Materials Science and l:ngineering, A 145 ( 1991 179-86 79

Formation of Fe-A1203 interfaces by hot pressing

M. A. Smith and D. P. Pope University ql'Pem~syh,ania. 1)~Tmrtment ~!f Material,~ Science and Engineering, 323/ Wulnut Street, l'hiladelphia, I'A 1~104 (U.S.A.)

(Received November 20, 1990: in revised fi~rm March 7, 1991 )

Abstract

Bonds of substantial strength may be formed between sapphire ((. t-Al203) and high purity iron. Samples were fabricated under reducing conditions using hot pressing, at a pressure of 10 MPa and temperatures of 850-1450 °C in a vacuum of 3 x 10 ~' Torr or better. The interface microstructure was characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. The strongest bonds were observed to be accompanied by the development of a thin layer of oxide (75-450 A thick) at the interface which was determined by electron diffraction studies to be leO.

1. Introduction

Metal-ceramic interfaces have been the sub- ject of extensive research in recent years. Appli- cations such as high temperature engine components, catalysts and alloys resistant to high temperature oxidation have placed increasing emphasis on metal-ceramic interfacial phe- nomena. Numerous studies have examined the various physical, chemical, microstructural and mechanical properties of metal-ceramic inter- faces. Although great strides have been made in these areas, much more work is needed before a general understanding of these phenomena can be reached.

The focus of the present work is on the micro- structure and microchemistry of Fe-AI203 inter- faces. The system is of considerable practical importance since iron is a major constituent in many Al203-forming oxidation-resistant high temperature alloys. Spalling of these Al203 coatings is a major concern to industry since it results in a vastly reduced service life for the alloy. The results of this work indicate that the development of an interracial layer of FeO results in a strong bond between iron and A1203.

Previous studies have been performed on this system. In one, by Sakata et al. [ 1 ] the iron surface was pre-oxidized and quenched to give an FeO layer. These were then hot pressed with AI203 under a 10 ~ Pa vacuum at 1250 °C for 1 h. The result was the formation of an FeA120 ~ interlayer

approximately 6 gm thick. This phase is expected to form, based on the phase diagram for the system. In other studies by Nicholas [2] and by Pilliar and Nutting [3], only a limited examination of the interface was performed. Thus much remains unknown about the structure of Fe- AI20 ~ interfaces. In this study, we have investi- gated the formation of the interfacial bond between pure iron and sapphire with the inten- tion of determining the microstructure of the interfacial region.

2. Experimental procedure

Iron foils were produced by cold rolling 99.98% pure iron to a thickness of 25-150 p~m. The foils were ultrasonically degreased in ace- tone, washed in dry ethanol, dried and then ground to a 600 grit finish. The washing proce- dure was then repeated. This resulted in a bright oxide-free surface. Sapphire single crystals oriented to (1120) and polished to an optical quality finish were obtained from Saphikon Inc. These were cleaned in the same way as the iron.

The samples were arranged as sapphire/Fe/ sapphire sandwiches and hot pressed at 10 MPa and temperatures ranging from 850 to 1450 °C in a graphite furnace (Fig. 1 ). Tantalum inserts were placed between the graphite pressing rams and the sapphire to prevent reaction between them. The vacuum was maintained at better than

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Fig. 1. (a) Typical hot-pressed specimen; (b) hot-pressing furnace.

3 × 10-6 Torr at the pressing temperature. The presence of graphite heating elements assured that very low oxygen partial pressures were main- tained.

Interfacial microstructures were characterized via optical microscopy, scanning electron micros- copy (SEM) and transmission electron micros- copy (TEM). The interfacial chemistry was initially evaluated by fracturing the specimens in air and examining the surfaces with scanning Auger microscopy. Later, an impact stage was utilized which allowed in situ fracture of the specimens in the scanning Auger microscope. The vacuum in the chamber was maintained at 5 × 10 -=0 Torr or better. The FeO layer was identified using electron diffraction. Auger depth- profiling techniques were used to estimate the oxide layer thickness. Sputter rates were cali- brated using SiO 2 layers of known thickness. Correlation between this rate and that of FeO was estimated from the known sputter yields of ele- mental iron and silicon. Estimates of the thick- ness were further refined on the basis of the TEM images of the interfaces of some samples.

