fe, B and C as a function of temperature between 200 ° C and 600 ° C. The peak heights are normalized relative to the internal reference provided by the gold holder of the sample. A continuous decrease of the B signal was observed between 200 ° C and 360 ° C which is the
i
~/)
8
--
v
o
o °0
0
@
0 0
3.2. Fe85B/5 x
The crystallization of this amorphous alloy was studied by Tete et al. [9]. For alloys of low content in B like the one used in this study, crystallization occurs in two stages: the first one takes place at
200
300
' ~ - ' ~
400
x
×
500 (~ oc) ~
Fig. 7. Fe, B - variation of the Auger signal of the elements with temperature. © D, increasing temperature; • II, after annealing at 560 ° C.
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys temperature where the crystallization starts. Above 4 5 0 ° C - i.e., the end of crystallization - the B peak increased again up to its initial value restored at 600 ° C. By decreasing temperatures this value remains stable and was measured at 3 5 0 ° C and 200 o C. The Fe signal did not change markedly and could be considered as constant within the experimental accuracy, which is very poor at the highest temperatures. The content in carbon remained almost constant. 3.2.2. Initial rate of oxidation The measurement of the initial rate of oxygen uptake was carried out in the following way: the sample was first prepared at the temperature of the experiment according to the previous test of cleanliness and stability with the time of the Auger spectrum. A low pressure of oxygen (3 × 10 -s Torr) was then admitted in the U H V chamber and the variation of the Auger signal of oxygen was recorded for a few minutes. As previously mentioned, the peak height was normalized relative to the peak height of gold under the same conditions. It can be seen in fig. 8 that the initial rate of oxygen uptake of the F e - B alloy did not depend very much on the temperature and almost kept within the experimental accuracy.
o
d hA
"
o
o o
2;0
o
~ 300
•
o
4~o
o
5~o
~oc) e>
Fig. 8. Fe, B - initial rate of oxygen untake versus temperature. ©, increasing temperature; O, after annealing at 560 o C; zx,pure Fe.
157
_ + /
tO t~ hA tO ,<
~
1
~10 2
e 4
~"
0B 6
--O-Exp x iO-3 (L_)
Fig. 9. Fe, B - oxygen uptake on an amorphous sample at 200 o C. Variation of the Auger signal of the various elements with exposure, o, Fe; +, O; rn B; • I~, B oxide.
This result is in good agreement with the work of Blakely [10] and is consistent with other experiments carried out separately on a pure Fe ribbon and that are not reported here for the sake of simplicity. However, let us emphasize the fact that the initial rate of oxygen uptake had the same value for both ribbons of the F e - B alloy - amorphous and annealed - and for pure Fe submitted to the same experiment. 3.2.3. Oxidation In these experiments we followed the variation of the Auger spectrum for two samples of the Fe85B]5 alloy, one amorphous and the other annealed under vacuum at 560°C. The oxidation was carried out at 2 0 0 ° C with an exposure to oxygen as large as 10 000 L. This variation of the different signals in the case of the amorphous sample is shown in fig. 9. It can be inferred from the Auger spectra that the B peak corresponding to the B metal disappeared for very low exposures and were replaced by two other peaks shifted to lower energy. There is a little ambiguity concerning the position of the first satellite, which is very close to the position of t h e sulfur peak. However, taking into account the care we had taken when cleaning the sample and the parallel variation of the two quantities, it seems most likely to assign both peaks to B oxide.
J. Fusy, P. Pareja / Surface oxidation behaviour of alloys
158
N(E) cts/s +
J
f
300
0~0~
200
.o--
m-m
m--
2
EXPxl0-3
4
Fig. 10. Fe, B - oxygen u p t a k e on a crystalline s a m p l e at 200 o C. V a r i a t i o n of the A u g e r signal of the various elements with exposure. ©, Fe; + , 0; D, B; • V1 B oxide.
The two signals exhibit a slight m a x i m u m between around 550-750 ° C and then decrease regularly with increasing exposure. Fig. 10 relates to the sample previously annealed at 560 o C in order to achieve its crystallization. In that case, the peak assigned to the B metal
N(F)
I
cls/s
2O0
N
100
.
200
,
.-
195
190
,
..
•
Eb(e.v)
Fig. 11. Fe, B - oxygen u p t a k e on a n a m o r p h o u s s a m p l e at 200 o C. XPS spectra of B 1 s at different exposures.
100
.
".
200
'
195
-
"'.'"'." " i
190
Eb(e.v)
Fig, 12. Fe, B - o x y g e n u p t a k e on a crystallized s a m p l e at 200 ° C. XPS spectra of B 1 s at different exposures.
decreased rapidly but without totally disappearing even for the largest exposures. The two statellites corresponding to B oxide were still the main species and followed a parallel variation: an increase up to saturation obtained for an exposure of about 2000 L. In both cases, the Fe signals decreased while that of B increased. It could be observed that this decrease is steeper in the case of the amorphous sample and one can even imagine a slight minim u m corresponding to the B maximum. Let us recall that such a phenomenon - a m a x i m u m of B and minimum of Fe - had been observed in our previous work [8] on Metglas. Figs. 11 and 12 show the XPS spectra of B ls before and during the oxidation of amorphous and annealed samples. It must be noted that even on a clean amorphous sample, a little part of the B was oxidized, as demonstrated by the presence of a second feature shifted by 4.0 eV. When exposed to oxygen, the XPS line centered at 188 eV disappeared progressively. For the crystallized sample the second peak was not observable on the XPS spectrum of the
J. Fusy, P. Pareja /Surface oxidation behaviour of alloys N(E) cts/s
2x104
f
.,. "
730
'
,'./
720
N(E)
I cts/s
xo 'L' , , , . ,
159
i
I i 'i
..
