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AES and EPMA studies of aluminum and aluminum-gold carbon composite interfaces
surface science ELSEVIER
Surface Science 352-354(1996) 839-844
AES and EPMA studies of aluminum and aluminum-gold carbon composite interfaces J.F. Silvain *, Y. Le Petitcorps, M. Lahaye, M. Turner Institut de Chimie et de la Mati~re Condens£e de Bordeaux (ICMCB)-CNRS, Universit£ Bordeaux I, Chateau Brivazac, Av. du Docteur A. Schweitzer, F-33608 Pessac Cedex, France
Received 5 September 1995; accepted for publication 31 October 1995
Abstract
The chemical composition of an AS7G0.6-C interface has been investigated using an Auger and electron probe micro-analysis. An Al4C 3 interphase is formed between the AS7G0.6 matrix and carbon substrate at temperature of 600°C. The presence of a thin gold layer between the AS7G0.6 and the carbon has been found to limit the diffusion of carbon into AS7G0.6, and thereby, eliminate the formation of the interphase. The same results were obtained using a model or a real system. Keywords: Aluminum; Auger electron spectroscopy; Carbon; Clusters; Compound formation; Gold; Ion bombardment; Ion etching;
Sputtering; X-my emission
1. Introduction
Graphite/aluminum composites are promising composites for structural applications due to their potentially high specific properties [1 ]. The fabricat i o n o f such composites by the infiltration of molten aluminum into carbon fibre preforms poses three major difficulties: (1) the lack of wetting of the fibre with the molten aluminum (2) the deterioration of the fibre properties during fabrication and (3) the formation of brittle reaction products between the fibre and the matrix. To overcome these difficulties,
* Corresponding author. Fax: + 33 56 848321; e-mail: [email protected].
fibre surface coatings can be applied by various methods such as electroless deposition, electroplating, vapour deposition and plasma spraying [2]. Nickel, copper and gold (which are chemically stable with carbon and can be deposited by an electrochemical or chemical methods) are three types of widely used surface coatings for carbon fibres used in metal matrix composite. However, gold has the advantage of a better oxidation resistance than nickel and copper. For this reason, a gold fibre coating for graphite/aluminum composites was investigated. Because most metal matrix composites can be regarded as non-equilibrium systems during their fabrication and their high temperature service, a gradient of chemical potential exists at the F / M (fibre/matrix) and F / c o a t i n g / M interfaces. This
0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0039-6028(95)01223-0
840
J.F. Silvain et aL / Surface Science 352-354 (1996) 839-844
difference in chemical potential provides the driving force for diffusion a n d / o r chemical reaction when the composites are heated to elevated temperature
2.2. Model composite produced by physical vapour deposition (PVD)
[3].
2.2.1. Materials
In order to obtain some information about the interfacial reactions occurring at the A1/C and A 1 / A u / C interfaces, two different composites have been characterized (a) a real composite prepared by squeeze casting of aluminum into a gold coated carbon fibre preform and (b) a model composite prepared by PVD deposition of aluminum and alum i n u m / g o l d thin films on a flat polished carbon substrates. The composition of the phases formed at the interface, under controlled conditions, were analyzed by electron probe micro analysis (EPMA) line profiles on the real composite and Auger electron spectroscopy (AES) depth profiles on the model composite. By comparing the experimental results of the two systems, it was found that the analysis of the model composite can give relevant information regarding the M / F chemical interactions of the real composite quicker and at lower costs than the analysis of the real composite.
Aluminum alloy and gold thin films were deposited on thick carbon polished substrates by sputtering using a magnetron sputterer. The targets were placed at a distance of 50 cm from the substrate surface and a temperature of 20°C was maintained during the deposition. The rather high deposition rate prevented oxygen contamination of the films. The average composition of the AS7G0.6 layer, measured by EPMA, was analogue with the target. Some model composites were annealed under a controlled atmosphere (pressure inside the annealing chamber lower than 10 -5 mbar) for 30 min at 500 and 600°C.
