On the atomic structure of the Nb/Al 2 O 3 interface and the growth of Al 2 O 3 particles
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M. Ruhle Materials Department, University of California-Santa Barbara, Santa Barbara, California 93106 (Received 8 August 1988; accepted 13 March 1989)
The growth mechanism for small precipitates of A12O3 formed by internal oxidation in the Nb-Al 2 O 3 interface is studied in detail. The observations show that the Nb (001)/Al2O3 (00.1) interface is almost atomically flat and that there are no interface compounds. We suggest that the final layer on the A12O3 side of this interface consists of oxygen atoms. The effects of image forces on misfit dislocations are found to result in a standoff distance between dislocation cores and the interface, in good agreement with the recent theory. The implications of this for the strength of metal-ceramic bonding are discussed.
I. INTRODUCTION The study of the atomic structure at metal-ceramic interfaces is important for both fundamental and applied reasons. The niobium-sapphire system studied here has applications including Josephson junctions and structural ceramics, and has been widely studied as a model system.1"3 Structural ceramics must be bonded to metals. The applicability of the ceramic depends mainly on the fracture resistance of the bond. An understanding of the adhesion between the dissimilar materials requires that structural details are available down to the atomic level. Recently it has become possible to prepare extremely well-characterized interfaces, and to study their structure at atomic resolution using high-resolution electron microscopy (HREM). This technique is now capable, in favorable cases, of determining the atomic structure of an interface of known composition.4 It is therefore able to answer such questions as the location of misfit dislocation cores, the presence or absence of reaction layers, and the degree of surface roughness at interfaces (on an atomic scale). It is also possible in some cases to determine precisely the atomic structure at the interface plane (see Ref. 5 for a review). In this paper we report a study of the growth mechanism of A12O3 particles formed by internal oxidation in Nb. We also discuss the atomic structure of the Al2O3/Nb interface and find that, in agreement with recent elasticity calculations,6 the misfit dislocations do not occur exactly at the interface. Rather, due to image forces, their cores are set back a small "standoff" distance into the Nb. The implications of this finding for the strength of metalceramic interfaces generally are discussed. II. EXPERIMENTAL The Nb-Al alloy was internally oxidized by holding niobium containing 3 at. % Al at 1450 °C for 40 min at a 972
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J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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pressure of 5 X 10 5 mbar oxygen. This produces the A12O3 precipitates, as described elsewhere. 2 The final specimens for the electron microscope were prepared by the argon ion bombardment of cut and dimpled samples. They suffer little radiation damage. However, because of the slightly different thinning rate of Nb and A12O3, the thickness on the
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