Evaluation of the interfacial strength is qualita- tive. Samples were fractured by extending the end of the samples over a sharp edge and striking the cantilevered portion. For specimens denoted as

ion mill cover plate

Ta shield

specimen

Fig. 2. Schematic diagram of cover plates used in the ion mill.

having strong adhesion, fracture of the interface and subsequent exposure of the sapphire and metal surfaces was limited to very small areas. Removal of larger areas required near pulveriza- tion of the sapphire substrates. Those termed as exhibiting moderate adhesion could be removed over somewhat larger areas and with consider- ably less effort. Those termed as having poor adhesion could be removed with little difficulty and with little or no damage to the substrate. Finally, it is noted that fracture proceeded in a manner which left both surfaces smooth and the metal mirror-like, indicating that little or no plastic deformation had occurred.

Samples for TEM examination of the interface were thinned by mechanical grinding to 125 ~tm, further thinned to 50 ~tm by dimpling with 1 ~tm diamond and then ion milled to electron trans- parency. Liquid-nitrogen cooling was used to minimize beam damage. Tantalum shields (Fig. 2) were used to limit milling to two 90 ° zones centred perpendicular to the interface. Sputtering without these shields results in preferential sput- tering of the iron. Details of a similar technique are described in the literature [4].

Samples for electron diffraction studies of the oxide in the transmission electron microscope were prepared by first separating the metal foils from the sapphire. The foils were then mechani- cally back thinned to electron transparency using the dimpler and 0.25 ~m diamond. Ion milling was not used since it was found to cause severe damage to the oxide layer.

3. Results and discussion

Samples hot pressed at 850°C for up to 12 h showed the development of no interfacial layer

and no adhesion. They were not considered further. An increase in temperature to 1250°C resulted in the formation of an interracial layer. The formation of this layer was found to be time dependent. After 1 h, the layer that formed was quite patchy (Fig. 3). Growth of the layer appears to coincide with the areas in which the metal has made good contact with the substrate and is notably absent from areas where grinding marks on the metal are still present. Further, the metal grain boundaries appear to be relatively low energy nucleation sites for the layer since growth begins there and proceeds across the grains. The

adhesion of these specimens was fair. Increasing the pressing time to 5 h resulted in a more com- plete layer (Fig. 4) with many fewer grinding marks still visible and better foil-substrate

I J

w TRANSPARENT SAPPHIRE

(a/

Fig. 3. (a) Schematic view of optical microscopy examina- tion; (b) incomplete oxide interlayer (arrow) prepared at 125(I °C for I h (viewed through the transparent sapphire).

Fig. 4. Incomplete oxide interlayer prepared at 1250 °C for 5 h (viewed through the transparent sapphire substrate): (a) ~ample interior; (b) sample edge where local curvature pre- vents Fe A120~ contact.

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contact. The adhesion was noticeably improved. After 12 h, the layer was generally found to be complete and strong adhesion was noted (Fig. 5). An increase in temperature to 1450 °C results in a complete film in less than 1 h (Fig. 6). In all cases, some tensile fracture of the sapphire due to thermal stresses was observed; however, these exhibited no tendency to spall. This phenomenon

has been discussed by Evans [5]. Furthermore, samples with areas as large as 1 c m 2 c a n be prepared which are completely free from cracks.

Fig. 5. Completed oxide interlayer prepared at 1250 °C for Fig. 6. Completed oxide interlayer prepared at 1450 °C for 12 h: (a) sample interior; (b) edge. 1 h: (a) sample interior; (b) edge.

As has been stated, evaluation of the interface strength is qualitative. Attempts to measure the adhesion quantitatively met with only limited success. Peel testing was attempted; however, there were several problems. First, in order to perform a peel test, only one of the iron surfaces may be bonded to sapphire. In this geometry, it was found that the oxide layer formed only as patches in the interface of single sapphi re-Fe bonds. Furthermore, some buckling of the foils from the surface was seen in areas which lacked the interlayer. Second, it was found that the metal layer thickness had to be 25 ~tm or less or the residual thermal stresses, which depend critically on the specimen dimensions [5, 6], would shatter the sapphire and render the specimen unusable. Subsequent tests on the foil specimens 25 ~m thick failed because it was found that the foils would tear before the interface would fracture. However, it was found that sufficient load could be applied to fracture the interface in areas with only small interfacial islands. In this situation, brittle interfacial fracture would occur suddenly over the entire area of the island. In areas without an interlayer, the metal was observed to peel away at unmeasurably low forces in areas without the interlayer.