2. Experimental procedure 2.1. Real composites produced by squeeze casting 2.1.1. Materials The composite was produced by a squeeze casting process which is one of the most important casting methods presently used [4,5]. The matrix was an aluminum alloy with 7 at% silicon and 0.6 at% magnesium (AS7G0.6). The T700 carbon preforms, consisting of either coated (electrochemical deposition of dense 0.3 /zm gold film) or uncoated fibres, were infiltrated by the molten aluminium alloy matrix. The pressures applied to the system were up to 100 MPa.
2.1.2. Method After cooling, ingots were cut using a diamond saw and subsequently polished to a mirror finish for E P M A observations. The microprobe used was a CAMECA with a spatial resolution of 0.5 /zm.
2.2.2. Method As deposited and annealed composites were analyzed, using the combination of ion etching with the inherent sensibility of AES, in order to profile the elemental concentration of each model composite with depth. The static pressure of the argon used was around 2 × 10 - v mbar and the etching rate was approximately 10 n m / m i n . The AES used was a Physical Electronics 590. The energy of the incident electrons was 5 keV. The data was taken in the conventional dN(E)/d(E) mode. The energy windows scanned in this multiplexing technique were for each element: O: 495-515 eV, AI: 1375-1405 eV, Au: 130-165 eV, C: 250-275 eV, and Si: 1595-1620 eV. The elemental sensitivity factor which was used for the quantitative analysis has both advantages and disadvantages. The advantages include the elimination of standards and the insensitivity to surface roughness and while the disadvantages include a neglect of variation in the backscattering factor and escape depth with material. Because the data was taken in the dN(E)/d(E) mode, the sensitivity factor was based on peak to peak heights. However, the peak to peak heights are not necessarily invariant to the matrix material; a shape variation of the N(E) curve leads to a variation, in the energy position a n d / o r the shape, of the dN(E)/d(E) curve. In particular, the energy position of the minimum a n d / o r the maximum peaks are changed. Although the dN(E)/d(E) peak position (i n respect to energy) of each element or compound is based on
J.F. Silvain et a l . / Surface Science 352-354 (1996) 839-844
it's minimum position, the maximum peak position (which seems to be more compound sensitive), is used in this study in order to identify the different aluminum species present after annealing the model composites. For example, the peak shape for a pure metal can be different than that of its oxide or compound. Some problems can therefore arise in the calculation of atomic concentration of the surface oxide or the compound interface, hence, the measured atomic concentration of these species can vary from the real one.
841
Carbon
Au/AS7G0.6
C
0
C
C
Total line profile : 3.07 microns
3. R e s u l t s
A1 - Au Inteaphas¢
3.1. Real composite
Si rich intcrphasc
AS7G0.6
Fig. 2. EPMAqualitative line profile across the F/M interface for a AS7G0.6/Au/C real compositeannealed at 600°C for 96 h.
Fig. 1 shows an EPMA qualitative line profile across the F / M interface for an AS7G0.6/C composite annealed at 600°C for 96 h (this rather long annealing time of 96 h was used in order to accentuate the formation of the reaction zone for subsequent analysis). The slopes of the carbon and aluminum profiles at the F / M interface shows the formation of a C-A1 compound. This interracial reaction zone also display a depletion of silicon which is rejected at the reaction zone/matrix interface. Fig. 2 shows an EPMA line profile across the F / M interface for an A S 7 G 0 . 6 / A u / C composite
AS7G0.6
Carbon fibre
annealed at 600°C for 96 h. Several features can be observed in this figure: 1. Even if the electron diffusion is more important at the AS7G0.6/C interface than at the C / A u interface, the abrupt slope of the carbon line profile can be associated with a sharp interface, and in consequence, an absence of carbon diffusion inside the matrix resulting in the absence of A14C3 formation. 2. An A1-Au interphase between the carbon fibre and the AS7G0.6 matrix. There is an absence of silicon in this A1-Au interphase region. 3. An A1-Si rich region at the A1-Au interface; the Si, which is rejected from the A1-Au region, segregates to the A1-Au matrix border. The gold atoms do not appear to diffuse inside the AS7G0.6 matrix.