"
710
700
Eb(e.v)
730
Fig. 13. Fe, B - oxygen uptake on an amorphous sample at 200 o C. XPS spectra of Fe 2p at different exposures.
720
710
700
Eb[ev)
Fig. 15. Pure Fe - XPS spectra of Fe 2p at different oxygen exposures.
clean sample. On exposure to oxygen, the peak at 188 eV disappeared as another appeared, shifted by 4.1 eV. Figs. 13 and 14 show the change of the Fe 2p~/2 and Fe 2p3/2 spectra on exposure to oxygen of the two amorphous and crystallized samples. Fig. 15 is given for comparison and relates to the experiments mentioned above and carried out on pure Fe. In each case, a shift of 3.55 eV to higher energy was observed, allowing characterization of Fe III [11].
NIE) cts/s
3xi04
2xi04
~
N
I×104
730
7½0
710
'E b(e.v)
Fig. 14. Fe, B - oxygen uptake on a crystallized sample at 200 ° C. XPS spectra of Fe 2p at different exposures.
4. Conclusion
Let us recall the aim of this work. In a previous study carried out on Metglas, we had observed that the oxygen uptake occurred at a higher rate on the surface of the corresponding crystallized alloy obtained by in situ annealing. But a careful examination of this phenomenon led us to establish a correlation between the actual initial content of phosphorus of the surface and its ability to fix oxygen.
160
3". Fusy, P. Pareja /Surface oxidation behaviour of alloys
W e have then r e p r o d u c e d the same m e a s u r e m e n t s on two different F e - b a s e d alloys, one having P in its f o r m u l a a n d the o t h e r not. In the case of FevsP15C10, we have o b s e r v e d exactly as for M e t g l a s - that the rate of o x y g e n u p t a k e d e p e n d s m o r e on the superficial c o n t e n t in p h o s p h o r u s at the b e g i n n i n g of the o x i d a t i o n t h a n on the crystalline state. Moreover, because of the strong segregation of this element, this key factor is very difficult to control. In the case of the second alloy - FessB15 which contains n o p h o s p h o r u s - we o b s e r v e d a very small change in the initial rate of oxygen u p t a k e according to the temperature. This change was of the o r d e r of m a g n i t u d e of the e x p e r i m e n t a l accuracy, although the superficial c o n t e n t in m e t a l l o i d u n d e r w e n t a very large change, p a s s i n g t h r o u g h a m a r k e d m i n i m u m in the range of t e m p e r a t u r e where crystallization occurred. T h e i n d e p e n d e n c e of the rate of oxygen u p t a k e with t e m p e r a t u r e is otherwise k n o w n for p o l y c r y s t a l l i n e F e as well as for single crystals of the same metal. W e can also note that we m e a s u r e d exactly the s a m e rate for the a m o r p h o u s a n d a n n e a l e d s a m p l e s a n d for p u r e F e in the same range of temperature. Finally, we have n o t o b s e r v e d with a n y o f the three alloys used in these studies - M e t g l a s [8], Fe75P15C10 (this work) - a n y difference in the
superficial reactivity t o w a r d oxygen which could n o t be e x p l a i n e d b y a c h a n g e in the surface c o m p o s i t i o n d u e to the segregation of the p h o s p h o r u s which acts as a n i n h i b i t o r of the reaction.
References [1] R.B. Diegle, N.R. Sorensen, T. Turu and M.R. Lataniesen, Treatise on Material Science and Technology, Vol. 23, ed. J.C. Scully (Academic Press, New York, 1983). [2] M.T. Thomas and D.R. Baer, Proc. 4th Int. Conf. Rapidly Quenched Metals. Vol. II (Japan. Inst. of Metals, Sendal', 1981). [3] D.R. Baer and M.T. Thomas, J. Vac. Sci. Technol. 18 (1981) 722. [4] D.R. Baer, D.A. Petersen, L.R. Pederson and M.T. Thomas, J. Vac. Sci. Technol. 20 (1982) 957. [5] P.P. Karve, S.K. Kulkarni and A.S. Nigavekar, Sol. St. Commun. 49 (1984) 719. [6] P.P. Karve, M.G. Thube, S.K. Kulkarni and A.S. Nigavekar, Sol. St. Commun. 50 (1984) 1027. [7] M.G. Thube, C.V. Dharmadhikari, G.A. Dixit, S.K. Kulkarni and A.S. Nigavekar, Surface Sci. 182 (1987) 439. [8] J. Fusy and P. Pareja, J. Non-Cryst. Solids 89 (1987) 131. [9] K. Dehghan, J.M. Dubois, G. Le Caer and C. Tete, J. Non-Cryst. Solids. 65 (1984) 87. [10] S.E. Greco, J.P. Roux and J.M. Blakely, Surface Sci. 55 (1982) 203. [11] C.R. Brundle, E. Silverman and R.J. Madix, J. Vac. Sci. Technol. 16 (1979) 474.