SEM analysis of the impact fractured surfaces showed them to be smooth and relatively feature- less (Fig. 7). Auger analysis of the segregated interlayer showed that it contained only iron and oxygen (Fig. 8(a)). Surface coverages of segre- gated carbon and occasionally silicon were also observed, but no aluminium was ever seen in the layer. Depth profiles of the oxide layer performed in the scanning Auger microscope showed the thickness of the oxide to be linearly proportional to the metal thickness (Fig. 9). Error bars in the diagram reflect uncertainties in exact metal thick- ness and the finite sputter times between Auger surface scans. No significant difference in surface chemistry or oxide thickness was observed between specimens fractured in the Auger and those fractured in air. Electron diffraction studies revealed the oxide layer to be composed of fine- grained (less than 0.5 ~tm) polycrystalline FeO (Fig. 10). The sapphire surfaces showed no trace of iron (Fig. 8(c)). Therefore it was concluded that fracture occurs at the interface between FeO and A1203.

The presence of wustite at the interface is diffi- cult to account for owing to the low oxygen con- tent of the vacuum (Po~ = 10-21 Tort at 1250 °C).

Fig. 7. (a) SEM image of an iron fracture surface prepared at 1250 °C for 12 h: (b) A120~ fracture surface.

Exposure of the fracture surface to the vacuum at 1250 °C results in rapid reduction to iron. Fur- thermore, the oxide is expected to transform to Fe~O4 below 565 °C. However, it can be retained metastably if sufficient cooling rates are employed.

Additionally, the iron foils were known to con- tain 18 wt.ppm O and the results in Fig. 9 suggest that the oxide layer might have come from the iron. However, the oxide thicknesses observed require a bulk level of approximately 100 wt.ppm, irrespective of any oxygen which remains in the iron. Thus it is probably not due to segrega- tion of oxygen from the foil. Instead, it appears that a partial reduction of A1203 is occurring. The oxide film-metal thickness dependence also

84

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Fig. 8. Auger spectrum of (a) the iron fracture surface, (b) the iron surface after 10 s sputter (20 A) and (c) AI203 frac- ture surface (specimen prepared at 1250 °C for 12 h).

appears to support such a conclusion. The driving force for reactions of this type has been discussed in detail by Klomp [7], who showed that both vacuum environment and the free energy of solution of the components in each other may offset an otherwise positive free energy of reaction. Such calculations have been attempted for this system but uncertain and incomplete thermodynamic data have prevented conclusive results.

1

v

1 0 0 200

metal thickness (microns)

Fig. 9. Oxide interlayer thickness vs . metal thickness for specimens prepared at 1250 °C for 12 h.

Fig. 10. Electron diffraction pattern from the polycrystalline FeO interlayer showing a (100) texture.

Under more oxidizing conditions the forma- tion of an iron spinel layer (FeA1204) is predicted by the thermodynamics. Indeed, such a layer has been observed both in previous studies [1], as well as by the present authors when sufficient oxygen partial pressures are maintained (Fig. 11 ).

Less obvious is the role of kinetics in these processes. FeO is, as previously noted, unstable under the fabrication conditions. However, in unfractured specimens, heat treatments in the hot-pressing vacuum at 1250-1450 °C and in dry

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FeO FeAI204 7 Fig. I 1. SEM images: (a) an iron fracture surface with indentalion~ from FeAI20 ~ crystals (specimen produced at 1250 °C for 12 h): (b) AI30~ fracture surface with FeAI30~ crystals.

hydrogen at 800-1100°C have produced no appreciable reduction of the layer. Further, the phase diagram predicts that this layer should react to give the iron spinel layer, discussed above, when it is in contact with sapphire. Failure to observe it may be the result of several different effects. One possibility is that the spinel layer does form during hot pressing but decomposes during cooling. The other is that its failure to form is the result of a growth instability due to an interracial kinetics barrier, as has been discussed by Goesele and Tu [8]. In this view, the failure to observe a predicted equilibrium product can be accounted for if the oxide layer is less than a critical thickness. Indeed, this may explain why the iron aluminate is observed when additional oxygen is available. This theory has only been discussed for binary systems, but such effects have also been postulated to occur in ternary systems [9].