c
3.2. Model composite 3.2.1. AS7GO.6 / C Si
0
AI
A1
Si
Total line proffie : 2.31 microns
AS7G0.6
Reaction zone A14C3
Fig. 1. E P M A q u a l i t a t i v e line profile a c r o s s the F / M i n t e r f a c e f o r a A S 7 G 0 . 6 / C real c o m p o s i t e a n n e a l e d at 6 0 0 ° C for 9 6 h.
A E S has been performed on AS7G0.6/C model composite samples for which AS7G0.6 was deposited on carbon substrates and annealed at 600°C for 30 min. Fig. 3a shows the AES depth profile where four regions can be observed: (1) an A1 oxide region at the composite surface which is due to oxidation during the annealing process, (2) an A1 metal region, (3) an interphase zone where carbon
J.F. Silvain et a L / Surface Science 352-354 (1996) 839-844
842
a) Oxide . . . .
1{)(2
Reaction z o n e A L 4 C3
AI m at ri x
I . . . .
] . . . .
I . . . .
! . . . .
] . . . .
] . . . .
Carbon Substrate I . . . .
~ . . . .
[ H 5 n i
80.
60.
r5 "<
40.
\
20.
c
/½ . . . . . . . . . . .
0
10
20
30
3.3. AS7GO.6 / Au / C
oO
o 40
50
60
specifically o f the carbide during the annealing process. With respect to the m i n i m u m of the Auger A1 curves, the energy shift difference between the metal (1397 eV) and the oxide (1390 eV) is 7 eV. This experimental value, which is much smaller than the value quoted in the literature of 18 eV [6], is due to the reduction of the oxide, under the electron beam, during the Auger analysis.
. . . . . .
70
Figs. 4a and 4b show respectively the A E S depth profile of the non-annealed and the annealed (600°C,
..
80
90
100
Depth into Specimen (nm)
b) 7
_
A
........
B
I. . . . .
a
AI layer
100
C
.
.
~ .
.
A u layer
~ .
.
~ .
.
T .
C substntte
.
~ .
.
;
,
.
~ .
.
I., .I . . . . . . . . . . . . . 80-
6 -
5-" .
o
AI metal
C
60-
r.=_
4-c~
Al OXl
40-
3-" 20-
1" Oj 0
.... 1370
i .... 1375
i .... 1380
f • . ~ 1385 1390
30 1395
6O
9O
I
L
I
ff
120
150
180
210
240
Depth into specimen (nm)
1400
Electron Energy (eV) Fig. 3. (a) In depth AES profile of a AS7G0.6/C model composite annealed at 600°C during 30 min. (b) AI KLL Auger spectra of the same composite taken in three different regions as defined in (a): the oxide, the metal and the carbide layers. and aluminum react to form AlaC 3 carbide, and (4) the carbon substrate. Fig. 3b shows the d N ( E ) / d ( E ) A1 profile associated with the three regions (going from the oxide surface to the carbon substrate) defined previously. These plots describe the evolution, during the sputtering time, o f the aluminum peak shape. The energy shifts, of .the maximum position of the peak, observed for the three aluminum species, represented by A = A1 oxide, B = A1 carbide and C = A1 metal in Fig. 3b represents the difference in the chemical environment of the aluminum atoms. T h e s e shifts occur do to the formation o f the oxide and more
b
Oxide
°t:o 30
AI-Au inlerplmse
C
subsn'at¢
/ 60 90 t20 150 180 Depth into Specimen (nm)
210
z40
Fig. 4. In depth AES profile of (a) the non-annealed AS7G0.6/Au/C model composite and (b) the annealed one (600°C, 30 rain).
J.F. Silvain et aL / Surface Science 352-354 (1996) 839-844
30 min) model composites. Fig. 4a shows clearly the sequence of the two layers which have been deposited on a polished carbon substrate (60 nm of gold followed by 90 nm of AS7G0.6). The relative abrupt slopes of the carbon and gold line profiles at the C / A u interface and likewise that of the Au and AI line profiles at the Au/AS7G0.6 interface, shows the quasi-absence of diffusion of these elements during the deposition process. In order to highlight the diffusion process after deposition, annealing of the composite (600°C, 30 min) has been performed (cf. Fig. 4b). From the AES depth profiles the following information can be noted: (1) An oxidation zone of the AS7GO.6 free surface in the model composite, induced by he presence of oxygen inside the annealing chamber. (2) An A1-Au compound with a constant composition in the A1-Au interphase. From the A.C. calculation the compound formed has a composition (55 at% A1, 35 at% Au) close to that of A12Au. This agrees with the results found for the real composite for which a diffusion of the two species (A1 and Au) is visible inside the carbon substrate. (3) The slope of the carbon line profile, which is equivalent for the annealed and non annealed sam~ pies (see Figs. 4a and 4b), shows that the carbon
A
$
........
D
B
C
I. ,[[. . .[. , .I . . . . . . . .
7-
6-: 5AI
(180
rim)
3 2 1 0
. . . .
1370
i
. . . .
1375
i
. . . .
1380
i
. . . .
1385
[
. . . .
1390
1395
Electron Energy (eV) Fig. 5. AI KLL Auger spectra of the A S 7 G 0 . 6 / A u / C model composite taken in three different regions as defined in Fig. 4b: the near carbon substrate, the C / A I - A u interface and the AI-Au bulk interphase.
843
does not react with the matrix and therefore the A1-Au interphase seems to inhibit the formation of any carbide reaction product. This is clearly visible in Fig. 5 where the dN(E)/d(E) profile obtained in the different regions of Fig. 4b (oxide surface, A1-Au compound) shows just one aluminum species (D around 1381 eV) which does not correspond to any species found in Fig. 3b. The energy shift of the maximum position of the peak is attributed to the A1-Au compound.
4. Summary
Three points will be summarized in this study of the formation of an A S 7 G 0 . 6 / C and AS7G0.6/ A u / C interfaces: - The formation of an aluminum carbide interphase for both the real and model AS7G0.6/C composites after annealing at 600°C. This formation can be divided into three steps: (1) the dissolution of carbon atoms from the graphite fibre, (2) the diffusion of carbon into the AS7G0.6 matrix, and (3) the reaction of carbon and aluminum to form A14C 3. This carbide formation in A1/C composites must be overcome to avoid the degradation of the carbon fibres (via carbon diffusion) and the formation of a fragile A14C 3 interphase. - The absence of formation of the above mentioned carbide for the A S 7 G O . 6 / A u / C composite. In this composite, the gold layer inhibits the carbon diffusion into the matrix and thereby any C-A1 reaction. However, the properties of the A12 Au compound found after annealing must be further studied in order to understand its influence on the final properties of the composite. - Similar results were found for both the real and the model composites. In effect, because the same sequence of reaction zones were found after annealing for both types of composites. We can conclude from such results that the model composite can be very helpful in giving an idea as to the chemical reactions which can take place at a F / M interface in a real composite. An important benefit of the presented results is that this model can be used for any system for which reaction occurs during either fabrication or use and must be characterized.
844
J.F. Silvain et a L / Surface Science 352-354 (1996) 839-844
References [1] J.P. Rocher, J.M. Quenisset and R. Naslain, J. Mater. Sci. Lett. 4 (1985) 1527. [2] K. Honjo and A. Shindo, J. Mater. Sci. 21 (1986) 2043. [3] T.W. Chou, A. Kelly and A. Okura, Composite 7 (1985) 201.
[4] Y. Abi, S. Horikiri, K. Fujimura and E. Ichiki, Proc. of ICCM-IV, Ed. T. Hayashi (1982) p. 1427. [5] T.W. Clyne and J.F. Mason, Metall. Trans. A 18 (1987) 1519. [6] Handbook of Auger Electron Spectroscopy (Physical Electronic, Eden Prairie, MN, 1978).