TEM bright field images revealed the presence of an oxide layer as expected. Thickness measurements of the oxide roughly conform with those estimated from the Auger depth profiles and were used to correct the thickness estimates. As shown in the micrographs the interfaces are straight and parallel (Fig. 12). The two-tone shading of the oxide layer is due to partial reduc- tion of the interlayer by the ion mill. Electron dif- fraction studies reveal the orientation relationship between iron and AI203 to be (110)11(1120). The [111] zone axis of the iron foil

AI203

FeO

Fe

Fig. 12. (a) T E M bright field image of the interfacial area; /b) electron diffraction pat tern from iron 4 ° from [1 i 1 ] zone axis; c c) electron diffraction pattern from AI_~O~ ([0001] zone axis). The specimen was prepared at 1450 °C for 1 h.

86

AI203

Fo

Fig. 13. (a) TEM bright field image of a direct Fe-A1203 bond; (b) electron diffraction pattern from the interface, (110)H(1120), [1 i 1] rotated 4 ° from [0001] in the interfacial plane. The specimen was prepared at 1450 °C for 5 h.

was found to be rotated 4 ° from the [0001] zone axis of the sapphire in the interface plane. The wurzite interlayer was found to possess a fairly strong (100) texture (Fig. 10).

One highly unusual specimen has been pre- pared which has no resolvable oxide interlayer (Fig. 13). Why an interfacial bond can form in this sample without an interracial oxide is not known but we believe that the oxide layer may have been reduced during ion milling (since the oxide layer was observed optically prior to milling). The same orientation relationship between iron and

A1203 is found to hold for this specimen as for those with oxide interlayers. This result suggests that direct sapphire - Fe bonding may be possible if oxide formation can be suppressed or if the oxide can be reduced by subsequent treatment. Failure to observe this direct bonding macro° scopically can be attributed to thermal mismatch strains which have been shown to be critically dependent on the sample dimensions [5, 6]. Because of this, direct bonding may be limited to very small areas and to very thin metal films. Finally, the ability of this bond to survive TEM sample preparation suggests that such bonds may be quite strong in the absence of residual thermal stresses.

4. Conclusions

Hot isostatic pressing of iron and sapphire under reducing conditions leads to strong inter- facial bonding. The adhesion is associated with the formation of a thin layer of FeO. The possi- bility of direct Fe -AI203 bonding is less clear. Whereas bulk unoxidized areas show little adhe- sion, there is some evidence to suggest that such bonding may occur over small areas of the sur- face. Finally, it is pointed out that the vacuum environment plays a pivotal role in both the microstructural and microchemical details of the interface.

References

1 K. Sakata, K. Honma, K. Ogawa, O. Watanabe and K. Nii, J. Mater. Sci., 21 (1986)4463.

2 M. Nicholas,,/. Mater. Sci., 3 (1968) 571. 3 R. M. Pilliar and J. Nutting, Philos. Mag., 16 (1967) 181. 4 U. Helmersson and J. E. Sundgren, J. Electron Microsc. Tech., 4 (1986) 361. 5 H.E. Evans, Mater. Sci. Eng., A120 (1989) 139. 6 M. Shimada, K. Suganuma, T. Okamoto and M. Koizumi,

Proc. Conf. on Ceramic Microstructures '86; Role of Inter- faces, Plenum, New York, 1987, p. 409,

7 J.T. Klomp, in L. C. DuFour, C. Monty and G. Petot-Ervas (eds.), Surfaces and Interfaces of Ceramic Materials, Kluwer Academic, Dordrecht, 1989, p. 375.

8 U. Goesele and K. N. Tu, J. Appl. Phys., 53(4) (1982) 3252.

9 K.J. Schultz, X. Y. Zheng and Y. A. Chang, J. Mater. Res., 4(6)(1989) 1462.