Surface Science 352-354(1996) 839-844
AES and EPMA studies of aluminum and aluminum-gold carbon composite interfaces J.F. Silvain *, Y. Le Petitcorps, M. Lahaye, M. Turner Institut de Chimie et de la Mati~re Condens£e de Bordeaux (ICMCB)-CNRS, Universit£ Bordeaux I, Chateau Brivazac, Av. du Docteur A. Schweitzer, F-33608 Pessac Cedex, France
Received 5 September 1995; accepted for publication 31 October 1995
Abstract
The chemical composition of an AS7G0.6-C interface has been investigated using an Auger and electron probe micro-analysis. An Al4C 3 interphase is formed between the AS7G0.6 matrix and carbon substrate at temperature of 600°C. The presence of a thin gold layer between the AS7G0.6 and the carbon has been found to limit the diffusion of carbon into AS7G0.6, and thereby, eliminate the formation of the interphase. The same results were obtained using a model or a real system. Keywords: Aluminum; Auger electron spectroscopy; Carbon; Clusters; Compound formation; Gold; Ion bombardment; Ion etching;
Sputtering; X-my emission
1. Introduction
Graphite/aluminum composites are promising composites for structural applications due to their potentially high specific properties [1 ]. The fabricat i o n o f such composites by the infiltration of molten aluminum into carbon fibre preforms poses three major difficulties: (1) the lack of wetting of the fibre with the molten aluminum (2) the deterioration of the fibre properties during fabrication and (3) the formation of brittle reaction products between the fibre and the matrix. To overcome these difficulties,
* Corresponding author. Fax: + 33 56 848321; e-mail: [email protected].
fibre surface coatings can be applied by various methods such as electroless deposition, electroplating, vapour deposition and plasma spraying [2]. Nickel, copper and gold (which are chemically stable with carbon and can be deposited by an electrochemical or chemical methods) are three types of widely used surface coatings for carbon fibres used in metal matrix composite. However, gold has the advantage of a better oxidation resistance than nickel and copper. For this reason, a gold fibre coating for graphite/aluminum composites was investigated. Because most metal matrix composites can be regarded as non-equilibrium systems during their fabrication and their high temperature service, a gradient of chemical potential exists at the F / M (fibre/matrix) and F / c o a t i n g / M interfaces. This
0039-6028/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0039-6028(95)01223-0
840
J.F. Silvain et aL / Surface Science 352-354 (1996) 839-844
difference in chemical potential provides the driving force for diffusion a n d / o r chemical reaction when the composites are heated to elevated temperature
2.2. Model composite produced by physical vapour deposition (PVD)
[3].
2.2.1. Materials
In order to obtain some information about the interfacial reactions occurring at the A1/C and A 1 / A u / C interfaces, two different composites have been characterized (a) a real composite prepared by squeeze casting of aluminum into a gold coated carbon fibre preform and (b) a model composite prepared by PVD deposition of aluminum and alum i n u m / g o l d thin films on a flat polished carbon substrates. The composition of the phases formed at the interface, under controlled conditions, were analyzed by electron probe micro analysis (EPMA) line profiles on the real composite and Auger electron spectroscopy (AES) depth profiles on the model composite. By comparing the experimental results of the two systems, it was found that the analysis of the model composite can give relevant information regarding the M / F chemical interactions of the real composite quicker and at lower costs than the analysis of the real composite.
Aluminum alloy and gold thin films were deposited on thick carbon polished substrates by sputtering using a magnetron sputterer. The targets were placed at a distance of 50 cm from the substrate surface and a temperature of 20°C was maintained during the deposition. The rather high deposition rate prevented oxygen contamination of the films. The average composition of the AS7G0.6 layer, measured by EPMA, was analogue with the target. Some model composites were annealed under a controlled atmosphere (pressure inside the annealing chamber lower than 10 -5 mbar) for 30 min at 500 and 600°C.
2. Experimental procedure 2.1. Real composites produced by squeeze casting 2.1.1. Materials The composite was produced by a squeeze casting process which is one of the most important casting methods presently used [4,5]. The matrix was an aluminum alloy with 7 at% silicon and 0.6 at% magnesium (AS7G0.6). The T700 carbon preforms, consisting of either coated (electrochemical deposition of dense 0.3 /zm gold film) or uncoated fibres, were infiltrated by the molten aluminium alloy matrix. The pressures applied to the system were up to 100 MPa.
2.1.2. Method After cooling, ingots were cut using a diamond saw and subsequently polished to a mirror finish for E P M A observations. The microprobe used was a CAMECA with a spatial resolution of 0.5 /zm.
2.2.2. Method As deposited and annealed composites were analyzed, using the combination of ion etching with the inherent sensibility of AES, in order to profile the elemental concentration of each model composite with depth. The static pressure of the argon used was around 2 × 10 - v mbar and the etching rate was approximately 10 n m / m i n . The AES used was a Physical Electronics 590. The energy of the incident electrons was 5 keV. The data was taken in the conventional dN(E)/d(E) mode. The energy windows scanned in this multiplexing technique were for each element: O: 495-515 eV, AI: 1375-1405 eV, Au: 130-165 eV, C: 250-275 eV, and Si: 1595-1620 eV. The elemental sensitivity factor which was used for the quantitative analysis has both advantages and disadvantages. The advantages include the elimination of standards and the insensitivity to surface roughness and while the disadvantages include a neglect of variation in the backscattering factor and escape depth with material. Because the data was taken in the dN(E)/d(E) mode, the sensitivity factor was based on peak to peak heights. However, the peak to peak heights are not necessarily invariant to the matrix material; a shape variation of the N(E) curve leads to a variation, in the energy position a n d / o r the shape, of the dN(E)/d(E) curve. In particular, the energy position of the minimum a n d / o r the maximum peaks are changed. Although the dN(E)/d(E) peak position (i n respect to energy) of each element or compound is based on
J.F. Silvain et a l . / Surface Science 352-354 (1996) 839-844
it's minimum position, the maximum peak position (which seems to be more compound sensitive), is used in this study in order to identify the different aluminum species present after annealing the model composites. For example, the peak shape for a pure metal can be different than that of its oxide or compound. Some problems can therefore arise in the calculation of atomic concentration of the surface oxide or the compound interface, hence, the measured atomic concentration of these species can vary from the real one.
841
Carbon
Au/AS7G0.6
C
0
C
C
Total line profile : 3.07 microns
3. R e s u l t s
A1 - Au Inteaphas¢
3.1. Real composite
Si rich intcrphasc
AS7G0.6
Fig. 2. EPMAqualitative line profile across the F/M interface for a AS7G0.6/Au/C real compositeannealed at 600°C for 96 h.
Fig. 1 shows an EPMA qualitative line profile across the F / M interface for an AS7G0.6/C composite annealed at 600°C for 96 h (this rather long annealing time of 96 h was used in order to accentuate the formation of the reaction zone for subsequent analysis). The slopes of the carbon and aluminum profiles at the F / M interface shows the formation of a C-A1 compound. This interracial reaction zone also display a depletion of silicon which is rejected at the reaction zone/matrix interface. Fig. 2 shows an EPMA line profile across the F / M interface for an A S 7 G 0 . 6 / A u / C composite
AS7G0.6
Carbon fibre
annealed at 600°C for 96 h. Several features can be observed in this figure: 1. Even if the electron diffusion is more important at the AS7G0.6/C interface than at the C / A u interface, the abrupt slope of the carbon line profile can be associated with a sharp interface, and in consequence, an absence of carbon diffusion inside the matrix resulting in the absence of A14C3 formation. 2. An A1-Au interphase between the carbon fibre and the AS7G0.6 matrix. There is an absence of silicon in this A1-Au interphase region. 3. An A1-Si rich region at the A1-Au interface; the Si, which is rejected from the A1-Au region, segregates to the A1-Au matrix border. The gold atoms do not appear to diffuse inside the AS7G0.6 matrix.
c
3.2. Model composite 3.2.1. AS7GO.6 / C Si
0
AI
A1
Si
Total line proffie : 2.31 microns
AS7G0.6
Reaction zone A14C3
Fig. 1. E P M A q u a l i t a t i v e line profile a c r o s s the F / M i n t e r f a c e f o r a A S 7 G 0 . 6 / C real c o m p o s i t e a n n e a l e d at 6 0 0 ° C for 9 6 h.
A E S has been performed on AS7G0.6/C model composite samples for which AS7G0.6 was deposited on carbon substrates and annealed at 600°C for 30 min. Fig. 3a shows the AES depth profile where four regions can be observed: (1) an A1 oxide region at the composite surface which is due to oxidation during the annealing process, (2) an A1 metal region, (3) an interphase zone where carbon
J.F. Silvain et a L / Surface Science 352-354 (1996) 839-844
842
a) Oxide . . . .
1{)(2
Reaction z o n e A L 4 C3
AI m at ri x
I . . . .
] . . . .
I . . . .
! . . . .
] . . . .
] . . . .
Carbon Substrate I . . . .
~ . . . .
[ H 5 n i
80.
60.
r5 "<
40.
\
20.
c
/½ . . . . . . . . . . .
0
10
20
30
3.3. AS7GO.6 / Au / C
oO
o 40
50
60
specifically o f the carbide during the annealing process. With respect to the m i n i m u m of the Auger A1 curves, the energy shift difference between the metal (1397 eV) and the oxide (1390 eV) is 7 eV. This experimental value, which is much smaller than the value quoted in the literature of 18 eV [6], is due to the reduction of the oxide, under the electron beam, during the Auger analysis.
. . . . . .
70
Figs. 4a and 4b show respectively the A E S depth profile of the non-annealed and the annealed (600°C,
..
80
90
100
Depth into Specimen (nm)
b) 7
_
A
........
B
I. . . . .
a
AI layer
100
C
.
.
~ .
.
A u layer
~ .
.
~ .
.
T .
C substntte
.
~ .
.
;
,
.
~ .
.
I., .I . . . . . . . . . . . . . 80-
6 -
5-" .
o
AI metal
C
60-
r.=_
4-c~
Al OXl
40-
3-" 20-
1" Oj 0
.... 1370
i .... 1375
i .... 1380
f • . ~ 1385 1390
30 1395
6O
9O
I
L
I
ff
120
150
180
210
240
Depth into specimen (nm)
1400
Electron Energy (eV) Fig. 3. (a) In depth AES profile of a AS7G0.6/C model composite annealed at 600°C during 30 min. (b) AI KLL Auger spectra of the same composite taken in three different regions as defined in (a): the oxide, the metal and the carbide layers. and aluminum react to form AlaC 3 carbide, and (4) the carbon substrate. Fig. 3b shows the d N ( E ) / d ( E ) A1 profile associated with the three regions (going from the oxide surface to the carbon substrate) defined previously. These plots describe the evolution, during the sputtering time, o f the aluminum peak shape. The energy shifts, of .the maximum position of the peak, observed for the three aluminum species, represented by A = A1 oxide, B = A1 carbide and C = A1 metal in Fig. 3b represents the difference in the chemical environment of the aluminum atoms. T h e s e shifts occur do to the formation o f the oxide and more
b
Oxide
°t:o 30
AI-Au inlerplmse
C
subsn'at¢
/ 60 90 t20 150 180 Depth into Specimen (nm)
210
z40
Fig. 4. In depth AES profile of (a) the non-annealed AS7G0.6/Au/C model composite and (b) the annealed one (600°C, 30 rain).
J.F. Silvain et aL / Surface Science 352-354 (1996) 839-844
30 min) model composites. Fig. 4a shows clearly the sequence of the two layers which have been deposited on a polished carbon substrate (60 nm of gold followed by 90 nm of AS7G0.6). The relative abrupt slopes of the carbon and gold line profiles at the C / A u interface and likewise that of the Au and AI line profiles at the Au/AS7G0.6 interface, shows the quasi-absence of diffusion of these elements during the deposition process. In order to highlight the diffusion process after deposition, annealing of the composite (600°C, 30 min) has been performed (cf. Fig. 4b). From the AES depth profiles the following information can be noted: (1) An oxidation zone of the AS7GO.6 free surface in the model composite, induced by he presence of oxygen inside the annealing chamber. (2) An A1-Au compound with a constant composition in the A1-Au interphase. From the A.C. calculation the compound formed has a composition (55 at% A1, 35 at% Au) close to that of A12Au. This agrees with the results found for the real composite for which a diffusion of the two species (A1 and Au) is visible inside the carbon substrate. (3) The slope of the carbon line profile, which is equivalent for the annealed and non annealed sam~ pies (see Figs. 4a and 4b), shows that the carbon
A
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Electron Energy (eV) Fig. 5. AI KLL Auger spectra of the A S 7 G 0 . 6 / A u / C model composite taken in three different regions as defined in Fig. 4b: the near carbon substrate, the C / A I - A u interface and the AI-Au bulk interphase.
843
does not react with the matrix and therefore the A1-Au interphase seems to inhibit the formation of any carbide reaction product. This is clearly visible in Fig. 5 where the dN(E)/d(E) profile obtained in the different regions of Fig. 4b (oxide surface, A1-Au compound) shows just one aluminum species (D around 1381 eV) which does not correspond to any species found in Fig. 3b. The energy shift of the maximum position of the peak is attributed to the A1-Au compound.
4. Summary
Three points will be summarized in this study of the formation of an A S 7 G 0 . 6 / C and AS7G0.6/ A u / C interfaces: - The formation of an aluminum carbide interphase for both the real and model AS7G0.6/C composites after annealing at 600°C. This formation can be divided into three steps: (1) the dissolution of carbon atoms from the graphite fibre, (2) the diffusion of carbon into the AS7G0.6 matrix, and (3) the reaction of carbon and aluminum to form A14C 3. This carbide formation in A1/C composites must be overcome to avoid the degradation of the carbon fibres (via carbon diffusion) and the formation of a fragile A14C 3 interphase. - The absence of formation of the above mentioned carbide for the A S 7 G O . 6 / A u / C composite. In this composite, the gold layer inhibits the carbon diffusion into the matrix and thereby any C-A1 reaction. However, the properties of the A12 Au compound found after annealing must be further studied in order to understand its influence on the final properties of the composite. - Similar results were found for both the real and the model composites. In effect, because the same sequence of reaction zones were found after annealing for both types of composites. We can conclude from such results that the model composite can be very helpful in giving an idea as to the chemical reactions which can take place at a F / M interface in a real composite. An important benefit of the presented results is that this model can be used for any system for which reaction occurs during either fabrication or use and must be characterized.
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J.F. Silvain et a L / Surface Science 352-354 (1996) 839-844
References [1] J.P. Rocher, J.M. Quenisset and R. Naslain, J. Mater. Sci. Lett. 4 (1985) 1527. [2] K. Honjo and A. Shindo, J. Mater. Sci. 21 (1986) 2043. [3] T.W. Chou, A. Kelly and A. Okura, Composite 7 (1985) 201.
[4] Y. Abi, S. Horikiri, K. Fujimura and E. Ichiki, Proc. of ICCM-IV, Ed. T. Hayashi (1982) p. 1427. [5] T.W. Clyne and J.F. Mason, Metall. Trans. A 18 (1987) 1519. [6] Handbook of Auger Electron Spectroscopy (Physical Electronic, Eden Prairie, MN, 1978).